Dwarf planet

A dwarf planet is a small planetary-mass object that orbits the Sun. It is big enough to be shaped like a ball by its own gravity, but not big enough to clear other objects out of its path like the eight main planets of our Solar System. The most famous dwarf planet is Pluto, which used to be called a planet until scientists created the "dwarf planet" category in 2006.

Dwarf planets can still have interesting geology. In 2015, space missions to Ceres and Pluto showed that these worlds can be active and changing. This makes them exciting targets for scientists who study planets.

Right now, astronomers think there are at least nine big enough objects to be dwarf planets. In order from biggest to smallest, they are Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. Some of these have moons, which helps scientists learn about their sizes and what they are made of. Only Sedna has not been visited by a spacecraft and does not have any known moons, making it harder to study.

History of the concept

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

Near true-colour image of Pluto and its moon Charon. Separation to scale

In 1801, astronomers found Ceres, a small object between Mars and Jupiter. For many years, these objects were called planets. But as more were found, they were called asteroids instead.

When Pluto was discovered in 1930, it was called the ninth planet. In the 1990s, astronomers found many similar objects in a region called the Kuiper belt. This showed that Pluto was part of a larger group.

In 2006, the International Astronomical Union made a new group called dwarf planet. This group includes objects like Pluto, Ceres, and Eris. These objects are round but share space with other objects. This changed how we think about planets in our solar system.

Name

The term dwarf planet describes small bodies that orbit the Sun and are big enough to be round, but not big enough to be full planets. Scientists created this name in 2006 because Pluto did not fit the definition of a regular planet. Other names for these objects include planetoid and quasi-planet.

In different languages, these objects have various names. For example, in French they are called planète naine, in Spanish planeta enano, and in Japanese junwakusei, meaning "quasi-planets." The International Astronomical Union introduced the term plutoid for trans-Neptunian dwarf planets, but not all groups accept this name.

Criteria

The idea of a dwarf planet started because scientists had different ways to think about what makes a planet. Some look at how a body moves in space. Others look at its shape and what it is made of.

From a space-movement point of view, planets clear out their area by pulling in or pushing away smaller objects. Dwarf planets, like Ceres in the asteroid belt and Pluto in the Kuiper belt, are not big enough to do this.

From a geology point of view, a dwarf planet is big enough for its own gravity to make it round, like a small world. This round shape happens when the body’s gravity is strong enough to push down high spots and fill in low spots. This makes it smooth overall. This is different from smaller objects, which might look more like lumpy rocks.

Because of these different ideas, scientists made the category of dwarf planets. This describes objects that are big enough to be round but not big enough to clear out their space.

Planetary discriminants

Body

⁠m/M_🜨 ⁠ ^[†]

Λ ^[‡]

µ ^[§]

Π ^[#]

Mercury

0.055

1.95×10³

9.1×10⁴

1.3×10²

Venus

0.815

1.66×10⁵

1.35×10⁶

9.5×10²

Earth

1.53×10⁵

1.7×10⁶

8.1×10²

Mars

0.107

9.42×10²

1.8×10⁵

5.4×10¹

Ceres

0.00016

8.32×10⁻⁴

0.33

4.0×10⁻²

Jupiter

317.7

1.30×10⁹

6.25×10⁵

4.0×10⁴

Saturn

95.2

4.68×10⁷

1.9×10⁵

6.1×10³

Uranus

14.5

3.85×10⁵

2.9×10⁴

4.2×10²

Neptune

17.1

2.73×10⁵

2.4×10⁴

3.0×10²

Pluto

0.0022

2.95×10⁻³

0.077

2.8×10⁻²

Eris

0.0028

2.13×10⁻³

0.10

2.0×10⁻²

Sedna

0.0002

3.64×10⁻⁷

1.6×10⁻⁴

Planetary discriminants of the planets ( white ), and of the largest known dwarf planet ( light purple ) in each orbital population (asteroid belt, Kuiper belt, scattered disc, sednoids). All other known objects in these populations have smaller discriminants than the one shown.

[†]	Mass in M_🜨, the unit of mass equal to that of Earth ( 5.97 × 10²⁴ kg ).
[‡]	Λ is the capacity to clear the neighbourhood (greater than 1 for planets) by Stern & Levison (2002): Λ = k m² a^{^{⁠− +3/2⁠}}, where k = 0.0043 for m in units of yottagrams (10¹⁸ metric tons) and a in astronomical units (AU), where a is the body's semi-major axis.
[§]	µ is Soter's planetary discriminant, which he finds greater than 100 for planets. µ = ⁠m/ M − m ⁠ , where m is the mass of the body, and M is the aggregate mass of all the bodies that occupy its orbital zone.
[#]	Π is the capacity to clear the neighbourhood (greater than 1 for planets) by Margot. Π = k m a^{^{⁠− +9/8⁠}}, where k = 807 for units of Earth masses and AU.

Population of dwarf planets

Main article: List of possible dwarf planets

A dwarf planet is a special object that goes around the Sun. It is big enough to be pulled into a round shape by its own gravity. But it is not big enough to push other objects out of its path, so it is not called a regular planet like Earth or Mars.

The most well-known dwarf planets are Ceres, Pluto, and Eris. Ceres was found in 1801 and was once thought to be a planet. Pluto was discovered in 1930 and was considered a planet for many years until it was changed to a dwarf planet in 2006. Eris, found in 2005, is even bigger than Pluto and was also named a dwarf planet. Other likely dwarf planets include Haumea and Makemake, which were officially named in 2008. Scientists think there may be many more dwarf planets beyond Neptune, in a region called the Kuiper Belt.

Orbital attributes
Name	Region of the Solar System	Semi-major axis (AU)	Orbital period (years)	Mean orbital speed (km/s)	Inclination to ecliptic	Orbital eccentricity	Planetary discriminant
Ceres	Asteroid belt	2.768	4.604	17.90	10.59°	0.079	0.3
Orcus	Kuiper belt (resonant – 2:3)	39.40	247.3	4.75	20.58°	0.220	0.003
Pluto	Kuiper belt (resonant – 2:3)	39.48	247.9	4.74	17.16°	0.249	0.08
Salacia	Kuiper belt (cubewano)	42.18	274.0	4.57	23.92°	0.106	0.003
Haumea	Kuiper belt (resonant – 7:12)	43.22	284.1	4.53	28.19°	0.191	0.02
Quaoar	Kuiper belt (cubewano)	43.69	288.8	4.51	7.99°	0.040	0.007
Makemake	Kuiper belt (cubewano)	45.56	307.5	4.41	28.98°	0.158	0.02
Gonggong	Scattered disc (resonant – 3:10)	67.49	554.4	3.63	30.74°	0.503	0.01
Eris	Scattered disc	67.86	559.1	3.62	44.04°	0.441	0.1
Sedna	Detached	506.8	≈ 11,400	≈ 1.3	11.93°	0.855

Other attributes
Name	Diameter relative to the Moon	Diameter (km)	Mass relative to the Moon	Mass (×10²¹ kg)	Density (g/cm³)	Rotation period (hours)	Moons	Albedo	H
Ceres	27%	939.4±0.2	1.3%	0.93835±0.00001	2.16	9.1	0	0.09	3.33
Orcus	26%	910+50 −40	0.8%	0.55±0.01	1.4±0.2	13±4	1	0.23+0.02 −0.01	2.19
Pluto	68%	2377±3	17.7%	13.03±0.01	1.85	6d 9.3h	5	0.52	−0.45
(Charon)	35%	1212±1	2.2%	1.59±0.01	1.70±0.01	6d 9.3h	–	0.38	1
Salacia	24%	846±21	0.7%	0.49±0.01	1.50±0.12	6.1	1	0.04	4.27
Haumea	≈ 45%	≈ 1560	5.5%	4.01±0.04	≈ 1.8	3.9	2	≈ 0.66	0.23
Quaoar	32%	1098±2	1.9%	1.21±0.01	1.75±0.01	17.7	1 (2?)	0.11±0.01	2.42
Makemake	41%	1430±14	3.7%	2.69±0.20	1.76±0.17	22.8	1	0.81+0.03 −0.05	−0.20
Gonggong	35%	1230±50	2.4%	1.75±0.07	1.74±0.16	22.4±0.2?	1	0.14±0.01	1.86
Eris	67%	2326±12	22.4%	16.38±0.14	2.43±0.05	15d 18.9h	1	0.96±0.04	−1.21
Sedna	26%	906+314 −258	≈ 1%?	≈ 1?	?	10±3	0?	0.41+0.393 −0.186	1.52

Exploration

Only two missions have visited dwarf planets. On March 6, 2015, the Dawn spacecraft orbited Ceres, the first spacecraft to visit a dwarf planet. On July 14, 2015, the New Horizons probe flew by Pluto and its moons.

Ceres shows signs of active geology, such as salt deposits and cryovolcanos. Pluto has water-ice mountains and a nitrogen-ice glacier, along with a thin atmosphere. Ceres may have brine moving below its surface, and Pluto might have a hidden ocean.

Similar objects

Many objects in space look like dwarf planets. Some of these are called former dwarf planets. They might have been round in the past but changed after big hits or moving into orbit around another planet. For example, Triton, a moon of Neptune, is thought to have been a dwarf planet before Neptune caught it.

Others are large moons that are big enough to be round, like dwarf planets, but they go around planets instead of the Sun. These are called planetary-mass moons. One example is Charon, the biggest moon of Pluto. Some people think Pluto and Charon together could be called a double dwarf planet, but officially Charon is just a moon of Pluto.

Former dwarf planets

Vesta, a large object in the asteroid belt, used to be round but is not anymore because of big hits. Triton, a moon of Neptune, is heavier than some dwarf planets and is thought to have been a dwarf planet before Neptune caught it. Phoebe, another moon of Neptune, was also round but is not anymore.

A monochrome mosaic of Triton, from images by Voyager 2. Triton is thought to be a captured dwarf planet.

Planetary-mass moons

Main article: Planetary-mass moon

Some moons are big enough to be round like dwarf planets, even though they orbit planets. These are called planetary-mass moons. Seven of these moons are heavier than the dwarf planets Eris or Pluto. Some scientists call these "satellite planets". The word planemo means any object with a mass like a planet, whether it goes around the Sun or a planet.

Charon

There has been talk about whether the Pluto–Charon system should be called a double dwarf planet. At one time, scientists thought both could be planets together. But now, the group that decides these things says Charon is just a moon of Pluto. They might change this idea later. Right now, it is not clear if Charon is still round. Also, whether two objects count as a pair depends on their masses and how far apart they are.