Planet

• 1. Introduction

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  A planet is a large, rounded astronomical body, and for centuries the word meant something humble: a point of light that wandered. In antiquity, the Greek word planḗtai referred to the Sun, the Moon, and five lights visible to the naked eye that drifted across the fixed stars. Those five were Mercury, Venus, Mars, Jupiter, and Saturn. Earth was not among them. It sat, everyone agreed, at the unmoving center of everything. The story of how that list grew, shrank, and fractured into competing definitions runs through Babylonian clay tablets, a telescope, a vote in 2006, and the discovery of more than 5,900 worlds beyond our own Sun. How does a cloud of gas become a world? Where does a planet end and a star begin? And why do astronomers still argue over whether Pluto qualifies? These are the questions the wandering lights left behind.

• 2. Birth From A Disk

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  It is not known with certainty how planets are formed, but the prevailing account is the nebular hypothesis. An interstellar cloud collapses out of a nebula, and a young protostar forms at the core, surrounded by a rotating protoplanetary disk of gas and dust. Through accretion, a process of sticky collision, dust particles steadily accumulate mass into ever-larger bodies. Local concentrations called planetesimals form, and their gravity draws in more material until they collapse inward into protoplanets. After a planet reaches a mass somewhat larger than that of Mars, it begins to accumulate an extended atmosphere, which increases its capture rate of planetesimals through atmospheric drag. Depending on the accretion history of solids and gas, the result may be a giant planet, an ice giant, or a terrestrial planet. When the protostar ignites into a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, and Poynting-Robertson drag. The energetic impacts of smaller planetesimals, along with radioactive decay, heat the growing planet until it at least partially melts. Denser materials sink toward the core, leaving lighter ones near the surface. Whether a star forms planets at all appears to depend on its metallicity, the abundance of elements heavier than helium. A metal-rich population I star is more likely to host a substantial planetary system than a metal-poor population II star.

• 3. The Eight And Beyond

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  Jupiter is the largest planet in the Solar System at 318 Earth masses, while Mercury is the smallest at 0.055 Earth masses. The terrestrials, Mercury, Venus, Earth, and Mars, are largely composed of rock and metal, with Earth the largest among them. The gas giants Jupiter and Saturn are primarily hydrogen and helium, and Saturn is one third as massive as Jupiter, at 95 Earth masses. The ice giants Uranus and Neptune are primarily made of water, methane, and ammonia, with thick atmospheres of hydrogen and helium, and they carry only 14 and 17 Earth masses respectively. Beyond the eight lie the dwarf planets, gravitationally rounded but unable to clear their orbits of other bodies. Ceres is the largest object in the asteroid belt, between Mars and Jupiter. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt beyond Neptune. Gonggong and Eris orbit in the scattered disc further out, and Sedna is the largest known detached object, never coming close enough to the Sun to interact with the classical planets. All of them are smaller than Mercury, with Pluto the largest dwarf planet and Eris the most massive. At least nineteen moons are large enough to take ellipsoidal shapes, including Earth's Moon, the four Galilean satellites of Jupiter, Titan and Iapetus at Saturn, Miranda and Oberon at Uranus, Neptune's Triton, and Pluto's Charon. Mimas is the smallest object generally agreed to be a geophysical planet, at about six millionths of Earth's mass.

• 4. Worlds Around Other Suns

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  In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12, generally considered the first definitive detection of exoplanets. Researchers suspect those worlds formed from a disk left over from the supernova that produced the pulsar. The first confirmed exoplanet around an ordinary main-sequence star came on the 6th of October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced 51 Pegasi b around the star 51 Pegasi. Analysis of gravitational microlensing data suggests a minimum average of 1.6 bound planets for every star in the Milky Way. In 2011, the Kepler space telescope team reported Kepler-20e and Kepler-20f, the first Earth-sized exoplanets around a Sun-like star. One in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone, which suggests the nearest would be within 12 light-years of Earth. Many exoplanets show features the Solar System lacks. Hot Jupiters such as 51 Pegasi b orbit very close to their stars and may evaporate into chthonian planets, the leftover cores. Ultra-short period planets can orbit in less than a day, while COCONUTS-2b sits thousands of AU from its star and takes more than a million years to orbit. There are super-Earths and mini-Neptunes, with masses between Earth and Neptune, that have no counterpart at home. Gliese 581c, with a mass 5.5 to 10.4 times that of Earth, drew attention as possibly habitable before later studies placed it too close to its star.

• 5. Motion And Spin

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  WASP-17b orbits in the opposite direction to its star's rotation, a striking exception to the Solar System, where every planet circles the Sun counter-clockwise as seen from above the Sun's north pole. A planet's year, its sidereal period, lengthens with distance from its star, since a farther planet travels a longer path at a slower speed. No orbit is perfectly circular, so each planet swings between its closest approach, the perihelion, and its farthest, the aphelion, speeding up as it falls inward and slowing as it climbs away. Eccentricity measures how elongated an orbit is, and while the major planets keep nearly circular paths, comets and many Kuiper belt objects swing on exceedingly elliptical ones. Axial tilt gives planets their seasons. Jupiter's tilt is very small, so its seasonal variation is minimal, while Uranus is tilted so extremely it lies virtually on its side, leaving its hemispheres in continual sunlight or darkness near its solstices. Earth's Moon is tilted 6.687 degrees, the biggest exception among the planetary-mass moons. Rotation varies just as widely. Venus takes 243 days to turn once and rotates clockwise, one of only two exceptions along with Uranus, while the giant planets spin in only a few hours. Mercury is tidally locked into a 3:2 spin-orbit resonance, rotating three times for every two trips around the Sun. Haumea rotates so fast it has been distorted into a triaxial ellipsoid, while the exoplanet Tau Boötis b and its star appear mutually tidally locked.

• 6. Skins Of Gas And Iron

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  Gravity pulls planets into a roughly spherical shape, though rotation flattens them: Earth's equatorial diameter is 43 km larger than its pole-to-pole diameter, making it an oblate spheroid. A planet's defining physical trait is that its gravity dominates the forces binding its structure, forcing it into hydrostatic equilibrium and a spheroidal form. Every planet began entirely fluid, then differentiated, with denser materials sinking to form a core wrapped in a mantle. The terrestrial planets have cores of iron and nickel and mantles of silicates, while Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen. All Solar System planets except Mercury hold substantial atmospheres, and the threshold for retaining light hydrogen and helium sits near 2.0 Earth masses, placing Earth and Venus near the maximum size for rocky planets. Venus runs hottest of all by surface temperature, hotter even than Mercury, the result of a runaway greenhouse effect, and its surface pressure is about 92 times that of Earth. Mars, by contrast, has a surface pressure less than 1 percent of Earth's, too low for liquid water. Titan carries the only dense nitrogen-rich atmosphere in the Solar System besides Earth's, and its conditions sit near the triple point of methane just as Earth's sit near that of water. A planet's magnetic field signals that it is still geologically alive, generated by flows of conducting material that carve a magnetosphere out of the solar wind. Jupiter's field is the strongest in the Solar System, so intense it poses a serious health risk to future crewed missions to its moons inward of Callisto.

• 7. The Wandering Definition

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  Pluto was discovered in 1930 and immediately accepted as the ninth major planet, after early observations suggested it was larger than Earth. The discovery of its large moon Charon in 1978 revealed that Pluto was only 0.2 percent the mass of Earth. The Copernican count of primary planets had stood until 1781, when William Herschel discovered Uranus, and Neptune followed in 1846, its position predicted from its gravitational pull on Uranus. The 19th century had seen a similar reckoning when Ceres, Pallas, Juno, and Vesta were each called planets after their discoveries between 1801 and 1807, then reclassified as asteroids once many more turned up sharing the same region between Mars and Jupiter. During the 1990s and early 2000s, objects approaching Pluto's size were found across the Kuiper belt, and Pluto turned out to be one small body among thousands. The announcement of Eris in 2005, an object 27 percent more massive than Pluto, forced the issue, since calling Pluto a planet would have demanded the same for Eris. In August 2006 the International Astronomical Union adopted a definition proposed by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi. It required a planet to orbit the Sun, to be rounded by its own gravity, and to have cleared its neighborhood. Bodies meeting the first two conditions but not the third became dwarf planets, placing Ceres, Pluto, and Eris in that category.

• 8. A Definition Still Contested

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  In planetary geology, an object earns the name planet by geophysical traits rather than its address in space. The hallmark is hydrostatic equilibrium, the stable round shape a body takes once its mantle becomes plastic under its own weight, a mass generally below what is needed to clear an orbit. By this standard Ceres and Pluto are planets, and many planetary geologists treat the nineteen known planetary-mass moons as satellite planets, including Earth's Moon and Pluto's Charon. Astronomer Jean-Luc Margot proposed a mathematical criterion that yields a value, pi, greater than 1 for objects that can clear their orbit. The eight planets and all known exoplanets score above 100, while Ceres, Pluto, and Eris score 0.1 or less. In 2024, Margot and collaborators revised the criterion with a uniform clearing timescale of 10 billion years or 13.8 billion years to accommodate planets orbiting brown dwarfs. The boundary at the heavy end is just as unsettled. Brown dwarfs can fuse deuterium above 13 Jupiter masses, while true hydrogen burning and red dwarf status begin near 80 Jupiter masses. Deuterium makes up less than 0.0026 percent of the galaxy's hydrogen, and most brown dwarfs finished burning theirs long before discovery, so some catalogues list objects up to 60 Jupiter masses as planets. The 2018 IAU working definition for exoplanets draws the line at 13 Jupiter masses, the limiting mass for fusing deuterium, while noting the definition could be expected to evolve as knowledge improves.