Dwarf planets and candidate dwarf planets [Science Photo Lab]

Pluto is the best known dwarf planet, but it's not the only one. Here's a little tour of the five recognized by the International Astronomical Union (IAU).

Ceres
First stop: Ceres, the inner Solar System's only dwarf planet. Discovered in 1801, it was named for a Roman agricultural goddess. As later 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 farther away from the Sun than 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 studied Ceres from March 2015 to June 2017.

Pluto
Next stop: Pluto, the gateway to the Kuiper Belt. Here is a diagram of the Solar System out to the Kuiper Belt. The Kuiper Belt is similar to the asteroid belt, but it's much bigger, and it 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 orbit 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 orbit a common center of gravity, each keeping the same side facing the other. Pluto rotates once in the time it takes Charon to make one orbit, so a month and a day on Pluto are the same length – about six Earth days.

There are also four tiny moons. We've learned something about them from NASA's New Horizons, but the data still leaves plenty of room for speculation.

Haumea
We now move on to Haumea (how-MAY-uh) whose year is 284 Earth years long. It orbits the Sun at an average distance of 43 AU. However 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 large 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 no one has worked out what could have produced the resurfacing. Haumea also has a red spot that's a mystery. 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.

Makemake
Next, we're off to Makemake (MAH-kay-MAH-kay), named for a fertility god in 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 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.

Eris
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 one of the most distant known natural objects 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, just 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 (IAU) left Pluto no longer the ninth planet, and meant that Eris failed 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 and be grateful!