As Ceres reaches opposition this week, back garden telescopes will be able to catch it in the constellation of Aquarius! But how do dwarf planets like Ceres fit into our picture of the solar system? Although not as big or flashy as the larger planets, dwarf planets have a lot to offer the world of science. As recent findings show, Ceres could even hold clues to extra-terrestrial life.
What is a dwarf planet?
Dwarf planets are typically much smaller than planets, but this isn’t actually how they are classified. Firstly, to claim the word planet at all, an object needs to be rounded in shape. This includes both planets and dwarf planets. A lot of smaller objects don’t have enough gravity to pull their material into this shape, so are classed as asteroids instead.
So how do we differentiate between planets and dwarf planets? A planet is able to do something called clearing its orbit. This means that over time, the planet has managed to carve a path through all the debris of space, leaving no similar objects at roughly the same distance from the Sun. It has a nice, clear orbit to move through on its own. Dwarf planets can’t do this; they have to share their orbit with other objects.
There are currently 5 official dwarf planets in our solar system. Ceres is situated in the Asteroid Belt between Mars and Jupiter. The other four, Pluto, Eris, Haumea and Makemake, are all what’s known as Trans Neptunian Objects or TNOs. That’s basically a fancy way of saying they are situated beyond the orbit of Neptune, the furthest planet from the Sun.
Ceres is the closest dwarf planet to us. It shares the Asteroid Belt with lots of smaller objects and debris, which are leftover from planet formation in the early solar system. Despite this, Ceres contains 25% of the total mass of the Asteroid Belt combined – and it’s 1/3 the size of the Earth. It was discovered in 1801 by Giuseppe Piazzi, the first indication of the Asteroid Belt. Then in 2006, it was classified as a dwarf planet, and in 2015 it became the first dwarf planet to be visited by a spacecraft.
Perhaps the most beloved of the dwarf planets, Pluto was discovered by Clyde Tombaugh in 1930. It is 1/6 the size of the Earth, and was long considered a planet. However, when similar worlds were discovered beyond Neptune, the planetary criteria was set down and in 2006, Pluto was reclassified as a dwarf planet. 5 moons orbit Pluto: Charon, Hydra, Kerberos, Nix and Styx. Along with Charon, Pluto is sometimes referred to as a ‘double planet’, because Charon is large enough to have a noticeable gravitational effect on the dwarf planet.
Discovered in 2003 by M.E. Brown, C.A. Trujillo, and D. Rabinowitz, Eris is about the same size as Pluto. However it orbits the Sun 3 times further out. This means it takes sunlight 9 hours to travel from the Sun to Eris – compared with the 8 minutes it takes to get to the Earth! Although its day is of a similar length to ours, it takes 557 Earth years to orbit the Sun once. You wouldn’t get many birthdays on Eris! Eris has one moon, known as Dysnomia.
Makemake is the second brightest object in the Kuiper Belt as seen from Earth, after Pluto. It’s only a little smaller than Pluto, and was discovered in 2005 by M.E. Brown, C.A. Trujillo, and D. Rabinowitz. Appearing reddish-brown in colour, Makemake is too far away to make out details from afar. Its orbit takes 305 Earth years to complete. Makemake has 1 provisional moon, but this discovery has yet to be confirmed.
Haumea is the furthest from the Sun of the 5 dwarf planets. Discovered in 2003, its oddly elliptical shape sets it apart from the others. It looks like that because it is one of the fastest spinning large objects in the solar system, so gets distorted by its own motion! Haumea is just under 10 times smaller than the Earth, and despite its tiny size, has 2 moons: Namaka and Hi’iake. They may have broken off from Haumea itself due to a massive impact billions of years ago.
Have you got a favourite dwarf planet?