Pluto is the best-known dwarf planet, but it's not the only one. Here's a little tour of all five.
First stop: Ceres, the inner Solar System's only dwarf planet. Discovered in 1801, it was named it for a Roman agricultural goddess. In common with Pluto, Ceres was considered a planet for a long time – until it was obvious that it shared its orbit with many other small bodies.
Although Ceres is by far the biggest asteroid, it's the smallest dwarf planet, only a third the size of the Moon. It orbits at nearly 3 AU. The astronomical unit (AU) is the distance from the Earth to the Sun, so Ceres is almost three times as far away from the Sun as we are. The Cererian year is over four and a half Earth years, though the day is only nine hours long. Despite its distance from the Sun, the surface of Ceres can warm up to -38°C (-36°F), but the annual mean temperature is -100°C (-148°F).
Scientists think that Ceres has a rocky core surrounded by a layer of water ice, topped by a thin crust. Without the gravitational pull of Jupiter, which disrupted the process of planet formation in the asteroid belt, Ceres might well have become another rocky planet.
NASA's Dawn mission is currently studying Ceres.
Next stop: Pluto, the gateway to the Kuiper Belt. [Click for a diagram of the Solar System.] The Kuiper Belt is similar to the asteroid belt, but it's much bigger and has icy bodies instead of rocky ones. The eight planets orbit the Sun in the same plane, called the ecliptic, and in fairly circular orbits. But out beyond Neptune, objects often have highly elliptical orbits that cross the ecliptic at steep angles. [Click to see a comparison of the orbits of the planets and dwarf planets.]
Pluto takes 248 years to get around the Sun, at an average distance of nearly 40 AU, in an orbit that's tilted at 17 degrees to the ecliptic. Of course, it's much colder than Ceres, and even the maximum temperature is only -218°C (-360°F).
Pluto and its large moon Charon each keep the same side facing the other. Charon orbits Pluto in the same time it takes Pluto to rotate once, so a month and a day on Pluto are the same length - about six Earth days. Charon was the name of the ferryman who rowed the dead across the river Styx to Pluto's realm.
There are also four tiny moons. When the data from NASA's New Horizons arrives, we should know more about them.
We now move on to Haumea (how-MAY-uh) whose year is 282 Earth years long. It orbits the Sun at an average distance of 43 AU, but it originated far beyond the Kuiper Belt, out in the scattered disk. A collision broke it into smaller pieces, set it spinning, and sent it into the Kuiper Belt. In Hawaiian mythology, Haumea is the goddess of fertility and childbirth. Many children sprang from different parts of her body, rather like the dwarf planet's two moons and some other icy bodies that broke off it.
Haumea is the fastest-spinning body in the Solar System, taking less than four hours to rotate once. Planets that spin – including Earth – bulge at the equator, but Haumea's spin is so fast that its diameter at the equator is twice that at the poles. It's egg-shaped.
Oddly, Haumea seems to be covered by crystalline ice, which suggests a comparatively young surface. But we have no plausible idea about what could have produced the resurfacing. Haumea also has a mysterious red spot. We don't know what it is, what caused it or even how big or bright it is. That's because it was detected through infrared measurements, so we can't actually see it.
Next, we're off to Makemake (MAH-kay-MAH-kay), named for a fertility god of the Rapa Nui mythology of Easter Island. On average it's 46 AU from the Sun and the mean surface temperature has been estimated at -240°C (-400°F). It takes 310 years to orbit the Sun.
Makemake is the third largest dwarf planet, probably about three-quarters Pluto's size. It's also fairly bright so it could be seen in a really good amateur telescope. Ten years after Makemake's discovery, it was found to have a small dark moon. The moon hasn't yet been named.
Our final stop is Eris, so we must leave the Kuiper Belt and proceed to the scattered disk. Except for long-period comets, Eris is currently the most distant known natural object in the Solar System. Its average distance from the Sun is 68 AU, but it varies from 38 to 98 AU, and its orbit is tilted 44 degrees to the ecliptic. Although its day of 26 hours is close to an Earth day, a year is 558 Earth years.
Eris is nearly the same size as Pluto, only a tiny bit smaller. Although Eris and Pluto are similar in size, Eris is more massive. This means that it has a higher proportion of rock than Pluto, therefore a different history.
It was Eris's discovery that prompted a heated debate about what a planet was. In 2006 a decision by the International Astronomical Union left Pluto no longer the ninth planet and Eris failing to become the tenth planet. Eris was the Greek goddess of discord and strife, so it was a name both classical and appropriate. The dwarf planet's moon was named after a daughter of Eris, Dysnomia (lawlessness).
This ends our tour. Let's get back to Earth, so nicely near to the Sun. And if your weather ever seems chilly, just think of those dwarf planets!