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149-441: A planet is a large, rounded astronomical body that is generally required to be in orbit around 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

298-463: 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

447-638: 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 the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of

596-516: 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

745-500: 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

894-433: A class of emission nebula associated with giant molecular clouds. These form as a molecular cloud collapses under its own weight, producing stars. Massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths . The region of ionized hydrogen surrounding the massive stars is known as an H II region while the shells of neutral hydrogen surrounding

1043-578: A cube. F weight = − ρ g A h {\displaystyle F_{\text{weight}}=-\rho gAh} By balancing these forces, the total force on the fluid is ∑ F = F bottom + F top + F weight = P bottom A − P top A − ρ g A h {\displaystyle \sum F=F_{\text{bottom}}+F_{\text{top}}+F_{\text{weight}}=P_{\text{bottom}}A-P_{\text{top}}A-\rho gAh} This sum equals zero if

1192-687: 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

1341-407: 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

1490-474: 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

1639-399: A molten planet or a rocky planet, but does not apply to a star or to a planet like the earth which has a dense metallic core. In 1737 Alexis Clairaut studied the case of density varying with depth. Clairaut's theorem states that the variation of the gravity (including centrifugal force) is proportional to the square of the sine of the latitude, with the proportionality depending linearly on

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1788-583: 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

1937-568: A nebular cloud the size of the Earth would have a total mass of only a few kilograms . Earth's air has a density of approximately 10 molecules per cubic centimeter; by contrast, the densest nebulae can have densities of 10 molecules per cubic centimeter. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffused that they can be detected only with long exposures and special filters. Some nebulae are variably illuminated by T Tauri variable stars. Originally,

2086-398: 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

2235-511: A particularly simple equilibrium solution of the Navier–Stokes equations. By plugging the energy–momentum tensor for a perfect fluid T μ ν = ( ρ c 2 + P ) u μ u ν + P g μ ν {\displaystyle T^{\mu \nu }=\left(\rho c^{2}+P\right)u^{\mu }u^{\nu }+Pg^{\mu \nu }} into

2384-407: 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

2533-546: 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 a significant impact on mythology , religious cosmology , and ancient astronomy . In ancient times, astronomers noted how certain lights moved across

2682-424: 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

2831-484: A rapid rotation and was in hydrostatic equilibrium, but that its shape became "frozen in" and did not change as it spun down due to tidal forces from its moon Weywot . If so, this would resemble the situation of Iapetus, which is too oblate for its current spin. Iapetus is generally still considered a planetary-mass moon nonetheless, though not always. Solid bodies have irregular surfaces, but local irregularities may be consistent with global equilibrium. For example,

2980-779: A relatively recently identified astronomical phenomenon. In contrast to the typical and well known gaseous nebulae within the plane of the Milky Way galaxy , IFNs lie beyond the main body of the galaxy. Most nebulae can be described as diffuse nebulae, which means that they are extended and contain no well-defined boundaries. Diffuse nebulae can be divided into emission nebulae , reflection nebulae and dark nebulae . Visible light nebulae may be divided into emission nebulae, which emit spectral line radiation from excited or ionized gas (mostly ionized hydrogen ); they are often called H II regions , H II referring to ionized hydrogen), and reflection nebulae which are visible primarily due to

3129-420: A relatively thin solid crust . In addition to the Sun, there are a dozen or so equilibrium objects confirmed to exist in the Solar System . For a hydrostatic fluid on Earth: d P = − ρ ( P ) g ( h ) d h {\displaystyle dP=-\rho (P)\,g(h)\,dh} Newton's laws of motion state that a volume of a fluid that is not in motion or that

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3278-408: A rotation period of 12.5 hours. Consequently, Vega is about 20% larger at the equator than from pole to pole. In his 1687 Philosophiæ Naturalis Principia Mathematica Newton correctly stated that a rotating fluid of uniform density under the influence of gravity would take the form of a spheroid and that the gravity (including the effect of centrifugal force ) would be weaker at the equator than at

3427-409: 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

3576-400: 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

3725-400: 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

3874-512: A subsurface ocean, which is not the definition of equilibrium used by the IAU (gravity overcoming internal rigid-body forces). Even larger bodies deviate from hydrostatic equilibrium, although they are ellipsoidal: examples are Earth's Moon at 3,474 km (mostly rock), and the planet Mercury at 4,880 km (mostly metal). In 2024, Kiss et al. found that Quaoar has an ellipsoidal shape incompatible with hydrostatic equilibrium for its current spin. They hypothesised that Quaoar originally had

4023-467: 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

4172-436: 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

4321-634: Is a change in pressure, and h is the height of the volume element—a change in the distance above the ground. By saying these changes are infinitesimally small, the equation can be written in differential form. d P = − ρ g d h {\displaystyle dP=-\rho g\,dh} Density changes with pressure, and gravity changes with height, so the equation would be: d P = − ρ ( P ) g ( h ) d h {\displaystyle dP=-\rho (P)\,g(h)\,dh} Note finally that this last equation can be derived by solving

4470-1439: Is a characteristic mass of the baryonic gas particles) and rearranging, we arrive at d d r ( k T B ( r ) ρ B ( r ) m B ) = − ρ B ( r ) G r 2 ∫ 0 r 4 π r 2 ρ M ( r ) d r . {\displaystyle {\frac {d}{dr}}\left({\frac {kT_{B}(r)\rho _{B}(r)}{m_{B}}}\right)=-{\frac {\rho _{B}(r)G}{r^{2}}}\int _{0}^{r}4\pi r^{2}\,\rho _{M}(r)\,dr.} Multiplying by r 2 / ρ B ( r ) {\displaystyle r^{2}/\rho _{B}(r)} and differentiating with respect to r {\displaystyle r} yields d d r [ r 2 ρ B ( r ) d d r ( k T B ( r ) ρ B ( r ) m B ) ] = − 4 π G r 2 ρ M ( r ) . {\displaystyle {\frac {d}{dr}}\left[{\frac {r^{2}}{\rho _{B}(r)}}{\frac {d}{dr}}\left({\frac {kT_{B}(r)\rho _{B}(r)}{m_{B}}}\right)\right]=-4\pi Gr^{2}\rho _{M}(r).} If we make

4619-535: Is a measure of the total mass of the cluster, with r {\displaystyle r} being the proper distance to the center of the cluster. Using the ideal gas law p B = k T B ρ B / m B {\displaystyle p_{B}=kT_{B}\rho _{B}/m_{B}} ( k {\displaystyle k} is the Boltzmann constant and m B {\displaystyle m_{B}}

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4768-486: 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 the Solar System, but in multitudes of other extrasolar systems. The consensus as to what counts as

4917-561: 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

5066-495: Is dependent on the redshift z {\displaystyle z} of the cluster and is given by ρ B ( 0 ) / ρ M ( 0 ) ∝ ( 1 + z ) 2 ( θ s ) 3 / 2 {\displaystyle \rho _{B}(0)/\rho _{M}(0)\propto (1+z)^{2}\left({\frac {\theta }{s}}\right)^{3/2}} where θ {\displaystyle \theta }

5215-412: Is expected to spawn a planetary nebula about 12 billion years after its formation. A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas forms

5364-418: Is in a state of constant velocity must have zero net force on it. This means the sum of the forces in a given direction must be opposed by an equal sum of forces in the opposite direction. This force balance is called a hydrostatic equilibrium. The fluid can be split into a large number of cuboid volume elements; by considering a single element, the action of the fluid can be derived. There are three forces:

5513-455: 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

5662-407: Is negligible. From the time of Isaac Newton much work has been done on the subject of the equilibrium attained when a fluid rotates in space. This has application to both stars and objects like planets, which may have been fluid in the past or in which the solid material deforms like a fluid when subjected to very high stresses. In any given layer of a star there is a hydrostatic equilibrium between

5811-618: Is stable up to a certain (critical) angular momentum (normalized by M G ρ r e {\displaystyle M{\sqrt {G\rho r_{e}}}} ), but in 1834 Carl Jacobi showed that it becomes unstable once the eccentricity reaches 0.81267 (or f {\displaystyle f} reaches 0.3302). Above the critical value the solution becomes a Jacobi, or scalene, ellipsoid (one with all three axes different). Henri Poincaré in 1885 found that at still higher angular momentum it will no longer be ellipsoidal but piriform or oviform . The symmetry drops from

5960-470: 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

6109-461: Is the nebular hypothesis , which posits that an interstellar cloud collapses out of 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

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6258-413: Is the angular width of the cluster and s {\displaystyle s} the proper distance to the cluster. Values for the ratio range from 0.11 to 0.14 for various surveys. The concept of hydrostatic equilibrium has also become important in determining whether an astronomical object is a planet , dwarf planet , or small Solar System body . According to the definition of planet adopted by

6407-432: Is the dwarf planet Ceres , which is icy, at 945 km, whereas the largest known body to have a noticeable deviation from hydrostatic equilibrium is Iapetus being made of mostly permeable ice and almost no rock. At 1,469 km Iapetus is neither spherical nor ellipsoid. Instead, it is rather in a strange walnut-like shape due to its unique equatorial ridge . Some icy bodies may be in equilibrium at least partly due to

6556-434: Is the icy moon Mimas at 396 km, whereas the largest icy object known to have an obviously non-equilibrium shape is the icy moon Proteus at 420 km, and the largest rocky bodies in an obviously non-equilibrium shape are the asteroids Pallas and Vesta at about 520 km. However, Mimas is not actually in hydrostatic equilibrium for its current rotation. The smallest body confirmed to be in hydrostatic equilibrium

6705-437: 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

6854-498: 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

7003-418: 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

7152-456: 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

7301-450: Is visible to the human eye from Earth would appear larger, but no brighter, from close by. The Orion Nebula , the brightest nebula in the sky and occupying an area twice the angular diameter of the full Moon , can be viewed with the naked eye but was missed by early astronomers. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth (10 to 10 molecules per cubic centimeter) –

7450-704: The Andromeda Galaxy is located. He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster . The supernovas that created the Crab Nebula , SN 1054 , was observed by Arabic and Chinese astronomers in 1054. In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by Johann Baptist Cysat in 1618. However,

7599-548: The Cape of Good Hope , most of which were previously unknown. Charles Messier then compiled a catalog of 103 "nebulae" (now called Messier objects , which included what are now known to be galaxies) by 1781; his interest was detecting comets , and these were objects that might be mistaken for them. The number of nebulae was then greatly increased by the efforts of William Herschel and his sister, Caroline Herschel . Their Catalogue of One Thousand New Nebulae and Clusters of Stars

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7748-459: The Eagle Nebula . In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars . The remaining material is then thought to form planets and other planetary system objects. Most nebulae are of vast size; some are hundreds of light-years in diameter. A nebula that

7897-644: The Einstein field equations R μ ν = 8 π G c 4 ( T μ ν − 1 2 g μ ν T ) {\displaystyle R_{\mu \nu }={\frac {8\pi G}{c^{4}}}\left(T_{\mu \nu }-{\frac {1}{2}}g_{\mu \nu }T\right)} and using the conservation condition ∇ μ T μ ν = 0 {\displaystyle \nabla _{\mu }T^{\mu \nu }=0} one can derive

8046-553: The Great Debate , it became clear that many "nebulae" were in fact galaxies far from the Milky Way . Slipher and Edwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light. In 1922, Hubble announced that nearly all nebulae are associated with stars and that their illumination comes from star light. He also discovered that

8195-636: The International Astronomical Union in 2006, one defining characteristic of planets and dwarf planets is that they are objects that have sufficient gravity to overcome their own rigidity and assume hydrostatic equilibrium. Such a body will often have the differentiated interior and geology of a world (a planemo ), though near-hydrostatic or formerly hydrostatic bodies such as the proto-planet 4 Vesta may also be differentiated and some hydrostatic bodies (notably Callisto ) have not thoroughly differentiated since their formation. Often

8344-463: The Sun , Moon , and five points of light visible to the naked eye that moved across 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

8493-1011: The Tolman–Oppenheimer–Volkoff equation for the structure of a static, spherically symmetric relativistic star in isotropic coordinates: d P d r = − G M ( r ) ρ ( r ) r 2 ( 1 + P ( r ) ρ ( r ) c 2 ) ( 1 + 4 π r 3 P ( r ) M ( r ) c 2 ) ( 1 − 2 G M ( r ) r c 2 ) − 1 {\displaystyle {\frac {dP}{dr}}=-{\frac {GM(r)\rho (r)}{r^{2}}}\left(1+{\frac {P(r)}{\rho (r)c^{2}}}\right)\left(1+{\frac {4\pi r^{3}P(r)}{M(r)c^{2}}}\right)\left(1-{\frac {2GM(r)}{rc^{2}}}\right)^{-1}} In practice, Ρ and ρ are related by an equation of state of

8642-528: 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

8791-454: 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

8940-459: The eccentricity by ϵ , {\displaystyle \epsilon ,} with he found that the gravity at the poles is where G {\displaystyle G} is the gravitational constant, ρ {\displaystyle \rho } is the (uniform) density, and M {\displaystyle M} is the total mass. The ratio of this to g 0 , {\displaystyle g_{0},}

9089-465: The principles of equilibrium of fluids . A hydrostatic balance is a particular balance for weighing substances in water. Hydrostatic balance allows the discovery of their specific gravities . This equilibrium is strictly applicable when an ideal fluid is in steady horizontal laminar flow, and when any fluid is at rest or in vertical motion at constant speed. It can also be a satisfactory approximation when flow speeds are low enough that acceleration

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9238-548: The ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The Sun will produce a planetary nebula and its core will remain behind in the form of a white dwarf . Objects named nebulae belong to four major groups. Before their nature was understood, galaxies ("spiral nebulae") and star clusters too distant to be resolved as stars were also classified as nebulae, but no longer are. Not all cloud-like structures are nebulae; Herbig–Haro objects are an example. Integrated flux nebulae are

9387-402: The weight of the volume element causes a force downwards. If the density is ρ , the volume is V and g the standard gravity , then: F weight = − ρ g V {\displaystyle F_{\text{weight}}=-\rho gV} The volume of this cuboid is equal to the area of the top or bottom, times the height – the formula for finding the volume of

9536-412: The 8-fold D 2h point group to the 4-fold C 2v , with its axis perpendicular to the axis of rotation. Other shapes satisfy the equations beyond that, but are not stable, at least not near the point of bifurcation . Poincaré was unsure what would happen at higher angular momentum, but concluded that eventually the blob would split in two. The assumption of uniform density may apply more or less to

9685-538: 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 was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained

9834-627: 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 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

9983-555: The H II region are known as photodissociation region . Examples of star-forming regions are the Orion Nebula , the Rosette Nebula and the Omega Nebula . Feedback from star-formation, in the form of supernova explosions of massive stars, stellar winds or ultraviolet radiation from massive stars, or outflows from low-mass stars may disrupt the cloud, destroying the nebula after several million years. Other nebulae form as

10132-634: 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

10281-451: 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

10430-400: 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

10579-486: 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;

10728-499: The TOV equilibrium equation, these are two equations (for instance, if as usual when treating stars, one chooses spherical coordinates as basis coordinates ( t , r , θ , φ ) {\displaystyle (t,r,\theta ,\varphi )} , the index i runs for the coordinates r and θ {\displaystyle \theta } ). The hydrostatic equilibrium pertains to hydrostatics and

10877-986: The assumption that cold dark matter particles have an isotropic velocity distribution, then the same derivation applies to these particles, and their density ρ D = ρ M − ρ B {\displaystyle \rho _{D}=\rho _{M}-\rho _{B}} satisfies the non-linear differential equation d d r [ r 2 ρ D ( r ) d d r ( k T D ( r ) ρ D ( r ) m D ) ] = − 4 π G r 2 ρ M ( r ) . {\displaystyle {\frac {d}{dr}}\left[{\frac {r^{2}}{\rho _{D}(r)}}{\frac {d}{dr}}\left({\frac {kT_{D}(r)\rho _{D}(r)}{m_{D}}}\right)\right]=-4\pi Gr^{2}\rho _{M}(r).} With perfect X-ray and distance data, we could calculate

11026-472: The atmosphere bound to Earth and maintaining pressure differences with altitude. Nebula A nebula ( Latin for 'cloud, fog'; pl. : nebulae , nebulæ , or nebulas ) is a distinct luminescent part of interstellar medium , which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust . Nebulae are often star-forming regions, such as in the Pillars of Creation in

11175-478: 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

11324-531: The axis of rotation depended only on the distance from the axis and was proportional to that distance, and the component in the direction toward the plane of the equator depended only on the distance from that plane and was proportional to that distance. Newton had already pointed out that the gravity felt on the equator (including the lightening due to centrifugal force) has to be r p r e g p {\displaystyle {\frac {r_{p}}{r_{e}}}g_{p}} in order to have

11473-610: The baryon density at each point in the cluster and thus the dark matter density. We could then calculate the velocity dispersion σ D 2 {\displaystyle \sigma _{D}^{2}} of the dark matter, which is given by σ D 2 = k T D m D . {\displaystyle \sigma _{D}^{2}={\frac {kT_{D}}{m_{D}}}.} The central density ratio ρ B ( 0 ) / ρ M ( 0 ) {\displaystyle \rho _{B}(0)/\rho _{M}(0)}

11622-645: The category of dwarf planet . Many planetary scientists have nonetheless continued to apply 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

11771-558: 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

11920-457: The collisions thereof) emits X-ray radiation. The absolute X-ray luminosity per unit volume takes the form L X = Λ ( T B ) ρ B 2 {\displaystyle {\mathcal {L}}_{X}=\Lambda (T_{B})\rho _{B}^{2}} where T B {\displaystyle T_{B}} and ρ B {\displaystyle \rho _{B}} are

12069-461: 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 a list of omens and their relationships with various celestial phenomena including the motions of the planets. The inferior planets Venus and Mercury and

12218-406: 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

12367-689: 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): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. After the fall of the Western Roman Empire , astronomy developed further in India and

12516-471: The different types of nebulae. Some nebulae form from gas that is already in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds , the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution . Star-forming regions are

12665-516: The diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them. Hydrostatic equilibrium Hydrostatic equilibrium is the distinguishing criterion between dwarf planets and small solar system bodies , and features in astrophysics and planetary geology . Said qualification of equilibrium indicates that the shape of the object is symmetrically rounded, mostly due to rotation , into an ellipsoid , where any irregular surface features are consequent to

12814-471: 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

12963-405: The emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter (including all O-type main sequence stars ), while nebulae with continuous spectra appear with cooler stars. Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transformed in some manner. There are a variety of formation mechanisms for

13112-491: The equilibrium shape is an oblate spheroid , as is the case with Earth. However, in the cases of moons in synchronous orbit, nearly unidirectional tidal forces create a scalene ellipsoid . Also, the purported dwarf planet Haumea is scalene due to its rapid rotation, though it may not currently be in equilibrium. Icy objects were previously believed to need less mass to attain hydrostatic equilibrium than rocky objects. The smallest object that appears to have an equilibrium shape

13261-510: The expelled gases, producing emission nebulae with spectra similar to those of emission nebulae found in star formation regions. They are H II regions , because mostly hydrogen is ionized, but planetary are denser and more compact than nebulae found in star formation regions. Planetary nebulae were given their name by the first astronomical observers who were initially unable to distinguish them from planets, and who tended to confuse them with planets, which were of more interest to them. The Sun

13410-480: The explosion lies in the center of the Crab Nebula and its core is now a neutron star . Still other nebulae form as planetary nebulae . This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8–10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and

13559-418: 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

13708-480: 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 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

13857-522: The first detailed study of the Orion Nebula was not performed until 1659 by Christiaan Huygens , who also believed he was the first person to discover this nebulosity. In 1715, Edmond Halley published a list of six nebulae. This number steadily increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751 to 1753, Nicolas-Louis de Lacaille cataloged 42 nebulae from

14006-485: The flattening ( f {\displaystyle f} ) and the ratio at the equator of centrifugal force to gravitational attraction. (Compare with the exact relation above for the case of uniform density.) Clairaut's theorem is a special case, for an oblate spheroid, of a connexion found later by Pierre-Simon Laplace between the shape and the variation of gravity. If the star has a massive nearby companion object then tidal forces come into play as well, distorting

14155-439: The fluid's velocity is constant. Dividing by A, 0 = P bottom − P top − ρ g h {\displaystyle 0=P_{\text{bottom}}-P_{\text{top}}-\rho gh} Or, P top − P bottom = − ρ g h {\displaystyle P_{\text{top}}-P_{\text{bottom}}=-\rho gh} P top − P bottom

14304-505: The force downwards onto the top of the cuboid from the pressure, P , of the fluid above it is, from the definition of pressure , F top = − P top A {\displaystyle F_{\text{top}}=-P_{\text{top}}A} Similarly, the force on the volume element from the pressure of the fluid below pushing upwards is F bottom = P bottom A {\displaystyle F_{\text{bottom}}=P_{\text{bottom}}A} Finally,

14453-499: The form f ( Ρ , ρ ) = 0, with f specific to makeup of the star. M ( r ) is a foliation of spheres weighted by the mass density ρ ( r ), with the largest sphere having radius r : M ( r ) = 4 π ∫ 0 r d r ′ r ′ 2 ρ ( r ′ ) . {\displaystyle M(r)=4\pi \int _{0}^{r}dr'\,r'^{2}\rho (r').} Per standard procedure in taking

14602-534: 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 ,

14751-621: 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

14900-465: 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

15049-414: The gravity if the fluid is not rotating, is asymptotic to as ϵ {\displaystyle \epsilon } goes to zero, where f {\displaystyle f} is the flattening: The gravitational attraction on the equator (not including centrifugal force) is Asymptotically we have: Maclaurin showed (still in the case of uniform density) that the component of gravity toward

15198-416: 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

15347-415: 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

15496-435: The largest member of the collection of icy bodies known as 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

15645-519: 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

15794-445: 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

15943-503: The light they reflect. Reflection nebulae themselves do not emit significant amounts of visible light, but are near stars and reflect light from them. Similar nebulae not illuminated by stars do not exhibit visible radiation, but may be detected as opaque clouds blocking light from luminous objects behind them; they are called dark nebulae . Although these nebulae have different visibility at optical wavelengths, they are all bright sources of infrared emission, chiefly from dust within

16092-504: 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

16241-478: 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

16390-406: The massive base of the tallest mountain on Earth, Mauna Kea , has deformed and depressed the level of the surrounding crust, so that the overall distribution of mass approaches equilibrium. In the atmosphere, the pressure of the air decreases with increasing altitude. This pressure difference causes an upward force called the pressure-gradient force . The force of gravity balances this out, keeping

16539-525: 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 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

16688-577: 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

16837-401: The nebulae. Planetary nebulae are the remnants of the final stages of stellar evolution for mid-mass stars (varying in size between 0.5-~8 solar masses). Evolved asymptotic giant branch stars expel their outer layers outwards due to strong stellar winds, thus forming gaseous shells while leaving behind the star's core in the form of a white dwarf . Radiation from the hot white dwarf excites

16986-642: The nonrelativistic limit the Tolman–Oppenheimer–Volkoff equation reduces to Newton's hydrostatic equilibrium: d P d r = − G M ( r ) ρ ( r ) r 2 = − g ( r ) ρ ( r ) ⟶ d P = − ρ ( h ) g ( h ) d h {\displaystyle {\frac {dP}{dr}}=-{\frac {GM(r)\rho (r)}{r^{2}}}=-g(r)\,\rho (r)\longrightarrow dP=-\rho (h)\,g(h)\,dh} (we have made

17135-664: The nonrelativistic limit, we let c → ∞ , so that the factor ( 1 + P ( r ) ρ ( r ) c 2 ) ( 1 + 4 π r 3 P ( r ) M ( r ) c 2 ) ( 1 − 2 G M ( r ) r c 2 ) − 1 → 1 {\displaystyle \left(1+{\frac {P(r)}{\rho (r)c^{2}}}\right)\left(1+{\frac {4\pi r^{3}P(r)}{M(r)c^{2}}}\right)\left(1-{\frac {2GM(r)}{rc^{2}}}\right)^{-1}\rightarrow 1} Therefore, in

17284-546: 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

17433-413: 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

17582-415: 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 a rogue planet ,

17731-489: 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

17880-756: 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

18029-409: The outward-pushing pressure gradient and the weight of the material above pressing inward. One can also study planets under the assumption of hydrostatic equilibrium. A rotating star or planet in hydrostatic equilibrium is usually an oblate spheroid , that is, an ellipsoid in which two of the principal axes are equal and longer than the third. An example of this phenomenon is the star Vega , which has

18178-417: 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

18327-592: 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

18476-494: The poles by an amount equal (at least asymptotically ) to five fourths the centrifugal force at the equator. In 1742, Colin Maclaurin published his treatise on fluxions, in which he showed that the spheroid was an exact solution. If we designate the equatorial radius by r e , {\displaystyle r_{e},} the polar radius by r p , {\displaystyle r_{p},} and

18625-456: The result of supernova explosions; the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are then ionized by the energy and the compact object that its core produces. One of the best examples of this is the Crab Nebula , in Taurus . The supernova event was recorded in the year 1054 and is labeled SN 1054 . The compact object that was created after

18774-401: The same pressure at the bottom of channels from the pole or from the equator to the centre, so the centrifugal force at the equator must be Defining the latitude to be the angle between a tangent to the meridian and axis of rotation, the total gravity felt at latitude ϕ {\displaystyle \phi } (including the effect of centrifugal force) is This spheroid solution

18923-416: 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

19072-448: 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

19221-440: 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" was derived. In ancient Greece , China , Babylon , and indeed all pre-modern civilizations, it was almost universally believed that Earth

19370-412: 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

19519-514: The spectra of about 70 nebulae. He found that roughly a third of them had the emission spectrum of a gas . The rest showed a continuous spectrum and were thus thought to consist of a mass of stars. A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster . Thus, the nebula radiates by reflected star light. In 1923, following

19668-468: 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,

19817-461: The star into a scalene shape when rotation alone would make it a spheroid. An example of this is Beta Lyrae . Hydrostatic equilibrium is also important for the intracluster medium , where it restricts the amount of fluid that can be present in the core of a cluster of galaxies . We can also use the principle of hydrostatic equilibrium to estimate the velocity dispersion of dark matter in clusters of galaxies. Only baryonic matter (or, rather,

19966-399: 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

20115-520: 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

20264-458: 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

20413-407: 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

20562-507: 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 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

20711-834: The temperature and density of the baryonic matter, and Λ ( T ) {\displaystyle \Lambda (T)} is some function of temperature and fundamental constants. The baryonic density satisfies the above equation d P = − ρ g d r {\displaystyle dP=-\rho g\,dr} : p B ( r + d r ) − p B ( r ) = − d r ρ B ( r ) G r 2 ∫ 0 r 4 π r 2 ρ M ( r ) d r . {\displaystyle p_{B}(r+dr)-p_{B}(r)=-dr{\frac {\rho _{B}(r)G}{r^{2}}}\int _{0}^{r}4\pi r^{2}\,\rho _{M}(r)\,dr.} The integral

20860-589: The term "nebula" was used to describe any diffused astronomical object , including galaxies beyond the Milky Way . The Andromeda Galaxy , for instance, was once referred to as the Andromeda Nebula (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher , Edwin Hubble , and others. Edwin Hubble discovered that most nebulae are associated with stars and illuminated by starlight. He also helped categorize nebulae based on

21009-432: 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

21158-656: The three-dimensional Navier–Stokes equations for the equilibrium situation where u = v = ∂ p ∂ x = ∂ p ∂ y = 0 {\displaystyle u=v={\frac {\partial p}{\partial x}}={\frac {\partial p}{\partial y}}=0} Then the only non-trivial equation is the z {\displaystyle z} -equation, which now reads ∂ p ∂ z + ρ g = 0 {\displaystyle {\frac {\partial p}{\partial z}}+\rho g=0} Thus, hydrostatic balance can be regarded as

21307-687: The trivial notation change h  =  r and have used f ( Ρ , ρ ) = 0 to express ρ in terms of P ). A similar equation can be computed for rotating, axially symmetric stars, which in its gauge independent form reads: ∂ i P P + ρ − ∂ i ln ⁡ u t + u t u φ ∂ i u φ u t = 0 {\displaystyle {\frac {\partial _{i}P}{P+\rho }}-\partial _{i}\ln u^{t}+u_{t}u^{\varphi }\partial _{i}{\frac {u_{\varphi }}{u_{t}}}=0} Unlike

21456-548: The type of light spectra they produced. Around 150 AD, Ptolemy recorded, in books VII–VIII of his Almagest , five stars that appeared nebulous. He also noted a region of nebulosity between the constellations Ursa Major and Leo that was not associated with any star . The first true nebula, as distinct from a star cluster , was mentioned by the Muslim Persian astronomer Abd al-Rahman al-Sufi in his Book of Fixed Stars (964). He noted "a little cloud" where

21605-414: 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

21754-442: Was published in 1786. A second catalog of a thousand was published in 1789, and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity rather than a more distant cluster. Beginning in 1864, William Huggins examined

21903-420: Was recognized as a planet when heliocentrism supplanted geocentrism during 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

22052-540: 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 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

22201-461: 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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