Dwarf planet

A dwarf planet is a small planetary-mass object that orbits the Sun directly. It is massive enough to be gravitationally rounded, but it does not have orbital dominance like the eight main planets of the Solar System. Dwarf planets are not the same as minor planets.

The best-known dwarf planet is Pluto. For decades, Pluto was called a planet. In 2006, scientists adopted the "dwarf" idea. Many planetary geologists think dwarf planets and planetary-mass moons are planets. However, since 2006, the IAU and many astronomers have left them off the list of planets.

Dwarf planets can be geologically active. This was shown in 2015 by the Dawn mission to Ceres and the New Horizons mission to Pluto. Because of this, planetary geologists are very interested in them.

Astronomers generally agree that at least the nine largest candidates are dwarf planets. From largest to smallest, these are Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. The tenth largest candidate, Salacia, is a borderline case because scientists are still not sure about it.

Of these ten, two have been visited by spacecraft (Pluto and Ceres). Seven others have at least one known moon. This helps scientists measure their masses and estimate their densities. Scientists can use mass and density in geophysical models to learn about these worlds. Only Sedna has not been visited and has no known moons, so its mass is hard to estimate accurately. Some astronomers include many smaller bodies too, but there is no agreement that they are likely to be dwarf planets.

Main articles: Geophysical definition of planet and IAU definition of planet

Starting in 1801, astronomers found Ceres and other bodies between Mars and Jupiter. For decades, these were thought to be planets. Around 1851, when there were 23 planets, astronomers began calling the smaller bodies asteroids, a Greek word meaning 'star-like,' and called them minor planets instead of major planets. When Pluto was found in 1930, most astronomers counted nine major planets, plus thousands of much smaller bodies (asteroids and comets). For almost 50 years, Pluto was thought to be bigger than Mercury. After its moon Charon was found in 1978, Pluto's mass could be measured accurately. It turned out to be much smaller than first thought. It was about one twentieth the mass of Mercury, making it by far the smallest planet. It was still more than ten times as massive as Ceres, the largest object in the asteroid belt, but it had only one-fifth the mass of Earth's Moon. Its unusual features, such as a large orbital eccentricity and a high orbital inclination, showed that it was a different kind of body from the other planets.

In the 1990s, astronomers found more objects in the same region as Pluto, now called the Kuiper belt, and some even farther away. Many shared Pluto's key orbital features, and Pluto came to be seen as the largest member of a new group called plutinos. Either these larger bodies would also have to be planets, or Pluto would have to be reclassified, as Ceres had been. Some astronomers stopped calling Pluto a planet. When Eris (then known as 2003 UB₃₁₃), a trans-Neptunian object, was found in January 2005, it was thought to be slightly larger than Pluto. Some people informally called it the tenth planet. The question was hotly debated at the IAU General Assembly in August 2006. Many people objected to the first draft, which had included Charon, Eris, and Ceres as planets. Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi then proposed a new middle category for round objects that had not cleared their orbits of planetesimals. This removed Charon, Pluto, Ceres, and Eris from the planets. Some scientists disagreed with the decision. Mike Brown, who discovered Eris, supports reducing the number of planets to eight. NASA said it would follow the new guidelines. Alan Stern, head of NASA's mission to Pluto, still calls Pluto a planet. He also counts other likely dwarf planets, as well as the larger moons, as planets.

Name

Several names are used for large objects that are smaller than planets. These include dwarf planet, planetoid, meso-planet (used for sizes between Mercury and Ceres), quasi-planet, and plutoid (for objects beyond Neptune). The term dwarf planet was first used for the smallest planets, and many planetary astronomers still use it that way. As early as 1838, it was also used as another word for asteroid. It was also the opposite of giant planet, which included the terrestrial planets and the Moon. In 2006, the IAU decided that dwarf planets are not planets. Michael E. Brown said that planetoid was "a perfectly good word" and that calling a non-planet a dwarf planet was "dumb." The name was chosen after a vote changed the draft's word planetoids to dwarf planet. This plan also aimed to keep Pluto a planet.

Most languages translate dwarf planet almost word for word. In Japanese, though, these objects are called junwakusei, which means "quasi-planets." In 2008, the IAU Executive Committee announced the term plutoid for dwarf planets beyond Neptune, but other IAU groups did not accept it. Earlier, scientists divided the Solar System into inner terrestrial planets, middle giant planets, and outer ice dwarfs, with Pluto as the main example. The term ice dwarf was also used for all trans-Neptunian minor planets or for the icy asteroids of the outer Solar System. One definition said an ice dwarf "is larger than the nucleus of a normal comet and icier than a typical asteroid." Since the Dawn mission, Ceres has also been called an ice dwarf. Scientists suggested names such as plutons and plutonian objects for Pluto's group, but neither name was kept. This may have been because geologists did not want confusion with their word pluton.

Criteria

Main article: Clearing the neighbourhood

Scientists have argued about how to define a planet. Some look at how objects move. They ask whether a body is big enough to clear other objects from its path around the Sun. Mercury through Neptune can do this, but asteroids and Kuiper belt objects cannot. Other scientists look at shape. A body can become round with much less mass than it needs to clear its path. In 2000, Alan Stern and Harold F. Levison introduced a value called Λ (lambda). It shows how likely a body is to clear the neighbourhood of its orbit. The smallest terrestrial planets and the largest asteroids and Kuiper belt objects were very different in this value. Steven Soter used this idea to argue that planets can clear smaller bodies from their orbits, but dwarf planets cannot. He also proposed a measure called µ (mu). Later, Jean-Luc Margot made a similar measure called Π (Pi).

Main article: Hydrostatic equilibrium

Other scientists study roundness instead. A body becomes round when its own gravity makes it plastic enough. Then high places sink and low places fill in. Small bodies are often irregular. Larger ones are shaped like potatoes until they reach hydrostatic equilibrium. At that point they are as round as they can be and form an ellipsoid. Fast spinning can make a body oblate or even scalene, as with Haumea. The Moon is tidally locked to Earth. Pluto and Charon are tidally locked to each other. Eris and Dysnomia are also tidally locked, and Orcus and Vanth probably are too. Dwarf planets do not have a set size limit. A distant object that is more massive than Mercury might not have cleared its neighbourhood. It would then fit the dwarf planet definition. In 2006, the IAU estimated that objects with a mass above 0.5×10²¹ kg and a radius greater than 400 km would normally become round. Salacia is a borderline case. Orcus is just above the limit. Several without a measured mass come close to the expected size limit. Sedna has not been measured, but it is likely similar in mass to Ceres.

Population of dwarf planets

Main article: List of possible dwarf planets

Scientists are not always sure whether a trans-Neptunian object (TNO) is big and soft enough to pull itself into a round shape. Because of this, it is hard to know how many TNOs fit the idea. Three objects were discussed when the dwarf planet category was created in 2006: Ceres, Pluto, and Eris. Most astronomers accept all three as dwarf planets, including some who still call them planets. Pluto is the only one studied closely enough to confirm that its shape fits. Ceres is close to fitting. Eris is thought to be a dwarf planet because it is heavier than Pluto.

Other large objects are also called dwarf planets. Haumea and Makemake were the only bodies that met an absolute magnitude limit set for naming likely dwarf planets. The Minor Planet Center shares this naming task. Astronomers also call Gonggong, Quaoar, Sedna, and Orcus dwarf planets. Scientists have proposed more candidates, such as Salacia, Máni, Varuna, Ixion, Chiminigagua, and Aya. Some researchers now wonder if the smallest of these bodies can become round.

Most likely dwarf planets

Main article: List of possible dwarf planets

Most of the bodies in these tables are considered likely dwarf planets by several research teams. These include Tancredi and the scientists who studied Gǃkúnǁʼhòmdímà and Varda. Salacia is shown in italics because it is a borderline case. Charon is included for comparison. It is a planetary-mass moon of Pluto.

Recent studies used the James Webb Space Telescope (JWST) to look at the surfaces of several bodies. The telescope uses near-infrared spectroscopy to study light from them. The studies found ethane, acetylene, and ethylene on some of them. This suggests their insides may have melted in the past. Quaoar's shape also seems to have stayed the same as it slowed its spin. This may be because of its moon Weywot, much like Iapetus.

Symbols

Main article: Astronomical symbol § Trans-Neptunian objects

Ceres and Pluto received planetary symbols because they were considered planets when they were discovered. Later, most astronomers stopped using these symbols. Symbols for other dwarf planets are mostly used by astrologers. Denis Moskowitz created them, and they are included in Unicode.

As of 2025, only two missions have explored dwarf planets up close. On March 6, 2015, the Dawn spacecraft entered orbit around Ceres. It was the first spacecraft to visit a dwarf planet. On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons.

Ceres shows signs of an active geology, such as salt deposits and cryovolcanos. Pluto has water-ice mountains floating in nitrogen-ice glaciers, and it has a thick atmosphere. Ceres seems to have salty water flowing beneath its surface. There is also evidence that Pluto has an underground ocean. Saturn's moon Phoebe and Neptune's moon Triton also show signs that they were once dwarf planets. New Horizons has taken distant images of several dwarf planet candidates. Quaoar has been proposed as a possible target for the two Shensuo probes from the China National Space Administration.

Similar objects

Some bodies look a lot like dwarf planets. Some are former dwarf planets. They may still be round or show signs of active geology. Others are planetary-mass moons. These have the right shape but do not orbit the Sun directly. Charon, the moon of Pluto, may be a binary dwarf planet. Some objects fit into more than one group. For example, Triton is both a former dwarf planet and a planetary-mass moon.

Former dwarf planets

Vesta is the second most massive body in the asteroid belt, after Ceres. It was once round in hydrostatic equilibrium. Huge impacts formed the Rheasilvia and Veneneia craters, and now it is not round. Triton is more massive than Eris or Pluto and has a round shape. It is thought to be a former dwarf planet that was captured by Neptune early in the Solar System's history. Phoebe is a captured centaur that is no longer round. It may have been round early in its history because of radiogenic heating.

Planetary-mass moons

Main article: Planetary-mass moon

At least nineteen moons have round shapes. They either formed as solid bodies or settled into a round shape under their own gravity. Seven are more massive than Eris or Pluto. They are not very different from dwarf planets, but they do not fit the IAU definition because they do not directly orbit the Sun. Alan Stern calls these satellite planets. They are one of three groups of planets, along with dwarf planets and classical planets. The term planemo also covers all three groups.

Charon

Some scientists have debated whether the Pluto–Charon system should be called a double dwarf planet. In a draft resolution for the IAU definition of planet, both Pluto and Charon were considered planets in a binary system. The IAU now says Charon is not a dwarf planet but a moon of Pluto. This could be reconsidered later. It is also no longer clear that Charon is round. Before Pluto and Charon can be formally called binary dwarf planets, scientists need a clear definition. The barycentre depends on the masses of the bodies and the distance between them.