In 2006, Pluto was famously demoted from a planet to a dwarf planet. It stays probably the most well-known dwarf planet right this moment, however there are others in our photo voltaic system, together with doubtlessly tons of that have not been found but.
However what, precisely, is a dwarf planet? And what number of dwarf planets are there?
A dwarf planet is a celestial physique that’s smaller than a planet however greater than an asteroid or different small rock within the photo voltaic system. They’re not planets as a result of they solely meet two of the three standards for one thing in house to be thought-about a real planet.
A planet is outlined by three qualities: It orbits the solar, it is principally spherical, and it is large sufficient for its gravity to have principally kicked massive objects from its orbit. Pluto and another recognized dwarf planets meet the primary two standards however not the third as a result of they orbit within the Kuiper Belt, a area past Neptune stuffed with small icy particles.
When Pluto was first found in 1930, it (and different related objects within the Kuiper Belt) have been so far-off and faint that they have been actually exhausting to identify, even with probably the most highly effective telescopes on the time. Certainly, the very qualities that make one thing a dwarf planet make these objects significantly exhausting to detect.
Associated: James Webb telescope discovers ‘a brand new type of local weather’ on Pluto, in contrast to the rest in our photo voltaic system
“There are some things that work in opposition to us recognizing them,” Mathew Yu, an astronomer at UCLA, instructed Reside Science in an e mail. “They’re far-off from the solar. The mirrored mild from them is faint in consequence. Some have low reflectivity, making mild reflecting off them fainter.”
Plus, “they transfer slowly throughout the sky on account of their massive distance from the solar,” he added. “They go across the solar as soon as each tons of of Earth years. So their relative motions to the background stars are small. For astronomers with a restricted observing time, these objects turn out to be exhausting to detect.”
Astronomers bought quite a bit higher at discovering objects within the Kuiper Belt within the Nineteen Nineties and 2000s as telescope know-how improved. And as soon as they may see a bit higher, they observed there have been much more rocks on the market with orbits just like Pluto’s. Between 2002 and 2005, astronomers discovered seven extra significantly massive rocks: Haumea, Makemake, Quaoar, Sedna, Orcus, Eris and Salacia.
The Worldwide Astronomical Union (IAU) — the group of astronomers that decides on official names and phrases for issues in house — determined in 2006 {that a} new class was wanted to explain these objects that have been fairly large however not sufficiently big to clear their orbits as a real planet does. Thus, the identify “dwarf planet” was born.
What number of dwarf planets are there?
With eight named massive rocks within the Kuiper Belt, you would possibly assume meaning there are eight dwarf planets. Nonetheless, not all astronomers agree on the quantity.
“Accepted numbers vary between round 10 to a couple hundred within the Kuiper Belt alone,” Yu stated. “Some estimate that there could also be as much as tens of 1000’s of them past the Kuiper Belt.”
The IAU has formally acknowledged solely 5 dwarf planets: Pluto, Haumea, Makemake and Eris within the Kuiper Belt, plus Ceres within the asteroid belt. We have despatched house probes to solely two of those: Pluto (visited by NASA’s New Horizons) and Ceres (visited by NASA’s Daybreak), each of which flew by these our bodies in 2015.
Astronomers who observe dwarf planets, nonetheless, have a tendency to acknowledge 9: the earlier 5, plus Quaoar, Sedna, Orcus and Gonggong. Gonggong was found in 2007 after the dwarf planet identify change, however it wasn’t confirmed till some new observations from NASA telescopes in 2016. The IAU remains to be contemplating proposals on these dwarfish our bodies.
There are a minimum of a handful extra objects which were proposed however want extra observations to substantiate their dimension and, subsequently, dwarf planet standing.
And in June 2025, scientists proposed a brand new doable dwarf planet: 2017 OF201. Astronomers discovered this significantly faint object by looking out by an previous pile of knowledge with new superior algorithms.
Should you depend the entire at the moment proposed dwarf planets plus the newcomer, that makes eighteen or extra dwarf planets within the photo voltaic system — and that is simply those we have seen. There are doubtless extra on the market, maybe even tons of or 1000’s, sometime to be revealed as our telescopes preserve getting higher.
