Although Pluto is the most well-known dwarf planet, it is only one of five dwarf planets that are officially recognized by the International Astronomical Union (IAU). The other four dwarf planets are Eris, Ceres, Haumea and Makemake. Of these five dwarf planets, only Ceres lies in the asteroid belt, the region of celestial objects containing asteroids and minor planets between the orbits of Mars and Jupiter. The other four dwarf planets can be found in the Kuiper Belt, another region of celestial objects outside the orbit of Neptune.
Artist's Conception of Ceres © NASA
Ceres was discovered on January 1, 1801 by Giuseppe Piazzi, a Sicilian astronomer. It was initially classified as a planet, then later an asteroid. In 2006, it was reclassified as a dwarf planet, the only one in the asteroid belt so far. As an asteroid, it is considered the largest of its kind ever discovered. Although Ceres is only approximately 930 km across (580 mi), one-fourteenth the size of Pluto, it contains 25% of the total mass of all objects in the asteroid belt (9.47 x 1020 kg). This dwarf planet is believed to have a dense inner core, a thick mantle of water ice and a surface composed of water ice, carbonates and clays. Ceres has an orbit inclination of 10.6°and takes 4.6 years to orbit the Sun. It completes one rotation in 9h 4min.
Artist's Conception of Eris © NASA
Eris is the most massive of the five officially recognized dwarf planets. It was discovered in January 2005 by a team of astronomers at the Palomar Observatory in California led by Michael Brown. Its diameter is estimated to be 2326 km, roughly the same as Pluto's (2302 km). But its mass of 1.67 x 1022 kg is 27% greater than Pluto's mass. It completes a single orbit around the Sun in 560 years and completes one rotation in 25.9 hours. Eris is believed to have the same physical composition as Pluto: a surface of methane ice and a rocky inner core. Eris has a tiny moon called Dysnomia, which completes one orbit around it in 16 days.
Makemake is another dwarf planet in the Kuiper Belt. It was first observed by a team lead by Michael Brown at the Palomar Observatory in California in March 2005, but it was not until 2008 when the IAU officially recognized it as a dwarf planet. It completes one orbit around the Sun in 310 years, longer than Pluto's 248 years, and completes one rotation in 7.7 h. Its estimated diameter of 1,360–1,480 km makes it the third largest Kuiper Belt object after Eris and Pluto. Makemake is actually bright enough to be seen by a high-end amateur telescope. It is the second brightest Kuiper Belt object after Pluto. Little is known about the dwarf planet's physical composition but its surface is believed to be composed of frozen nitrogen, methane and ethane. It was the discovery of Eris and Makemake that led to the demotion of Pluto from planet to dwarf planet.
Haumea was first observed in March 2003 by astronomers at the Sierra Nevada Observatory in Spain and officially announced in 2005 with the temporary name 2003 EL61. It was not until September 2008 when it was designated as a dwarf planet. Haumea's ellipsoid shape makes it unique among the known dwarf planets. This slightly elongated shape is believed to be due to its extremely rapid rotation (4 h), which is believed to have been caused by a massive impact that resulted in Haumea being the largest surviving piece. Haumea completes one orbit around the Sun in 282 years. Based on its spectra and color, the dwarf planet's surface appears to be composed of crystalline water ice. Haumea has two moons: Hi'iaka and Namaka, both discovered in 2005. Hi'iaka, with a diameter of 310 km, is the brighter of the two and completes one orbit around the dwarf planet in 49 days. Namaka is smaller, one-tenth the mass of Hi'iaka, and closer to the dwarf planet. It completes one orbit around the dwarf planet in 18 days.