168-677: Saturn is the sixth planet from the Sun and the second largest in the Solar System , after Jupiter . It is a gas giant , with an average radius of about nine times that of Earth . It has an eighth the average density of Earth, but is over 95 times more massive. Even though Saturn is almost as big as Jupiter, Saturn has less than a third the mass of Jupiter. Saturn orbits the Sun at a distance of 9.59 AU (1,434 million km ), with an orbital period of 29.45 years. Saturn's interior
336-469: A geodynamo that generates a magnetic field . Similar differentiation processes are believed to have occurred on some of the large moons and dwarf planets, though the process may not always have been completed: Ceres, Callisto, and Titan appear to be incompletely differentiated. The asteroid Vesta, though not a dwarf planet because it was battered by impacts out of roundness, has a differentiated interior similar to that of Venus, Earth, and Mars. All of
504-438: A magnetic moment 580 times that of Earth because of Saturn's greater size. Saturn's magnetic field strength is about a twentieth that of Jupiter. The outer atmosphere is generally bland and lacking in contrast, although long-lived features can appear. Wind speeds on Saturn can reach 1,800 kilometres per hour (1,100 miles per hour). The planet has a bright and extensive system of rings , composed mainly of ice particles, with
672-460: A nebula to create a young protostar orbited by a protoplanetary disk . Planets grow in this disk by the gradual accumulation of material driven by gravity , a process called accretion . The word planet comes from the Greek πλανήται ( planḗtai ) ' wanderers ' . In antiquity , this word referred to the Sun , Moon , and five points of light visible to the naked eye that moved across
840-430: A resonance with Titan, was discovered in 1848 by a British team. In 1899, William Henry Pickering discovered Phoebe, a highly irregular satellite that does not rotate synchronously with Saturn as the larger moons do. Phoebe was the first such satellite found and it took more than a year to orbit Saturn in a retrograde orbit . During the early 20th century, research on Titan led to the confirmation in 1944 that it had
1008-406: A rogue planet , is believed to be orbited by a tiny protoplanetary disc , and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses. The idea of planets has evolved over the history of astronomy, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in
1176-426: A star , stellar remnant , or brown dwarf , and is not one itself. The Solar System has eight planets by the most restrictive definition of the term: the terrestrial planets Mercury , Venus , Earth , and Mars , and the giant planets Jupiter , Saturn , Uranus , and Neptune . The best available theory of planet formation is the nebular hypothesis , which posits that an interstellar cloud collapses out of
1344-519: A triaxial ellipsoid . The exoplanet Tau Boötis b and its parent star Tau Boötis appear to be mutually tidally locked. The defining dynamic characteristic of a planet, according to the IAU definition, is that it has cleared its neighborhood . A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with
1512-504: A boundary, even though deuterium burning does not last very long and most brown dwarfs have long since finished burning their deuterium. This is not universally agreed upon: the exoplanets Encyclopaedia includes objects up to 60 M J , and the Exoplanet Data Explorer up to 24 M J . The smallest known exoplanet with an accurately known mass is PSR B1257+12A , one of the first exoplanets discovered, which
1680-535: A clearly defined eyewall . Eyewall clouds had not previously been seen on any planet other than Earth. For example, images from the Galileo spacecraft did not show an eyewall in the Great Red Spot of Jupiter. The south pole storm may have been present for billions of years. This vortex is comparable to the size of Earth, and it has winds of 550 km/h. Saturn has an intrinsic magnetic field that has
1848-547: A complex organic chemistry occurs. It is the only satellite with hydrocarbon lakes . On 6 June 2013, scientists at the IAA-CSIC reported the detection of polycyclic aromatic hydrocarbons in the upper atmosphere of Titan, a possible precursor for life . On 23 June 2014, NASA claimed to have strong evidence that nitrogen in the atmosphere of Titan came from materials in the Oort cloud , associated with comets , and not from
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#17327660007192016-689: A disk remnant left over from the supernova that produced the pulsar. The first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of 51 Pegasi b , an exoplanet around 51 Pegasi . From then until the Kepler space telescope mission, most of the known exoplanets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets
2184-516: A drag upon Saturn's magnetic field, slowing its rotation slightly relative to the rotation of the planet. An apparent oddity for Saturn is that it does not have any known trojan asteroids . These are minor planets that orbit the Sun at the stable Lagrangian points , designated L 4 and L 5 , located at 60° angles to the planet along its orbit. Trojan asteroids have been discovered for Mars , Jupiter, Uranus, and Neptune. Orbital resonance mechanisms, including secular resonance , are believed to be
2352-477: A hot interior, reaching 11,700 °C (21,100 °F) at its core, and radiates 2.5 times more energy into space than it receives from the Sun. Jupiter's thermal energy is generated by the Kelvin–Helmholtz mechanism of slow gravitational compression ; but such a process alone may not be sufficient to explain heat production for Saturn, because it is less massive. An alternative or additional mechanism may be
2520-556: A level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185 to 270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 190–235 K. Finally, the lower layers, where pressures are between 10 and 20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in aqueous solution. Saturn's usually bland atmosphere occasionally exhibits long-lived ovals and other features common on Jupiter. In 1990,
2688-442: A list of omens and their relationships with various celestial phenomena including the motions of the planets. The inferior planets Venus and Mercury and the superior planets Mars , Jupiter , and Saturn were all identified by Babylonian astronomers . These would remain the only known planets until the invention of the telescope in early modern times. The ancient Greeks initially did not attach as much significance to
2856-411: A mass 5.5–10.4 times the mass of Earth, attracted attention upon its discovery for potentially being in the habitable zone, though later studies concluded that it is actually too close to its star to be habitable. Planets more massive than Jupiter are also known, extending seamlessly into the realm of brown dwarfs. Exoplanets have been found that are much closer to their parent star than any planet in
3024-454: A mean density about 4 + 1 ⁄ 2 higher than that of water (i.e., about 4.5 g/cm ), about 20% below the modern value, but still significantly larger than the mean density of normal rock, suggesting for the first time that the interior of the Earth might be substantially composed of metal. Hutton estimated this metallic portion to occupy some 20 / 31 (or 65%) of
3192-479: A metallic or rocky core today, or a reaccumulation of the resulting debris. Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid . The terrestrial planets' mantles are sealed within hard crusts , but in
3360-411: A much more diffuse core, with a mass equal to about 17 Earths and a radius equal to about 60% of Saturn's entire radius. This is surrounded by a thicker, liquid metallic hydrogen layer, followed by a liquid layer of helium-saturated molecular hydrogen , which gradually transitions to a gas as altitude increases. The outermost layer spans about 1,000 km (620 mi) and consists of gas. Saturn has
3528-585: A multitude of similar-sized objects. As described above, this characteristic was mandated as part of the IAU 's official definition of a planet in August 2006. Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs . Gravity causes planets to be pulled into
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#17327660007193696-402: A negligible axial tilt as a result of their proximity to their stars. Similarly, the axial tilts of the planetary-mass moons are near zero, with Earth's Moon at 6.687° as the biggest exception; additionally, Callisto's axial tilt varies between 0 and about 2 degrees on timescales of thousands of years. The planets rotate around invisible axes through their centres. A planet's rotation period
3864-462: A peppered coating of approximately 7% amorphous carbon . The particles that make up the rings range in size from specks of dust up to 10 m. While the other gas giants also have ring systems, Saturn's is the largest and most visible. There is a debate on the age of the rings. One side supports that they are ancient, and were created simultaneously with Saturn from the original nebular material (around 4.6 billion years ago), or shortly after
4032-480: A phenomenon in the Solar System. Whereas temperatures on Saturn are normally −185 °C, temperatures on the vortex often reach as high as −122 °C, suspected to be the warmest spot on Saturn. A persisting hexagonal wave pattern around the north polar vortex in the atmosphere at about 78°N was first noted in the Voyager images. The sides of the hexagon are each about 14,500 km (9,000 mi) long, which
4200-413: A planet reaches a mass somewhat larger than Mars's mass, it begins to accumulate an extended atmosphere , greatly increasing the capture rate of the planetesimals by means of atmospheric drag . Depending on the accretion history of solids and gas, a giant planet , an ice giant , or a terrestrial planet may result. It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in
4368-426: A plausible base for future human exploration . Titan has the only nitrogen -rich planetary atmosphere in the Solar System other than Earth's. Just as Earth's conditions are close to the triple point of water, allowing it to exist in all three states on the planet's surface, so Titan's are to the triple point of methane . Planetary atmospheres are affected by the varying insolation or internal energy, leading to
4536-611: A plume on Enceladus found most of the ingredients to sustain life forms that live by methanogenesis . In April 2014, NASA scientists reported the possible beginning of a new moon within the A Ring , which was imaged by Cassini on 15 April 2013. Saturn is probably best known for the system of planetary rings that makes it visually unique. The rings extend from 6,630 to 120,700 kilometres (4,120 to 75,000 mi) outward from Saturn's equator and average approximately 20 metres (66 ft) in thickness. They are composed predominantly of water ice, with trace amounts of tholin impurities and
4704-461: A roughly spherical shape, so a planet's size can be expressed roughly by an average radius (for example, Earth radius or Jupiter radius ). However, planets are not perfectly spherical; for example, the Earth's rotation causes it to be slightly flattened at the poles with a bulge around the equator . Therefore, a better approximation of Earth's shape is an oblate spheroid , whose equatorial diameter
4872-430: A series of cloud features found in northern latitudes, nicknamed the "String of Pearls". These features are cloud clearings that reside in deeper cloud layers. Saturn's atmosphere exhibits a banded pattern similar to Jupiter's, but Saturn's bands are much fainter and are much wider near the equator. The nomenclature used to describe these bands is the same as on Jupiter. Saturn's finer cloud patterns were not observed until
5040-454: A significant impact on mythology , religious cosmology , and ancient astronomy . In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the " fixed stars ", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες ( planētes asteres ) ' wandering stars ' or simply πλανῆται ( planētai ) ' wanderers ' from which today's word "planet"
5208-403: A significantly lower mass than the gas giants (only 14 and 17 Earth masses). Dwarf planets are gravitationally rounded, but have not cleared their orbits of other bodies . In increasing order of average distance from the Sun, the ones generally agreed among astronomers are Ceres , Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , Eris , and Sedna . Ceres is the largest object in
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5376-407: A similar way; however, Triton was likely captured by Neptune, and Earth's Moon and Pluto's Charon might have formed in collisions. When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation , the solar wind , Poynting–Robertson drag and other effects. Thereafter there still may be many protoplanets orbiting
5544-566: A simple, symmetric shape—a magnetic dipole . Its strength at the equator—0.2 gauss (20 μT )—is approximately one twentieth of that of the field around Jupiter and slightly weaker than Earth's magnetic field. As a result, Saturn's magnetosphere is much smaller than Jupiter's. When Voyager 2 entered the magnetosphere, the solar wind pressure was high and the magnetosphere extended only 19 Saturn radii, or 1.1 million km (684,000 mi), although it enlarged within several hours, and remained so for about three days. Most probably,
5712-409: A smaller amount of rocky debris and dust . At least 146 moons orbit the planet, of which 63 are officially named; these do not include the hundreds of moonlets in the rings. Titan , Saturn's largest moon and the second largest in the Solar System, is larger (and less massive) than the planet Mercury and is the only moon in the Solar System that has a substantial atmosphere. Saturn is named after
5880-471: A terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests that the nearest would be expected to be within 12 light-years distance from Earth. The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation , which estimates
6048-521: A thick atmosphere—a feature unique among the Solar System's moons. Pioneer 11 made the first flyby of Saturn in September 1979, when it passed within 20,000 km (12,000 mi) of the planet's cloud tops. Images were taken of the planet and a few of its moons, although their resolution was too low to discern surface detail. The spacecraft also studied Saturn's rings, revealing the thin F-ring and
6216-560: A whole) based on a compilation of various measurements from the Cassini , Voyager , and Pioneer probes is 10 32 35 . Studies of the planet's C Ring yield a rotation period of 10 33 38 − 1 19 . In March 2007, it was found that the variation in radio emissions from the planet did not match Saturn's rotation rate. This variance may be caused by geyser activity on Saturn's moon Enceladus . The water vapor emitted into Saturn's orbit by this activity becomes charged and creates
6384-487: Is 5.15 × 10 kg ) and states that "compared with the weight of the globe this mighty sum dwindles to insignificance". Absolute figures for the mass of the Earth are cited only beginning in the second half of the 19th century, mostly in popular rather than expert literature. An early such figure was given as "14 septillion pounds" ( 14 Quadrillionen Pfund ) [ 6.5 × 10 kg ] in Masius (1859). Beckett (1871) cites
6552-777: Is Zȃzȇl ( Hebrew : זאזל , romanized : Zazl ). Zazel has been described as a great angel , invoked in Solomonic magic , who is "effective in love conjurations ". In Ottoman Turkish , Urdu , and Malay , the name of Zazel is 'Zuhal', derived from the Arabic language ( Arabic : زحل , romanized : Zuhal ). Saturn's rings require at least a 15-mm-diameter telescope to resolve and thus were not known to exist until Christiaan Huygens saw them in 1655 and published about this in 1659. Galileo , with his primitive telescope in 1610, incorrectly thought of Saturn's appearing not quite round as two moons on Saturn's sides. It
6720-438: Is 43 kilometers (27 mi) larger than the pole -to-pole diameter. Generally, a planet's shape may be described by giving polar and equatorial radii of a spheroid or specifying a reference ellipsoid . From such a specification, the planet's flattening, surface area, and volume can be calculated; its normal gravity can be computed knowing its size, shape, rotation rate, and mass. A planet's defining physical characteristic
6888-413: Is 74% of what it is at the poles and is lower than the surface gravity of Earth. However, the equatorial escape velocity , nearly 36 km/s , is much higher than that of Earth. Saturn is the only planet of the Solar System that is less dense than water—about 30% less. Although Saturn's core is considerably denser than water, the average specific density of the planet is 0.69 g/cm , because of
Saturn - Misplaced Pages Continue
7056-466: Is a standing wave pattern in the atmosphere. Polygonal shapes have been replicated in the laboratory through differential rotation of fluids. HST imaging of the south polar region indicates the presence of a jet stream , but no strong polar vortex nor any hexagonal standing wave. NASA reported in November 2006 that Cassini had observed a " hurricane -like" storm locked to the south pole that had
7224-420: Is a unit of mass equal to the mass of the planet Earth . The current best estimate for the mass of Earth is M 🜨 = 5.9722 × 10 kg , with a relative uncertainty of 10 . It is equivalent to an average density of 5515 kg/m . Using the nearest metric prefix , the Earth mass is approximately six ronnagrams , or 6.0 Rg. The Earth mass is a standard unit of mass in astronomy that
7392-561: Is delineated by a set of elements: Planets have varying degrees of axial tilt; they spin at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the Northern Hemisphere points away from its star, the Southern Hemisphere points towards it, and vice versa. Each planet therefore has seasons , resulting in changes to
7560-572: Is determined using laser ranging data from Earth-orbiting satellites, such as LAGEOS-1 . G M E can also be calculated by observing the motion of the Moon or the period of a pendulum at various elevations, although these methods are less precise than observations of artificial satellites. The relative uncertainty of G M E is just 2 × 10 , considerably smaller than the relative uncertainty for M E itself. M E can be found out only by dividing G M E by G , and G
7728-458: Is known as a stellar day . Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole . The exceptions are Venus and Uranus, which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise. Regardless of which convention
7896-566: Is known as) the gravitational constant . Early estimates on the mean density of the Earth were made by observing the slight deflection of a pendulum near a mountain, as in the Schiehallion experiment . Newton considered the experiment in Principia , but pessimistically concluded that the effect would be too small to be measurable. An expedition from 1737 to 1740 by Pierre Bouguer and Charles Marie de La Condamine attempted to determine
8064-412: Is known only to a relative uncertainty of 2.2 × 10 , so M E will have the same uncertainty at best. For this reason and others, astronomers prefer to use G M E , or mass ratios (masses expressed in units of Earth mass or Solar mass ) rather than mass in kilograms when referencing and comparing planetary objects. Earth's density varies considerably, between less than 2700 kg/m in
8232-411: Is longer than the diameter of the Earth. The entire structure rotates with a period of 10 39 24 (the same period as that of the planet's radio emissions) which is assumed to be equal to the period of rotation of Saturn's interior. The hexagonal feature does not shift in longitude like the other clouds in the visible atmosphere. The pattern's origin is a matter of much speculation. Most scientists think it
8400-444: Is measured indirectly by determining other quantities such as Earth's density, gravity, or gravitational constant. The first measurement in the 1770s Schiehallion experiment resulted in a value about 20% too low. The Cavendish experiment of 1798 found the correct value within 1%. Uncertainty was reduced to about 0.2% by the 1890s, to 0.1% by 1930. The figure of the Earth has been known to better than four significant digits since
8568-668: Is significantly deficient compared to the abundance of this element in the Sun. The quantity of elements heavier than helium ( metallicity ) is not known precisely, but the proportions are assumed to match the primordial abundances from the formation of the Solar System . The total mass of these heavier elements is estimated to be 19–31 times the mass of Earth, with a significant fraction located in Saturn's core region. Trace amounts of ammonia, acetylene , ethane , propane , phosphine , and methane have been detected in Saturn's atmosphere. The upper clouds are composed of ammonia crystals, while
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#17327660007198736-430: Is similar in composition to Earth, but is more dense. The examination of Saturn's gravitational moment , in combination with physical models of the interior, has allowed constraints to be placed on the mass of Saturn's core. In 2004, scientists estimated that the core must be 9–22 times the mass of Earth, which corresponds to a diameter of about 25,000 km (16,000 mi). However, measurements of Saturn's rings suggest
8904-470: Is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium . This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until
9072-437: Is the largest known detached object , a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet and Eris being
9240-500: Is the smallest object generally agreed to be a geophysical planet , at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Salacia ). An exoplanet is a planet outside the Solar System. As of 24 July 2024, there are 7,026 confirmed exoplanets in 4,949 planetary systems , with 1007 systems having more than one planet . Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over
9408-421: Is the smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet. Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. They differ from
9576-422: Is thought to be composed of a rocky core, surrounded by a deep layer of metallic hydrogen , an intermediate layer of liquid hydrogen and liquid helium , and an outer layer of gas. Saturn has a pale yellow hue, due to ammonia crystals in its upper atmosphere. An electrical current in the metallic hydrogen layer is thought to give rise to Saturn's planetary magnetic field , which is weaker than Earth's, but has
9744-492: Is tilted at an angle of 27° to the other rings and, like Phoebe , orbits in retrograde fashion. Some of the moons of Saturn, including Pandora and Prometheus , act as shepherd moons to confine the rings and prevent them from spreading out. Pan and Atlas cause weak, linear density waves in Saturn's rings that have yielded more reliable calculations of their masses. The observation and exploration of Saturn can be divided into three phases: (1) pre-modern observations with
9912-638: Is used to indicate the masses of other planets , including rocky terrestrial planets and exoplanets . One Solar mass is close to 333 000 Earth masses. The Earth mass excludes the mass of the Moon . The mass of the Moon is about 1.2% of that of the Earth, so that the mass of the Earth–Moon system is close to 6.0457 × 10 kg . Most of the mass is accounted for by iron and oxygen (c. 32% each), magnesium and silicon (c. 15% each), calcium , aluminium and nickel (c. 1.5% each). Precise measurement of
10080-457: Is used, Uranus has a retrograde rotation relative to its orbit. The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets contributes to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter
10248-530: Is variable, subject to both gain and loss due to the accretion of in-falling material, including micrometeorites and cosmic dust and the loss of hydrogen and helium gas, respectively. The combined effect is a net loss of material, estimated at 5.5 × 10 kg (5.4 × 10 long tons ) per year. This amount is 10 of the total earth mass. The 5.5 × 10 kg annual net loss is essentially due to 100,000 tons lost due to atmospheric escape , and an average of 45,000 tons gained from in-falling dust and meteorites. This
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#173276600071910416-401: Is well within the mass uncertainty of 0.01% ( 6 × 10 kg ), so the estimated value of Earth's mass is unaffected by this factor. Mass loss is due to atmospheric escape of gases. About 95,000 tons of hydrogen per year ( 3 kg/s ) and 1,600 tons of helium per year are lost through atmospheric escape. The main factor in mass gain is in-falling material, cosmic dust , meteors , etc. are
10584-455: The Huygens probe on 25 December 2004. Huygens descended onto the surface of Titan on 14 January 2005. Starting in early 2005, scientists used Cassini to track lightning on Saturn. The power of the lightning is approximately 1,000 times that of lightning on Earth. Planet A planet is a large, rounded astronomical body that is generally required to be in orbit around
10752-406: The A Ring ). The Cassini–Huygens space probe entered orbit around Saturn on 1 July 2004. In June 2004, it conducted a close flyby of Phoebe , sending back high-resolution images and data. Cassini 's flyby of Saturn's largest moon, Titan, captured radar images of large lakes and their coastlines with numerous islands and mountains. The orbiter completed two Titan flybys before releasing
10920-497: The Cavendish experiment of 1798. The mass of Earth is estimated to be: which can be expressed in terms of solar mass as: The ratio of Earth mass to lunar mass has been measured to great accuracy. The current best estimate is: The product of M E and the universal gravitational constant ( G ) is known as the geocentric gravitational constant ( G M E ) and equals (398 600 441 .8 ± 0.8) × 10 m s . It
11088-547: The Hubble Space Telescope imaged an enormous white cloud near Saturn's equator that was not present during the Voyager encounters, and in 1994 another smaller storm was observed. The 1990 storm was an example of a Great White Spot , a short-lived phenomenon that occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere's summer solstice . Previous Great White Spots were observed in 1876, 1903, 1933, and 1960, with
11256-666: The Kuiper belt . The discovery of other large objects in the Kuiper belt, particularly Eris , spurred debate about how exactly to define a planet. In 2006, the International Astronomical Union (IAU) adopted a definition of a planet in the Solar System, placing the four terrestrial planets and the four giant planets in the planet category; Ceres, Pluto, and Eris are in the category of dwarf planet . Many planetary scientists have nonetheless continued to apply
11424-473: The LHB (around 4.1 to 3.8 billion years ago). The other side supports that they are much younger, created around 100 million years ago. An MIT research team, supporting the latter theory, proposed that the rings are remnant of a destroyed moon of Saturn, named ″Chrysalis″ . Beyond the main rings, at a distance of 12 million km (7.5 million mi) from the planet is the sparse Phoebe ring. It
11592-518: The Milan area. This value was refined in 1827 by Edward Sabine to 4.77 g/cm , and then in 1841 by Carlo Ignazio Giulio to 4.95 g/cm . On the other hand, George Biddell Airy sought to determine ρ by measuring the difference in the period of a pendulum between the surface and the bottom of a mine. The first tests and experiments took place in Cornwall between 1826 and 1828. The experiment
11760-423: The asteroid belt , located between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt , which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc , which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna
11928-454: The climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice . Each planet has two in the course of its orbit; when one hemisphere has its summer solstice with its day being the longest, the other has its winter solstice when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of
12096-416: The mass–energy equivalence principle , although these changes are relatively negligible. Mass loss due to the combination of nuclear fission and natural radioactive decay is estimated to amount to 16 tons per year. An additional loss due to spacecraft on escape trajectories has been estimated at 65 tons per year since the mid-20th century. Earth lost about 3473 tons in the initial 53 years of
12264-505: The naked eye , (2) telescopic observations from Earth beginning in the 17th century, and (3) visitation by space probes , in orbit or on flyby . In the 21st century, telescopic observations continue from Earth (including Earth-orbiting observatories like the Hubble Space Telescope ) and, until its 2017 retirement , from the Cassini orbiter around Saturn. Saturn has been known since prehistoric times, and in early recorded history it
12432-413: The poles and bulging at the equator . Its equatorial radius is more than 10% larger than the polar radius: 60,268 km versus 54,364 km (37,449 mi versus 33,780 mi). Jupiter, Uranus , and Neptune , the other giant planets in the Solar System, are less oblate. The combination of the bulge and the rotation rate means that the effective surface gravity along the equator, 8.96 m/s ,
12600-620: The "weight of the earth" as "5842 quintillion tons " [ 5.936 × 10 kg ]. The "mass of the earth in gravitational measure" is stated as "9.81996×6370980 " in The New Volumes of the Encyclopaedia Britannica (Vol. 25, 1902) with a "logarithm of earth's mass" given as "14.600522" [ 3.985 86 × 10 ]. This is the gravitational parameter in m ·s (modern value 3.986 00 × 10 ) and not the absolute mass. Experiments involving pendulums continued to be performed in
12768-400: The 16th and 17th centuries. With the development of the telescope , the meaning of planet broadened to include objects only visible with assistance: the moons of the planets beyond Earth; the ice giants Uranus and Neptune; Ceres and other bodies later recognized to be part of the asteroid belt ; and Pluto , later found to be the largest member of the collection of icy bodies known as
12936-421: The 1933 storm being the best observed. The latest giant storm was observed in 2010. In 2015, researchers used Very Large Array telescope to study Saturnian atmosphere, and reported that they found "long-lasting signatures of all mid-latitude giant storms, a mixture of equatorial storms up to hundreds of years old, and potentially an unreported older storm at 70°N". The winds on Saturn are the second fastest among
13104-427: The 1960s ( WGS66 ), so that since that time, the uncertainty of the Earth mass is determined essentially by the uncertainty in measuring the gravitational constant . Relative uncertainty was cited at 0.06% in the 1970s, and at 0.01% (10 ) by the 2000s. The current relative uncertainty of 10 amounts to 6 × 10 kg in absolute terms, of the order of the mass of a minor planet (70% of the mass of Ceres ). Before
13272-535: The Cavendish experiment, with results (within standard uncertainty) ranging between 6.672 and 6.676 × 10 m /kg/s (relative uncertainty 3 × 10 ) in results reported since the 1980s, although the 2014 CODATA recommended value is close to 6.674 × 10 m /kg/s with a relative uncertainty below 10 . The Astronomical Almanach Online as of 2016 recommends a standard uncertainty of 1 × 10 for Earth mass, M E 5.9722(6) × 10 kg Earth's mass
13440-655: The Committee of Attraction to consider the matter, appointing Maskelyne, Joseph Banks and Benjamin Franklin amongst its members. The Committee despatched the astronomer and surveyor Charles Mason to find a suitable mountain. After a lengthy search over the summer of 1773, Mason reported that the best candidate was Schiehallion , a peak in the central Scottish Highlands . The mountain stood in isolation from any nearby hills, which would reduce their gravitational influence, and its symmetrical east–west ridge would simplify
13608-405: The Earth mass is difficult, as it is equivalent to measuring the gravitational constant , which is the fundamental physical constant known with least accuracy, due to the relative weakness of the gravitational force . The mass of the Earth was first measured with any accuracy (within about 20% of the correct value) in the Schiehallion experiment in the 1770s, and within 1% of the modern value in
13776-706: The Earth, but Bouguer stated that it was at least sufficient to prove that the Earth was not hollow . That a further attempt should be made on the experiment was proposed to the Royal Society in 1772 by Nevil Maskelyne , Astronomer Royal . He suggested that the experiment would "do honour to the nation where it was made" and proposed Whernside in Yorkshire , or the Blencathra - Skiddaw massif in Cumberland as suitable targets. The Royal Society formed
13944-580: The Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. After the fall of the Western Roman Empire , astronomy developed further in India and the medieval Islamic world. In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of
14112-588: The Moon. The smallest object in the Solar System generally agreed to be a geophysical planet is Saturn's moon Mimas, with a radius about 3.1% of Earth's and a mass about 0.00063% of Earth's. Saturn's smaller moon Phoebe , currently an irregular body of 1.7% Earth's radius and 0.00014% Earth's mass, is thought to have attained hydrostatic equilibrium and differentiation early in its history before being battered out of shape by impacts. Some asteroids may be fragments of protoplanets that began to accrete and differentiate, but suffered catastrophic collisions, leaving only
14280-551: The Roman god of wealth and agriculture , who was the father of the god Jupiter. Its astronomical symbol ( [REDACTED] ) has been traced back to the Greek Oxyrhynchus Papyri , where it can be seen to be a Greek kappa - rho ligature with a horizontal stroke , as an abbreviation for Κρονος ( Cronus ), the Greek name for the planet ( [REDACTED] ). It later came to look like a lower-case Greek eta , with
14448-452: The Solar System is to the Sun. Mercury, the closest planet to the Sun at 0.4 AU , takes 88 days for an orbit, but ultra-short period planets can orbit in less than a day. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. There are hot Jupiters , such as 51 Pegasi b, that orbit very close to their star and may evaporate to become chthonian planets , which are
14616-404: The Solar System planets except Mercury have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. Saturn's largest moon Titan also has a substantial atmosphere thicker than that of Earth; Neptune's largest moon Triton and the dwarf planet Pluto have more tenuous atmospheres. The larger giant planets are massive enough to keep large amounts of
14784-422: The Solar System's planets, after Neptune's. Voyager data indicate peak easterly winds of 500 m/s (1,800 km/h). In images from the Cassini spacecraft during 2007, Saturn's northern hemisphere displayed a bright blue hue, similar to Uranus. The color was most likely caused by Rayleigh scattering . Thermography has shown that Saturn's south pole has a warm polar vortex , the only known example of such
14952-437: The Solar System, but in multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassed asteroids , moons , and dwarf planets like Pluto , and there continues to be some disagreement today. The five classical planets of the Solar System , being visible to the naked eye, have been known since ancient times and have had
15120-489: The Solar System, whereas others are commonly observed in exoplanets. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates : counter-clockwise as seen from above the Sun's north pole. At least one exoplanet, WASP-17b , has been found to orbit in the opposite direction to its star's rotation. The period of one revolution of a planet's orbit is known as its sidereal period or year . A planet's year depends on its distance from its star;
15288-777: The Sun is over 1.4 billion kilometers (9 AU ). With an average orbital speed of 9.68 km/s, it takes Saturn 10,759 Earth days (or about 29 + 1 ⁄ 2 years) to finish one revolution around the Sun. As a consequence, it forms a near 5:2 mean-motion resonance with Jupiter. The elliptical orbit of Saturn is inclined 2.48° relative to the orbital plane of the Earth. The perihelion and aphelion distances are, respectively, 9.195 and 9.957 AU, on average. The visible features on Saturn rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions (as in Jupiter's case). Astronomers use three different systems for specifying
15456-437: The atmosphere and the rings. During the flyby, the probe's turnable camera platform stuck for a couple of days and some planned imaging was lost. Saturn's gravity was used to direct the spacecraft's trajectory towards Uranus. The probes discovered and confirmed several new satellites orbiting near or within the planet's rings, as well as the small Maxwell Gap (a gap within the C Ring ) and Keeler gap (a 42 km-wide gap in
15624-512: The atmosphere. Jupiter has 318 times Earth's mass , and Saturn is 95 times Earth's mass. Together, Jupiter and Saturn hold 92% of the total planetary mass in the Solar System. Despite consisting mostly of hydrogen and helium, most of Saturn's mass is not in the gas phase , because hydrogen becomes a non-ideal liquid when the density is above 0.01 g/cm , which is reached at a radius containing 99.9% of Saturn's mass. The temperature, pressure, and density inside Saturn all rise steadily toward
15792-480: The atmospheric dynamics that affect the day-night temperature difference are complex. One important characteristic of the planets is their intrinsic magnetic moments , which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change
15960-408: The background of the stars—namely, Mercury, Venus, Mars, Jupiter, and Saturn. Planets have historically had religious associations: multiple cultures identified celestial bodies with gods, and these connections with mythology and folklore persist in the schemes for naming newly discovered Solar System bodies. Earth itself was recognized as a planet when heliocentrism supplanted geocentrism during
16128-416: The calculations. Its steep northern and southern slopes would allow the experiment to be sited close to its centre of mass , maximising the deflection effect. Nevil Maskelyne , Charles Hutton and Reuben Burrow performed the experiment, completed by 1776. Hutton (1778) reported that the mean density of the Earth was estimated at 9 / 5 that of Schiehallion mountain. This corresponds to
16296-440: The cause of the missing Saturnian trojans. Saturn has 146 known moons , 63 of which have formal names. It is estimated that there are another 100 ± 30 outer irregular moons larger than 3 km (2 mi) in diameter. In addition, there is evidence of dozens to hundreds of moonlets with diameters of 40–500 meters in Saturn's rings, which are not considered to be true moons. Titan , the largest moon, comprises more than 90% of
16464-559: The collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk . Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets . After
16632-533: The concept of isostasy, which limits the ability to accurately measure ρ , by either the deviation from vertical of a plumb line or using pendulums. Despite the little chance of an accurate estimate of the average density of the Earth in this way, Thomas Corwin Mendenhall in 1880 realized a gravimetry experiment in Tokyo and at the top of Mount Fuji . The result was ρ = 5.77 g/cm . The uncertainty in
16800-557: The core, which causes hydrogen to be a metal in the deeper layers. Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium, with trace amounts of various volatiles . Analysis of the distortion shows that Saturn is substantially more centrally condensed than Jupiter and therefore contains much more material denser than hydrogen near its center. Saturn's central regions are about 50% hydrogen by mass, and Jupiter's are about 67% hydrogen. This core
16968-427: The correct value. The average density of the Earth was not accurately known. Earth was assumed to consist either mostly of water ( Neptunism ) or mostly of igneous rock ( Plutonism ), both suggesting average densities far too low, consistent with a total mass of the order of 10 kg . Isaac Newton estimated, without access to reliable measurement, that the density of Earth would be five or six times as great as
17136-414: The cross added at the top in the 16th century to Christianize this pagan symbol. The Romans named the seventh day of the week Saturday , Sāturni diēs , "Saturn's Day", for the planet Saturn. Saturn is a gas giant , composed predominantly of hydrogen and helium. It lacks a definite surface, though it is likely to have a solid core. The planet's rotation makes it an oblate spheroid —a ball flattened at
17304-406: The definition, regarding where exactly to draw the line between a planet and a star. Multiple exoplanets have been found to orbit in the habitable zones of their stars (where liquid water can potentially exist on a planetary surface ), but Earth remains the only planet known to support life . It is not known with certainty how planets are formed. The prevailing theory is that they coalesce during
17472-463: The density of Earth by measuring the period of a pendulum (and therefore the strength of gravity) as a function of elevation. The experiments were carried out in Ecuador and Peru, on Pichincha Volcano and mount Chimborazo . Bouguer wrote in a 1749 paper that they had been able to detect a deflection of 8 seconds of arc , the accuracy was not enough for a definite estimate on the mean density of
17640-403: The density of water, which is surprisingly accurate (the modern value is 5.515). Newton under-estimated the Earth's volume by about 30%, so that his estimate would be roughly equivalent to (4.2 ± 0.5) × 10 kg . In the 18th century, knowledge of Newton's law of universal gravitation permitted indirect estimates on the mean density of the Earth, via estimates of (what in modern terminology
17808-405: The diameter of the Earth (modern value 55%). With a value for the mean density of the Earth, Hutton was able to set some values to Jérôme Lalande 's planetary tables, which had previously only been able to express the densities of the major Solar System objects in relative terms. Henry Cavendish (1798) was the first to attempt to measure the gravitational attraction between two bodies directly in
17976-515: The direct measurement of the gravitational constant , estimates of the Earth mass were limited to estimating Earth's mean density from observation of the crust and estimates on Earth's volume. Estimates on the volume of the Earth in the 17th century were based on a circumference estimate of 60 miles (97 km) to the degree of latitude, corresponding to a radius of 5500 km (86% of the Earth's actual radius of about 6371 km ), resulting in an estimated volume of about one third smaller than
18144-473: The eight planets in the Solar System, only Venus and Mars lack such a magnetic field. Of the magnetized planets, the magnetic field of Mercury is the weakest and is barely able to deflect the solar wind . Jupiter's moon Ganymede has a magnetic field several times stronger, and Jupiter's is the strongest in the Solar System (so intense in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto ). The magnetic fields of
18312-529: The fact that dark gaps in the rings are bright when viewed at a high phase angle (towards the Sun), meaning that they contain fine light-scattering material. In addition, Pioneer 11 measured the temperature of Titan. In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, its rings and satellites. Surface features of various moons were seen for
18480-418: The farther a planet is from its star, the longer the distance it must travel and the slower its speed, since it is less affected by its star's gravity . No planet's orbit is perfectly circular, and hence the distance of each from the host star varies over the course of its year. The closest approach to its star is called its periastron , or perihelion in the Solar System, whereas its farthest separation from
18648-408: The first half of the 19th century. By the second half of the century, these were outperformed by repetitions of the Cavendish experiment, and the modern value of G (and hence, of the Earth mass) is still derived from high-precision repetitions of the Cavendish experiment. In 1821, Francesco Carlini determined a density value of ρ = 4.39 g/cm through measurements made with pendulums in
18816-496: The first time. Voyager 1 performed a close flyby of Titan, increasing knowledge of the atmosphere of the moon. It proved that Titan's atmosphere is impenetrable at visible wavelengths ; therefore no surface details were seen. The flyby changed the spacecraft's trajectory out of the plane of the Solar System. Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in
18984-419: The flybys of the Voyager spacecraft during the 1980s. Since then, Earth-based telescopy has improved to the point where regular observations can be made. The composition of the clouds varies with depth and increasing pressure. In the upper cloud layers, with temperatures in the range of 100–160 K and pressures extending between 0.5–2 bar , the clouds consist of ammonia ice. Water ice clouds begin at
19152-473: The formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot ), and holes in the atmosphere (on Neptune). Weather patterns detected on exoplanets include a hot region on HD 189733 b twice the size of the Great Red Spot, as well as clouds on the hot Jupiter Kepler-7b ,
19320-586: The generation of heat through the "raining out" of droplets of helium deep in Saturn's interior. As the droplets descend through the lower-density hydrogen, the process releases heat by friction and leaves Saturn's outer layers depleted of helium. These descending droplets may have accumulated into a helium shell surrounding the core. Rainfalls of diamonds have been suggested to occur within Saturn, as well as in Jupiter and ice giants Uranus and Neptune. The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The proportion of helium
19488-626: The giant planets have numerous moons in complex planetary-type systems. Except for Ceres and Sedna, all the consensus dwarf planets are known to have at least one moon as well. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa and Enceladus). The four giant planets are orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny ' moonlets ' whose gravity shapes and maintains their structure. Although
19656-467: The giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel and mantles of silicates . Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen . Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia , methane , and other ices . The fluid action within these planets' cores creates
19824-455: The god Saturnus the equivalent of the Greek god Cronus ; in modern Greek , the planet retains the name Cronus — Κρόνος : Kronos . The Greek scientist Ptolemy based his calculations of Saturn's orbit on observations he made while it was in opposition . In Hindu astrology , there are nine astrological objects, known as Navagrahas . Saturn is known as " Shani " and judges everyone based on
19992-442: The good and bad deeds performed in life. Ancient Chinese and Japanese culture designated the planet Saturn as the "earth star" ( 土星 ). This was based on Five Elements which were traditionally used to classify natural elements. In Hebrew , Saturn is called Shabbathai . Its angel is Cassiel . Its intelligence or beneficial spirit is 'Agȋȇl ( Hebrew : אגיאל , romanized : ʿAgyal ), and its darker spirit ( demon )
20160-417: The grounds that the internal physics of objects does not change between approximately one Saturn mass (beginning of significant self-compression) and the onset of hydrogen burning and becoming a red dwarf star. Beyond roughly 13 M J (at least for objects with solar-type isotopic abundance ), an object achieves conditions suitable for nuclear fusion of deuterium : this has sometimes been advocated as
20328-416: The interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet. Of
20496-404: The laboratory. Earth's mass could be then found by combining two equations; Newton's second law , and Newton's law of universal gravitation . In modern notation, the mass of the Earth is derived from the gravitational constant and the mean Earth radius by Where gravity of Earth , "little g", is Cavendish found a mean density of 5.45 g/cm , about 1% below the modern value. While
20664-520: The leftover cores. There are also exoplanets that are much farther from their star. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit (e.g. COCONUTS-2b ). Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in
20832-447: The light gases hydrogen and helium, whereas the smaller planets lose these gases into space . Analysis of exoplanets suggests that the threshold for being able to hold on to these light gases occurs at about 2.0 +0.7 −0.6 M E , so that Earth and Venus are near the maximum size for rocky planets. The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on
21000-508: The likelihood that a star will have planets. Hence, a metal-rich population I star is more likely to have a substantial planetary system than a metal-poor, population II star . According to the IAU definition , there are eight planets in the Solar System, which are (in increasing distance from the Sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses , whereas Mercury
21168-419: The lower level clouds appear to consist of either ammonium hydrosulfide ( NH 4 SH ) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion . This photochemical cycle is modulated by Saturn's annual seasonal cycle. Cassini observed
21336-537: The magnetic field is generated similarly to that of Jupiter—by currents in the liquid metallic-hydrogen layer called a metallic-hydrogen dynamo. This magnetosphere is efficient at deflecting the solar wind particles from the Sun. The moon Titan orbits within the outer part of Saturn's magnetosphere and contributes plasma from the ionized particles in Titan's outer atmosphere. Saturn's magnetosphere, like Earth's , produces aurorae . The average distance between Saturn and
21504-480: The magnetosphere of an orbiting hot Jupiter. Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is common in satellite systems (e.g. the resonance between Io, Europa , and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have natural satellites , often called "moons". Earth has one, Mars has two, and
21672-426: The mantle and crust. Minor contributions are from iron(II) oxide (5%), aluminium oxide (3%) and calcium oxide (2%), besides numerous trace elements (in elementary terms: iron and oxygen c. 32% each, magnesium and silicon c. 15% each, calcium , aluminium and nickel c. 1.5% each). Carbon accounts for 0.03%, water for 0.02%, and the atmosphere for about one part per million . The mass of Earth
21840-425: The mass in orbit around Saturn, including the rings. Saturn's second-largest moon, Rhea , may have a tenuous ring system of its own , along with a tenuous atmosphere . Many of the other moons are small: 131 are less than 50 km in diameter. Traditionally, most of Saturn's moons have been named after Titans of Greek mythology. Titan is the only satellite in the Solar System with a major atmosphere , in which
22008-440: The mass of the Earth is implied by stating the Earth's radius and density, it was not usual to state the absolute mass explicitly prior to the introduction of scientific notation using powers of 10 in the later 19th century, because the absolute numbers would have been too awkward. Ritchie (1850) gives the mass of the Earth's atmosphere as "11,456,688,186,392,473,000 lbs". ( 1.1 × 10 lb = 5.0 × 10 kg , modern value
22176-442: The materials that formed Saturn in earlier times. Saturn's moon Enceladus , which seems similar in chemical makeup to comets, has often been regarded as a potential habitat for microbial life . Evidence of this possibility includes the satellite's salt-rich particles having an "ocean-like" composition that indicates most of Enceladus's expelled ice comes from the evaporation of liquid salt water. A 2015 flyby by Cassini through
22344-548: The modern value for the Earth's mass has been entirely due to the uncertainty in the gravitational constant G since at least the 1960s. G is notoriously difficult to measure, and some high-precision measurements during the 1980s to 2010s have yielded mutually exclusive results. Sagitov (1969) based on the measurement of G by Heyl and Chrzanowski (1942) cited a value of M E = 5.973(3) × 10 kg (relative uncertainty 5 × 10 ). Accuracy has improved only slightly since then. Most modern measurements are repetitions of
22512-582: The most massive. There are at least nineteen planetary-mass moons or satellite planets—moons large enough to take on ellipsoidal shapes: The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior. Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas
22680-478: The most significant contributors to Earth's increase in mass. The sum of material is estimated to be 37 000 to 78 000 tons annually, although this can vary significantly; to take an extreme example, the Chicxulub impactor , with a midpoint mass estimate of 2.3 × 10 kg , added 900 million times that annual dustfall amount to the Earth's mass in a single event. Additional changes in mass are due to
22848-548: The number of intelligent, communicating civilizations that exist in the Milky Way. There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes , which have masses between that of Earth and Neptune. Objects less than about twice the mass of Earth are expected to be rocky like Earth; beyond that, they become a mixture of volatiles and gas like Neptune. The planet Gliese 581c , with
23016-414: The object collapses into a sphere. Mass is the prime attribute by which planets are distinguished from stars. No objects between the masses of the Sun and Jupiter exist in the Solar System, but there are exoplanets of this size. The lower stellar mass limit is estimated to be around 75 to 80 times that of Jupiter ( M J ). Some authors advocate that this be used as the upper limit for planethood, on
23184-401: The origins of planetary rings are not precisely known, they are believed to be the result of natural satellites that fell below their parent planets' Roche limits and were torn apart by tidal forces . The dwarf planets Haumea and Quaoar also have rings. No secondary characteristics have been observed around exoplanets. The sub-brown dwarf Cha 110913−773444 , which has been described as
23352-490: The other giant planets, measured at their surfaces, are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative to the planets' rotational axes and displaced from the planets' centres. In 2003, a team of astronomers in Hawaii observing the star HD 179949 detected a bright spot on its surface, apparently created by
23520-715: The other in perpetual night. Mercury and Venus, the closest planets to the Sun, similarly exhibit very slow rotation: Mercury is tidally locked into a 3:2 spin–orbit resonance (rotating three times for every two revolutions around the Sun), and Venus's rotation may be in equilibrium between tidal forces slowing it down and atmospheric tides created by solar heating speeding it up. All the large moons are tidally locked to their parent planets; Pluto and Charon are tidally locked to each other, as are Eris and Dysnomia, and probably Orcus and its moon Vanth . The other dwarf planets with known rotation periods rotate faster than Earth; Haumea rotates so fast that it has been distorted into
23688-482: The planet detected by Voyager 1 and Voyager 2 , System III has a rotation period of 10 39 22.4 (810.8°/d). System III has largely superseded System II. A precise value for the rotation period of the interior remains elusive. While approaching Saturn in 2004, Cassini found that the radio rotation period of Saturn had increased appreciably, to approximately 10 45 45 ± 36 . An estimate of Saturn's rotation (as an indicated rotation rate for Saturn as
23856-420: The planet have introduced free molecular oxygen . The atmospheres of Mars and Venus are both dominated by carbon dioxide , but differ drastically in density: the average surface pressure of Mars's atmosphere is less than 1% that of Earth's (too low to allow liquid water to exist), while the average surface pressure of Venus's atmosphere is about 92 times that of Earth's. It is likely that Venus's atmosphere
24024-597: The planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either continually in sunlight or continually in darkness around the time of its solstices . In the Solar System, Mercury, Venus, Ceres, and Jupiter have very small tilts; Pallas, Uranus, and Pluto have extreme ones; and Earth, Mars, Vesta, Saturn, and Neptune have moderate ones. Among exoplanets, axial tilts are not known for certain, though most hot Jupiters are believed to have
24192-589: The planets as the Babylonians. In the 6th and 5th centuries BC, the Pythagoreans appear to have developed their own independent planetary theory , which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star ( Hesperos ) and morning star ( Phosphoros ) as one and
24360-480: The positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete
24528-745: The rotation rate of Saturn. System I has a period of 10 14 00 (844.3°/d) and encompasses the Equatorial Zone, the South Equatorial Belt, and the North Equatorial Belt. The polar regions are considered to have rotation rates similar to System I . All other Saturnian latitudes, excluding the north and south polar regions, are indicated as System II and have been assigned a rotation period of 10 38 25.4 (810.76°/d). System III refers to Saturn's internal rotation rate. Based on radio emissions from
24696-765: The same ( Aphrodite , Greek corresponding to Latin Venus ), though this had long been known in Mesopotamia. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution . By the 1st century BC, during the Hellenistic period , the Greeks had begun to develop their own mathematical schemes for predicting
24864-469: The second millennium BC. The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year. Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the exact sciences . The Enuma anu enlil , written during the Neo-Assyrian period in the 7th century BC, comprises
25032-421: The size of Neptune and smaller, down to smaller than Mercury. In 2011, the Kepler space telescope team reported the discovery of the first Earth-sized exoplanets orbiting a Sun-like star , Kepler-20e and Kepler-20f . Since that time, more than 100 planets have been identified that are approximately the same size as Earth , 20 of which orbit in the habitable zone of their star—the range of orbits where
25200-506: The size of the Moon . Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way . In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12 . This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from
25368-413: The smaller planetesimals (as well as radioactive decay ) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by density, with higher density materials sinking toward the core . Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from
25536-470: The spin axis of the planet. There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours. The rotational periods of exoplanets are not known, but for hot Jupiters , their proximity to their stars means that they are tidally locked (that is, their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day,
25704-400: The star is called its apastron ( aphelion ). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy , just as a falling object on Earth accelerates as it falls. As the planet nears apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory . Each planet's orbit
25872-523: The star or each other, but over time many will collide, either to form a larger, combined protoplanet or release material for other protoplanets to absorb. Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies . The energetic impacts of
26040-480: The stars. He also theorized that the orbits of planets were elliptical . Aryabhata's followers were particularly strong in South India , where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. Earth mass An Earth mass (denoted as M 🜨 , M ♁ or M E , where 🜨 and ♁ are the astronomical symbols for Earth ),
26208-460: The subsequent impact of comets (smaller planets will lose any atmosphere they gain through various escape mechanisms ). With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity —an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 ( helium )—appears to determine
26376-412: The super-Earth Gliese 1214 b , and others. Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets. These planets may have vast differences in temperature between their day and night sides that produce supersonic winds, although multiple factors are involved and the details of
26544-550: The term planet more broadly, including dwarf planets as well as rounded satellites like the Moon. Further advances in astronomy led to the discovery of over five thousand planets outside the Solar System, termed exoplanets . These often show unusual features that the Solar System planets do not show, such as hot Jupiters —giant planets that orbit close to their parent stars, like 51 Pegasi b —and extremely eccentric orbits , such as HD 20782 b . The discovery of brown dwarfs and planets larger than Jupiter also spurred debate on
26712-433: The terrestrial planets in composition. The gas giants , Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses. The ice giants , Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane , and ammonia , with thick atmospheres of hydrogen and helium. They have
26880-456: The upper crust to as much as 13 000 kg/m in the inner core . The Earth's core accounts for 15% of Earth's volume but more than 30% of the mass, the mantle for 84% of the volume and close to 70% of the mass, while the crust accounts for less than 1% of the mass. About 90% of the mass of the Earth is composed of the iron–nickel alloy (95% iron) in the core (30%), and the silicon dioxides (c. 33%) and magnesium oxide (c. 27%) in
27048-487: Was a failure due to a fire and a flood. Finally, in 1854, Airy got the value 6.6 g/cm by measurements in a coal mine in Harton, Sunderland. Airy's method assumed that the Earth had a spherical stratification. Later, in 1883, the experiments conducted by Robert von Sterneck (1839 to 1910) at different depths in mines of Saxony and Bohemia provided the average density values ρ between 5.0 and 6.3 g/cm . This led to
27216-411: Was a major character in various mythologies. Babylonian astronomers systematically observed and recorded the movements of Saturn. In ancient Greek, the planet was known as Φαίνων Phainon , and in Roman times it was known as the "star of Saturn ". In ancient Roman mythology , the planet Phainon was sacred to this agricultural god, from which the planet takes its modern name. The Romans considered
27384-413: Was derived. In ancient Greece , China , Babylon , and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the apparently common-sense perceptions that Earth was solid and stable and that it
27552-416: Was found in 1992 in orbit around a pulsar . Its mass is roughly half that of the planet Mercury. Even smaller is WD 1145+017 b , orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea, and it is typically termed a minor planet. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b , with a mass (and radius) that is probably slightly higher than that of
27720-589: Was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians , who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa , a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as
27888-626: Was not until Huygens used greater telescopic magnification that this notion was refuted, and the rings were truly seen for the first time. Huygens also discovered Saturn's moon Titan ; Giovanni Domenico Cassini later discovered four other moons: Iapetus , Rhea , Tethys , and Dione . In 1675, Cassini discovered the gap now known as the Cassini Division . No further discoveries of significance were made until 1789 when William Herschel discovered two further moons, Mimas and Enceladus . The irregularly shaped satellite Hyperion , which has
28056-518: Was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries. To the Greeks and Romans, there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names):
28224-462: Was the result of a runaway greenhouse effect in its history, which today makes it the hottest planet by surface temperature, hotter even than Mercury. Despite hostile surface conditions, temperature, and pressure at about 50–55 km altitude in Venus's atmosphere are close to Earthlike conditions (the only place in the Solar System beyond Earth where this is so), and this region has been suggested as
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