In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12. [48] This discovery was confirmed and is generally considered to be the first definitive detection of exoplanets. Researchers suspect they formed from a disk remnant left over from the supernova that produced the pulsar. [163] If you put Saturn in a bathtub it would float as Saturn has an average density that is less than water. You'd just need to find a bathtub big enough… In 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets orbiting a Sun-like star, Kepler-20e and Kepler-20f. [167] [168] [169] 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 a terrestrial planet could sustain liquid water on its surface, given enough atmospheric pressure. [170] [171] [172] 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. [c] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of intelligent, communicating civilizations that exist in the Milky Way. [175] a b c d Basri, Gibor; Brown, Michael E. (2006). "Planetesimals to Brown Dwarfs: What is a Planet?" (PDF). Annual Review of Earth and Planetary Sciences. 34: 193–216. arXiv: astro-ph/0608417. Bibcode: 2006AREPS..34..193B. doi: 10.1146/annurev.earth.34.031405.125058. S2CID 119338327 . Retrieved 4 August 2008.

Approximately 4.5 billion years ago a dark cloud of gas and dust began to collapse. As it shrank, the cloud flattened into a swirling disk known as a solar nebula, according to NASA Science. a b c d e f Hatzes, Artie P.; Rauer, Heike (2015). "A Definition for Giant Planets Based on the Mass-Density Relationship". The Astrophysical Journal. 810 (2): L25. arXiv: 1506.05097. Bibcode: 2015ApJ...810L..25H. doi: 10.1088/2041-8205/810/2/L25. S2CID 119111221. When four satellites of Jupiter (the Galilean moons) and five of Saturn were discovered in the 17th century, they were thought of as "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists. [7] In 2016, researchers proposed the possible existence of a ninth planet, for now, dubbed "Planet Nine" or Planet X. The planet is estimated to be about 10 times the mass of Earth and to orbit the sun between 300 and 1,000 times farther than the orbit of the Earth.Netburn, Deborah (13 November 2015). "Why we need a new definition of the word 'planet' ". Los Angeles Times. The energetic impacts of 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. [145] 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. [146] (Smaller planets will lose any atmosphere they gain through various escape mechanisms. [147]) Yes, so many! If you had asked anyone just 30 years ago, the answer would have been "we don’t know". But since then we have discovered already more than 5,000 planets orbiting stars other than our sun (so-called exoplanets). And since often we find multiple of them orbiting the same star, we can count about 4,000 other solar systems. Do solar systems move? Neptune was discovered in 1846, its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might be another planet even closer to the Sun. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein's general relativity. [38] [39] 20th century

Raymond, C. A.; Ermakov, A. I.; Castillo-Rogez, J. C.; Marchi, S.; etal. (August 2020). "Impact-driven mobilization of deep crustal brines on dwarf planet Ceres". Nature Astronomy. 4 (8): 741–747. Bibcode: 2020NatAs...4..741R. doi: 10.1038/s41550-020-1168-2. ISSN 2397-3366. S2CID 211137608. Astronomers believe an object twice the size of Earth collided with Uranus roughly 4 billion years ago, causing Uranus to tilt. That tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years at a time. The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900km diameter. [61] There is general consensus on Ceres in the asteroid belt [62] and on the eight trans-Neptunians that probably cross this threshold – Quaoar, Sedna, Orcus, Pluto, Haumea, Eris, Makemake, and Gonggong. [63] Near-infrared spectroscopy by the James Webb Space Telescope in 2022 suggests that the threshold for internal geochemistry (which reveals itself in the presence of light hydrocarbons on the surface of these objects) is slightly higher, and that even Orcus (the smallest of those eight trans-Neptunians) is not a dwarf planet, although the others would be. [64]Mayor, Michel; Queloz, Didier (1995). "A Jupiter-mass companion to a solar-type star". Nature. 378 (6356): 355–359. Bibcode: 1995Natur.378..355M. doi: 10.1038/378355a0. S2CID 4339201. If you insist on including Pluto, it would come after Neptune on the list. Pluto is truly way out there and on a wildly tilted, elliptical orbit (two of the several reasons it was demoted). 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 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). [255] [256] [257] [258] [259] According to the IAU definition, there are eight planets in the Solar System, which are (in increasing distance from the Sun): [1] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses. [151] The IAU noted that this definition could be expected to evolve as knowledge improves. [75] A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects". [74]