The solar system may have a new smallest dwarf planet: Hygiea". Science News. Society for Science. 28 October 2019. Astronomers, however, are still hunting for another possible planet in our solar system, a true ninth planet, after mathematical evidence of its existence was revealed on Jan. 20, 2016. The alleged "Planet Nine," also called "Planet X," is believed to be about 10 times the mass of Earth and 5,000 times the mass of Pluto. Mosaic of the Valles Marineris hemisphere of Mars. The image was created from 102 Viking Orbiter images of Mars. (Image credit: NASA/JPL-Caltech) Ronan, Colin (1996). "Astronomy Before the Telescope". In Walker, Christopher (ed.). Astronomy in China, Korea and Japan. British Museum Press. pp.264–265.

Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project Retrieved on 11 July 2022. One of the most detailed images of Earth. The montage was created from photographs taken by the Visible/Infrared Imager Radiometer Suite (VIIRS) instrument onboard the Suomi NPP satellite. (Image credit: NASA) Neptune is approximately the same size as Uranus and is known for its supersonic strong winds. The planet is more than 30 times as far from the sun as Earth.In Korean, these names are more often written in Hangul rather than Chinese characters, e.g. 명왕성 for Pluto. In Vietnamese, calques are more common than directly reading these names as Sino-Vietnamese, e.g. sao Thuỷ rather than Thuỷ tinh for Mercury. Pluto is not sao Minh Vương but sao Diêm Vương " Yama star". [100] Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union". IAU. 2001. Archived from the original on 16 September 2006 . Retrieved 23 August 2008. Sachs, A. (2 May 1974). "Babylonian Observational Astronomy". Philosophical Transactions of the Royal Society. 276 (1257): 43–50 [45 & 48–49]. Bibcode: 1974RSPTA.276...43S. doi: 10.1098/rsta.1974.0008. JSTOR 74273. S2CID 121539390. Luu, Jane X.; Jewitt, David C. (1996). "The Kuiper Belt". Scientific American. 274 (5): 46–52. Bibcode: 1996SciAm.274e..46L. doi: 10.1038/scientificamerican0596-46. The Hubble Space Telescope captured this image of Saturn during its northern hemisphere summer on July 4, 2020. (Image credit: NASA, ESA, A. Simon (Goddard Space Flight Center), M.H. Wong (University of California, Berkeley), and the OPAL Team)

Mercury is the closest planet to the sun and the smallest planet in the solar system — it is only a little larger than Earth's moon. Mercury zips around the sun in only 88 days and because it is so close to our star (about two-fifths the distance between Earth and the sun). Main articles: Definition of planet and IAU definition of planet Euler diagram showing the IAU Executive Committee conception of the types of bodies in the Solar System It is smaller than Earth's moon; its orbit is highly elliptical, falling inside Neptune's orbit at some points and far beyond it at others; and Pluto's orbit doesn't fall on the same plane as all the other planets —instead, it orbits 17.1 degrees above or below. In the 1950s, Gerard Kuiper published papers on the origin of the asteroids. He recognised that asteroids were typically not spherical, as had previously been thought, and that the asteroid families were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta. [37] The hypothesized Planet Nine is estimated to be about 10 times the mass of Earth (Image credit: ESO/Tom Ruen/nagualdesign)

The sun accumulated about 99% of the available matter and the remaining material further from the sun formed smaller clumps inside the spinning disk. Some of these clumps gained enough mass that their gravity shaped them into spheres, becoming planets, dwarf planets and moons. Other leftover pieces became asteroids, comets and smaller moons that make up our solar system.

Pirani, Simona, et al. " Consequences of planetary migration on the minor bodies of the early solar system." Astronomy & Astrophysics 623 (2019): A169. Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians Apollo and Artemis);a b Metzger, Philip T.; Sykes, Mark V.; Stern, Alan; Runyon, Kirby (2019). "The Reclassification of Asteroids from Planets to Non-Planets". Icarus. 319: 21–32. arXiv: 1805.04115. Bibcode: 2019Icar..319...21M. doi: 10.1016/j.icarus.2018.08.026. S2CID 119206487. Kuhn, Thomas S. (1957). The Copernican Revolution. Harvard University Press. pp. 5–20. ISBN 978-0-674-17103-9. The idea of planets has evolved over its history, 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 definition as to what counts as a planet vs. other objects orbiting the Sun has changed several times, previously encompassing asteroids, moons, and dwarf planets like Pluto, [5] [6] [7] and there continues to be some disagreement today. [7] A planet's defining physical characteristic 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 the object collapses into a sphere. [216] NASA has a fleet of spacecraft observing the sun, such as the Parker Solar Probe, to learn more about its composition, and to make better predictions about space weather and its effect on Earth.