Ten Planets: Stories

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Likewise, objects such as Pallas and Vesta that developed planetary geology but are no longer round are also excluded, though some authors would include them regardless. The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the world itself. Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result. This definition is based in modern theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects.

Neptune was discovered in 1846, its position having been predicted thanks to its gravitational influence upon Uranus. Aaboe, Asger (1991), "The culture of Babylonia: Babylonian mathematics, astrology, and astronomy", in Boardman, John; Edwards, I.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. Multiple exoplanets have been found to orbit in the habitable zones of their stars, but Earth remains the only planet known to support life. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 ( helium)—appears to determine the likelihood that a star will have planets. Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science".

Some scientists who accept the IAU definition of "planet" use other terms for bodies satisfying geophysical planet definitions, such as "world". A "planet" [1] is a celestial body inside the Solar System that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. 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 the subsequent impact of comets. Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.

From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. Thereafter there still may be many protoplanets orbiting 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. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury.