Dwarf Planets

Illustration showing dwarf planet sizes compared to Earth.

An artist's concept showing the size of the best known dwarf planets compared to Earth and its moon (top). Eris is left center; Ceres is the small body to its right and Pluto and its moon Charon are at the bottom.

Dwarf planets are round and orbit the Sun just like the eight major planets. But unlike planets, dwarf planets are not able to clear their orbital path so there are similar objects at roughly the same distance from the Sun. A dwarf planet is much smaller than a planet (smaller even than Earth's moon), but it is not a moon. Pluto is the best known of the dwarf planets.

10 Need-to-Know Things About Dwarf Planets:

  1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planets Pluto and Eris, for would each be about the size of the head of a pin.
  2. Dwarf planets orbit our sun, a star. Most are located in the Kuiper Belt, a region of icy objects beyond the orbit of Neptune. Pluto, one of the largest and most famous dwarf planets, is about 5.9 billion km (3.7 billion miles) or 39.48 AU away from the sun. Dwarf planet Ceres is in the main asteroid belt between Mars and Jupiter.
  3. Days and years vary on dwarf planets. One day on Ceres, for example, takes about nine hours (the time it takes for Ceres to rotate or spin once). Ceres makes a complete orbit around the sun (a year in Ceresian time) in about 4.60 Earth years.
  4. Dwarf planets are solid rocky and/or icy bodies, The amount rock vs. ice depends on their location in the solar system.
  5. Many, but not all dwarf planets have moons.
  6. There are no known rings around dwarf planet.
  7. Dwarf planets Pluto and Eris, for example, have tenuous (thin) atmospheres that expand when they come closer to the sun and collapse as they move farther away.
  8. The first mission to a dwarf planet is Dawn (to Ceres).
  9. Dwarf planets cannot support life as we know it.
  10. Pluto was considered a planet until 2006. The discovery of a similar-sized worlds deeper in the distant Kuiper Belt sparked a debate that resulted in a new official definition of a planet that did not include Pluto.

Dwarf Planets: Overview

What is a planet? We've been asking that question at least since Greek astronomers came up with the word to describe the bright points of light that seemed to wander among fixed stars. Our solar system's planet count has soared as high as 15 before it was decided that some discoveries were different and should be called asteroids.

Many disagreed in 1930 when Pluto was added as our solar system's ninth planet. The debate flared again in 2005 when Eris -- about the same size as Pluto -- was found deep in a zone beyond Neptune called the Kuiper Belt. Was it the 10th planet? Or are Eris and Pluto examples of an intriguing, new kind of world?

The International Astronomical Union decided in 2006 that a new system of classification was needed to describe these new worlds, which are more developed than asteroids, but different than the known planets. Pluto, Eris and the asteroid Ceres became the first dwarf planets. Unlike planets, dwarf planets lack the gravitational muscle to sweep up or scatter objects near their orbits. They end up orbiting the sun in zones of similar objects such as the asteroid and Kuiper belts.

Our solar system's planet count now stands at eight. But the lively debate continues as we continue to explore and make new discoveries.

Diagram of the solar system showing Pluto and Eris' eccentric orbits.
Scientists expect to find more dwarf planets beyond the orbit of Neptune.
Known Dwarf Planets

1. Pluto
2. Ceres
3. Makemake
4. Haumea
5. Eris

List of Dwarf Planets

According to the International Astronomical Union, which sets definitions for planetary science, a dwarf planet is a celestial body that:

  • Orbits the sun.
  • Has enough mass to assume a nearly round shape.
  • Has not cleared the neighborhood around its orbit.
  • Is not a moon.

The main distinction between a dwarf planet and a planet is that planets have cleared the path around the sun while dwarf planets tend to orbit in zones of similar objects that can cross their path around the sun, such as the asteroid and Kuiper belts. Dwarf planets also are generally smaller than the planet Mercury.

The first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens or even more than 100 dwarf planets awaiting discovery.

Dwarf Planets: Featured Mission - Dawn

Goals: Dawn is designed to study the conditions and processes of the solar system's earliest epoch by investigating in detail two of the largest protoplanets remaining intact since their formations. The orbiter will visit both the asteroid Vesta and dwarf planet Ceres, two main asteroid belt worlds that followed very differently evolutionary paths.

Accomplishments: Dawn comprehensively mapped asteroid Vesta, revealing an exotic and diverse planetary building block. The findings are helping scientists unlock some of the secrets of how the solar system, including our own Earth, was formed. It is now en route to dwarf planet Ceres.



Color image of Pluto and its Moons.
Recent images returned from the New Horizons spacecraft as it nears its historic July 14 encounter with Pluto.

Pluto: Overview

Discovered in 1930, Pluto was long considered our solar system's ninth planet. But after the discovery of similar intriguing worlds deeper in the distant Kuiper Belt, icy Pluto was reclassified as a dwarf planet. This new class of worlds may offer some of the best evidence about the origins of our solar system.

10 Need-to-Know Things About Pluto:

  1. If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planet Pluto would be about the size of the head of a pin.
  2. Pluto orbits our sun, a star, at an average distance of 3.7 billion miles (5.9 billion kilometers) or 39.5 AU.
  3. One day on Pluto takes about 153 hours. That's the time it takes for Pluto to rotate or spin once. Pluto makes a complete orbit around the sun (a year in Plutonian time) in about 248 Earth years.
  4. It is thought that Pluto has a rocky core surrounded by a mantle of water ice with other ices coating its surface.
  5. Pluto has five known moons. Pluto is sometimes called a double-planet system due to the fact that its moon Charon is quite large and orbits close to its parent planet.
  6. There are no known rings around Pluto.
  7. Pluto has a thin, tenuous atmosphere that expands when it comes closer to the sun and collapses as it moves farther away -- similar to a comet.
  8. NASA's New Horizons spacecraft is the first mission sent to encounter the Pluto-system and other members of the Kuiper Belt.
  9. Scientists do not think Pluto can support life as we know it. Although, some scientists believe it is possible Pluto could possess a hidden ocean under its surface.
  10. Pluto was considered a planet from 1930, when it was first discovered, until 2006. The discovery of similar-sized worlds deeper in the distant Kuiper Belt sparked a debate which resulted in a new official definition of a planet. The new definition did not include Pluto.

Pluto: Read More

Pluto is classified as a dwarf planet and is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system. These icy, rocky bodies are called Kuiper Belt objects or transneptunian objects.

Pluto is about two-thirds the diameter of Earth's Moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's Moon. Pluto is more massive than Ceres - the dwarf planet that resides in the asteroid belt between Mars and Jupiter - by a factor of 20.

Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the Sun. (One AU is the mean distance between Earth and the Sun: about 150 million km or 93 million miles.) From 1979 to 1999, Pluto was actually closer to the Sun than Neptune, and in 1989, Pluto came to within 29.7 AU of the Sun, providing rare opportunities to study this small, cold, distant world.

Pluto has a thin atmosphere with two distinct layers. Pluto's low gravity (about 6 percent of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere. Pluto becomes much colder during the part of each orbit when it is traveling far away from the sun. Earlier expectations that during this time the bulk of the planet's atmosphere would freeze are not withstanding observations made as Pluto moves out from the sun.

Pluto has a very large moon that is almost half its size named Charon, which was discovered in 1978. This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 19,591 km (12,173 miles).

The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is grayer than Pluto (which appears to have a slightly red hue), indicating that they have different surface compositions and structures.

Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets, but hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto -- this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus.

It isn't known whether Pluto has a magnetic field, but its small size, low density and slow rotation make one unlikely.

Because Pluto and Charon are so small and far away, they are extremely difficult to observe from Earth. In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years. Observations of these rare events allowed astronomers to make rudimentary maps of each body showing areas of relative brightness and darkness.

In 2005, scientists photographing Pluto with the Hubble Space Telescope in preparation for the New Horizons mission found two tiny moons orbiting in the same plane as Charon. These two moons, named Nix and Hydra, are two to three times farther away from Pluto than Charon.

n 2011 and 2012, scientists used Hubble to spot two more moons (originally designated P4 and P5). In 2013, the two moons were named Kerberos (P4) and Styx (P5).

How Pluto Got its Name

Pluto is the only world named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god the underworld. He forwarded the name to the Lowell Observatory and it was selected. Pluto's moons are named for other mythological figures associated with the underworld. Charon is named for the river Styx boatman who ferries souls in the underworld; Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology; and Styx is named for the mythological river that separates the world of the living from the realm of the dead.

Pluto's place in mythology can get a little muddled, so we asked Dr. Elizabeth Vandiver, chair of the Department of Classics in Whitman College in Walla Walla, Washington, to clarify the origins of the name: "Pluto is the name of the Roman god of the Underworld, equivalent to the Greek Hades. However, the Greek name "Plouton" (from which the Romans derived their name "Pluto") was also occasionally used as an alternative name for Hades. But Pluto is definitely the Roman spelling."

The symbol for dwarf planet Pluto is a monogram made up of P and L in Pluto (and also the initials of Percival Lowell, who predicted its discovery).

Pluto: Significant Dates

  • 1930: Clyde Tombaugh discovers Pluto.
  • 1977-1999: Pluto's lopsided orbit brings it slightly closer to the sun than Neptune. It will be at least 230 years before Pluto moves inward of Neptune's orbit for 20 years.
  • 1978: James Christy and Robert Harrington discover Pluto's unusually large moon, Charon.
  • 1988: Astronomers find Pluto has an atmosphere.
  • 2005: Scientists using the Hubble Space Telescope announce the discovery of two more moons. Named Nix and Hydra, the little moons may have formed at the same time as Charon did, perhaps all three splitting off from Pluto after a giant impact.
  • 2006: NASA's New Horizons mission launches on a mission to explore Pluto and the Kuiper Belt region beyond Neptune. The journey to Pluto will take more than nine years.
  • 2006: After the discovery of similar worlds in the Kuiper Belt, the International Astronomical Union reclassifies Pluto from planet to dwarf planet.
  • 2011-2012: Hubble observations reveal a fourth and fifth moon orbiting Pluto. They are named Kerberos and Nix.
  • 2015: The New Horizons mission team begins Pluto observations as the spacecraft approaches the dwarf planet. Closest approach is 14 July 2015.

Pluto: Featured Mission - New Horizons

Goals: NASA's New Horizons spacecraft is designed to make the first close-up study of Pluto and its moons and other icy worlds in the distant Kuiper Belt. The spacecraft has seven scientific instruments to study the atmospheres, surfaces, interiors and intriguing environments of Pluto and its distant neighbors.

Accomplishments: The spacecraft is still en route to its primary science target.


Kuiper Belt & Oort Cloud

Artist's concept of Eris and its moon. The sun is in the distance.
Artist's concept of Eris and its moon. The sun is in the distance. Image credit: Robert Hurt (IPAC).

The Kuiper Belt is a disc-shaped region of icy objects beyond the orbit of Neptune -- billions of kilometers from our sun. Pluto and Eris are the best known of these icy worlds. There may be hundreds more of these ice dwarfs out there. The Kuiper Belt and even more distant Oort Cloud are believed to be the home of comets that orbit our sun.

Kuiper Belt & Oort Cloud: Overview

  1. The Kuiper Belt and the Oort Cloud are regions of space. The known icy worlds and comets in both regions are much smaller than Earth's moon.
  2. The Kuiper Belt and the Oort Cloud surround our sun, a star. The Kuiper Belt is a doughnut-shaped ring, extending just beyond the orbit of Neptune from about 30 to 55 AU. The Oort Cloud is a spherical shell, occupying space at a distance between five thousand and 100 thousand AU.
  3. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. Short-period comets (which take less than 200 years to orbit the Sun) originate in the Kuiper Belt.
  4. There may be are hundreds of thousands of icy bodies larger than 100 km (62 miles) and an estimated trillion or more comets within the Kuiper Belt. The Oort Cloud may contain more than a trillion icy bodies.
  5. Some dwarf planets within the Kuiper Belt have thin atmospheres that collapse when their orbit carries them farthest from the sun.
  6. Several dwarf planets in the Kuiper Belt have tiny moons.
  7. The are no known rings around worlds in either region of space.
  8. The first mission to the Kuiper Belt is New Horizons. New Horizons will reach Pluto in 2015.
  9. Neither region of space is capable of supporting life as we know it.
  10. Both the Kuiper Belt and the Oort Cloud are named for the astronomers who predicted their existence during the 1950s: Gerard Kuiper and Jan Oort.

Kuiper Belt & Oort Cloud: Read More

In 1950, Dutch astronomer Jan Oort proposed that certain comets come from a vast, extremely distant, spherical shell of icy bodies surrounding the solar system. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the sun: about 150 million km or 93 million miles.) The outer extent of the Oort Cloud is believed to be in the region of space where the sun's gravitational influence is weaker than the influence of nearby stars.

The Oort Cloud probably contains 0.1 to 2 trillion icy bodies in solar orbit. Occasionally, giant molecular clouds, stars passing nearby, or tidal interactions with the Milky Way's disc disturb the orbits of some of these bodies in the outer region of the Oort Cloud, causing the object to fall into the inner solar system as a so-called long-period comet. These comets have very large, eccentric orbits and take thousands of years to circle the sun. In recorded history, they are observed in the inner solar system only once.

In contrast, short-period comets take less than 200 years to orbit the sun and they travel approximately in the plane in which most of the planets orbit. They are presumed to come from a disc-shaped region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper. (It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth.) The objects in the Oort Cloud and in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago.

The Kuiper Belt extends from about 30 to 55 AU and is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets.

Color image of clouds on Neptune.
The discovery of Eris -- which is similar in size to Pluto -- caused scientists to reconsider the definition of a planet.
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In 1992, astronomers detected a faint speck of light from an object about 42 AU from the sun -- the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,300 KBOs have been identified since 1992. (They are sometimes called Edgeworth-Kuiper Belt objects, and they are sometimes called transneptunian objects or TNOs for short.)

Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes.

One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons -- Hi'iaka and Namaka -- make up the family.

In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system. The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU -- instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud.

In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.

The discovery of Eris -- orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) -- forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category.

While no spacecraft has yet traveled to the Kuiper Belt, NASA's New Horizons spacecraft is scheduled to arrive at Pluto in 2015. The New Horizons mission team hopes to study one or more KBOs after its Pluto mission is complete.

How the Kuiper Belt and Oort Cloud Got Their Names

Both distant regions are named for the astronomers who predicted their existence -- Gerard Kuiper and Jan Oort. Objects discovered in the Kuiper Belt get their names from diverse mythologies. Eris is named for the Greek goddess of discord and strife. Haumea is named for a Hawaiian goddess of fertility and childbirth. Comets from both regions are generally named for the person who discovered them.

Significant Dates
  • 1943: Astronomer Kenneth Edgeworth suggests that a reservoir of comets and larger bodies resides beyond the planets.
  • 1950: Astronomer Jan Oort theorizes that a vast population of comets may exist in a huge cloud on the distant edges of our solar system.
  • 1951: Astronomer Gerard Kuiper predicts the existence of a belt of icy objects just beyond the orbit of Neptune.
  • 1992: After five years of searching, astronomers David Jewitt and Jane Luu discover the first KBO, 1992QB1.
  • 2002: Scientists using the 48-inch Oschin telescope at Palomar Observatory find Quaoar, the first large KBO hundreds of kilometers in diameter. This object was photographed in 1980, but was not noticed in those images.
  • 2004: Astronomers using the 48-inch Oschin telescope announce the discovery of Sedna (2003VB12).
  • 2005: Astronomers announce the discovery of 2003UB313. This object, later named Eris, is slightly larger than Pluto.
  • 2008: The Kuiper Belt object provisionally known as 2005FY9 ("Easterbunny") is recognized in July as a dwarf planet and named Makemake (pronounced MAHkeh-MAHkeh) after the Polynesian (Rapa Nui) creation god. In September, 2003EL61 ("Santa") was designated a dwarf planet and given the name Haumea after the Hawaiian goddess of fertility and childbirth.