Beyond the eight major planets that orbit our Sun, a new class of worlds has captured the attention of astronomers in recent years – dwarf planets. First officially recognized in 2006 by the International Astronomical Union (IAU), dwarf planets are celestial objects that are round and orbit the Sun, but have not cleared the neighborhood of their orbits like the larger planets have. To date, five dwarf planets have been identified, with many more likely awaiting discovery in the outer reaches of the solar system.

What Makes a Dwarf Planet?

The IAU established four criteria that an object must meet to be classified as a dwarf planet:

1. It must orbit the Sun
2. It must have enough mass to be rounded by its own gravity, giving it a nearly spherical shape
3. It must not have cleared the neighborhood of its orbit of other objects
4. It must not be a moon of another object

The first two criteria are shared with the larger planets. However, dwarf planets are too small to gravitationally dominate their orbital zones, so other objects like asteroids and comets can coexist in similar orbits. This is the key distinction between dwarf planets and regular planets.

The Five Known Dwarf Planets

Ceres

Ceres is the closest dwarf planet to the Sun and the only one located in the main asteroid belt between Mars and Jupiter. With a diameter of about 950 km, it is by far the largest object in the asteroid belt, containing about a third of the region’s total mass.

Ceres was the first asteroid discovered back in 1801, over a century before Pluto. It was considered a planet initially before being reclassified as an asteroid for many years. The Dawn spacecraft visited Ceres in 2015, revealing a dark, heavily cratered surface with mysterious bright spots, which scientists think are deposits of salt.

Pluto

Pluto is the most famous dwarf planet and the first to be visited by a spacecraft. Discovered in 1930, it was considered the ninth planet for over 75 years until the discovery of other similar-sized objects in its vicinity prompted the IAU to reconsider its planetary status.

With a diameter of 2372 km, Pluto is slightly smaller than Earth’s moon. It has a thin atmosphere and a complex surface with mountains, valleys, plains, and glaciers of nitrogen ice. Pluto’s largest moon Charon is big enough that Pluto and Charon are sometimes considered a double dwarf planet system. Pluto has four other smaller moons: Nix, Hydra, Kerberos and Styx.

In 2015, the New Horizons spacecraft made a historic flyby of Pluto, providing the first detailed images of this distant world and its moons. The data continues to revolutionize our understanding of Pluto.

Haumea

Haumea is one of the most unusual dwarf planets. With a diameter of about 1600 km on its longest axis, it has an elongated, football-like shape due to its rapid rotation – a day on Haumea lasts only 4 hours. This also makes it the least spherical of the dwarf planets.

Haumea has a very reflective surface of almost pure water ice, similar to the icy moons of the outer planets. It has two small moons, Hi’iaka and Namaka, named after Hawaiian goddesses. Haumea is located in the Kuiper Belt, a region of icy bodies beyond Neptune’s orbit.

Makemake

Makemake is another Kuiper Belt dwarf planet, about three quarters the size of Pluto. It has a reddish surface that is thought to be covered in frozen methane, similar to Pluto. However, unlike Pluto and Eris, Makemake does not appear to have an atmosphere.

Discovered in 2005, Makemake is named after the creator god of the Rapa Nui people of Easter Island. It has one known moon, nicknamed MK2. Because of its great distance and relatively small size, many of Makemake’s properties remain unknown.

Eris

Eris is the most massive dwarf planet, about 27% more massive than Pluto, although Pluto is slightly larger by volume. Its discovery in 2005 is what prompted the IAU to create the new “dwarf planet” category and reassess Pluto’s status.

Eris has a highly eccentric orbit that takes it up to 97 times farther from the Sun than Earth. At its most distant, it is the farthest known object in the solar system. Like Pluto, Eris has a thin atmosphere and a surface of nitrogen and methane ices. It has one known moon, Dysnomia.

Potential Dwarf Planets

Astronomers believe there could be hundreds or even thousands more dwarf planets awaiting discovery, especially in the Kuiper Belt and beyond. Some leading candidates include:

• Quaoar: A Kuiper Belt object about half the size of Pluto, with one known moon. It has a very dense rocky core.
• Sedna: A distant object with an extremely elongated orbit that takes about 11,400 years to complete. Its surface is one of the reddest in the solar system.
• Orcus: Sometimes called the “anti-Pluto” because it has a similar size and orbit but is opposite Pluto in the sky. It has one known moon, Vanth.
• Gonggong: A large, reddish object in the outer Kuiper Belt, with a suspected water ice surface and one moon.

As telescope technology improves, more of these distant, enigmatic worlds are likely to be found, providing new insights into the formation and evolution of our solar system.

Exploring Dwarf Planets

Due to their small size and distant orbits, dwarf planets are challenging to study from Earth. Space missions provide the best opportunity to learn about their properties and environments up close.

NASA’s New Horizons mission provided the first detailed look at Pluto and its moons. The spacecraft is now venturing deeper into the Kuiper Belt, and made a flyby of the smaller Kuiper Belt object Arrokoth in 2019.

The Dawn spacecraft orbited Ceres for over 3 years, mapping its surface and investigating its composition and internal structure before running out of fuel in 2018.

Future missions to explore dwarf planets could include orbiters, landers, or even sample return missions to bring back material for study on Earth. Such ambitious projects would greatly expand our knowledge of these intriguing worlds on the solar system’s frontier.

Summary

The study of dwarf planets is a relatively new field that has already yielded surprising discoveries and reshaped our understanding of the solar system. These small but fascinating worlds bridge the gap between planets and smaller bodies like asteroids and comets.