The term dwarf planet was adopted in 2006 by the International Astronomical Union (IAU), the organization that assigns designations to celestial bodies. With continually improving technologies and more Sun-orbiting bodies of all sizes being discovered all the time, how to designate them is a matter of great scientific debate. The only thing certain is that not all scientists agree.
The definition for planet that was adopted by the IAU is a celestial body that 1) orbits the Sun, 2) is nearly round in shape, 3) has cleared the neighborhood around its orbit, and 4) is not a satellite. Objects that are not massive enough to be rounded by their own gravity are defined as small solar system objects. Dwarf planets fall in-between.
The IAU currently recognizes five dwarf planets—Pluto, Eris, Ceres, Makemake, and Haumea. It is suspected that 40 more known solar system objects meet current the criteria for dwarf planets. When the entire Kuiper belt region is explored, there may be 200 more. If objects beyond the Kuiper belt are considered, there could be 2,000. Stay tuned – the definition may change again as we continue to better understand our solar system.
The upper and lower size and mass limits of dwarf planets have not been specific by the IAU.
Ceres is the dwarf planet closest to the Sun. It resides in the asteroid belt and orbits the Sun every 4.6 years. Eris is farthest from the Sun, located in the distant icy region called the scattered disk. It takes 557 years to orbit the Sun. The three remaining dwarfs are in the Kuiper belt, the massive region beyond Neptune that consists mainly of leftovers from the solar system's formation.
Photo: Three dwarf planets orbit in the Kuiper belt, the vast region between beyond Neptune's orbit located from 20 and 55 AUs (one AU is the distance from the Earth to the Sun, or 93 million miles).
An asteroid belt object such as Ceres is expected to be made of mainly rock and metal, while bodies from the Kuiper belt and beyond are composed largely of frozen ices of methane, ammonia, and water.
Photo: Keck telescope image of Haumea and its two moons, Hi'iaka above and Namaka below. Haumea is thought to be composed almost entirely of solid rock, without the thick ice mantle over a small rocky core typical of other known Kuiper belt objects.
- Pluto and Eris - Size Versus Matter
The International Astronomical Union currently recognizes five dwarf planets—Pluto, Eris, Ceres, Makemake, and Haumea. For Pluto it was a demotion from planet status, but for Ceres it was an upgrade in prominence. However, new observations suggest Pluto may regain its status as the largest known object orbiting beyond Neptune. Eris had been thought to be larger than Pluto but new data from astronomers in Chile disputes that.
Photo: A white arrow marks Pluto in this picture taken from the New Horizons spacecraft on Sept. 21, 2006. Seen at a distance of about 2.6 billion miles, Pluto is little more than a faint point of light among a dense field of stars. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
After its discovery in 2005, calculations from the Hubble and Spitzer space telescopes pegged Eris' width at about 1,491 miles. The new observations from three teams that watched through telescopes as Eris passed in front of a distant star and then measured precisely timed occulations show Eris is smaller than originally thought. The numbers are close enough for a photo-finish: Pluto is 1,455 miles wide to Eris' 1,454 miles. But the biggest surprise is the difference in their densities. Astronomers believe Eris is about 25 percent more massive than Pluto, which means it is composed of entirely different materials. Scientists expected Eris and Pluto to be similar in composition, almost twins, rather than totally different objects.
As always, this finding raises more questions. Why are they made of different stuff? Their surfaces appear to be similar, but Eris probably contains more rock and less ice. Did Eris form closer to the Sun? Did something else in the distant past occur to one of them, maybe more cosmic collisions? This new finding is changing scientists views about the outer solar system. When the New Horizons spacecraft arrives at Pluto for a close flyby in 2015, we will have a much better understanding of the Kuiper belt neighborhood and its frigid residents.
Photo: Eris and its moon Dysnomia, imaged by the Hubble Space Telescope
Photo: Artist's rendering of the dwarf planet Eris, which is more massive than Pluto but likely not as large.