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Eris is the largest and farthest away of the three known dwarf planets. It is larger than even Pluto, though not by much. It was discovered in 2005 when it was named 2003 UB313. The discoverers called it Xena, after a television warrior princess.

Eris is sometimes closer to the Sun than Pluto but most of the time it is much farther away. It takes 560 years for Eris to go round the Sun. Where Pluto is 30 to 49 times farther away from the Sun than the Earth, Xena is 38 to 98 times farther away.

Eris has a moon named Dysnomia.

Unlike the Sun, Moon and ordinary planets it does not follow the Zodiac when it crosses the sky. Even Pluto mostly follows the Zodiac.

The farther away a planet is, the slower it moves, which makes it harder to find. Although anyone with a good telescope who knows where to look can see it, Eris would never have been discovered without using cameras and computers.

The picture of Eris was taken by a telescope on top of a mountain in California in 2003. But it took nearly a year and a half before a computer spotted it as a possible planet. It almost missed it.

Ten computers at Caltech look through countless pictures of the night sky for anything moving against the background of stars. Most pictures turn up nothing. But a few do: astronomers then check these out for themselves.

Eris was discovered by Mike Brown, Chad Trujillo and David Rabinowitz on January 5th 2005 at 17:20 UTC.

They named it Xena, not after some forgotten goddess (that came later), but after a character from a television show about a warrior princess. They had always wanted to name something Xena.

They regarded it as a planet and so did NASA, the American government’s space department. So for a while it was known as the tenth planet in certain circles.

In other circles, however, some argued that Xena and even Pluto were not true planets at all: they were too small – smaller than the Moon even. In the 1990s all sorts of things – lost moons, would-be comets and so on – were found in the same region of space beyond Neptune.

To settle the argument, astronomers from all over the world met in 2006 and came up with the definition of what a planet is. In the end Pluto and Eris did not make the cut: now they were mere “dwarf planets”. While they are large enough to be round (the “planet” part) they were too small to remove other smaller bodies from their orbits (the “dwarf” bit).

It was because of all this strife that Xena got the name of Eris, the Greek goddess of strife, also known as Discordia. She was the daughter of Night (Nyx) and the sister of the god of war (Mars). She had a daughter named Dysnomia.

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On Tuesday Pluto was the ninth planet and now it is not. Pluto is two-thirds the size of the moon and takes almost 248 years to circle the sun. It is 40 times farther from the sun than the earth. Discovered in 1930, it was long considered to be the ninth planet till 2006.

In 2006 astronomers from all over the world met and came to the decision that Pluto is a mere dwarf planet: it is big enough to be round (the planet part) but not big enough to clear other bodies out of its orbit about the sun (the dwarf part).

In the 1990s astronomers found out that Pluto is just one of many other such bodies just beyond Neptune. The region is now known as the Kuiper belt. Pluto is in the middle of it and many other smaller bodies have been discovered there. In 2006 one of them, Eris (then known as Xena), was found to be larger than Pluto! The Kuiper belt is full of huge pieces of rock and ice that never managed to form themselves into a true planet, but sometimes they become comets.

Even when it was considered a planet it seemed more like a lost moon of Neptune than a true planet: it seemed very much like Triton, it is sometimes closer to the sun than Neptune, the eighth planet, and it does not always travel along the Zodiac like the other planets.

Pluto is so far away that light from the sun takes five hours to get there!

Pluto is so cold that our air would become snow there. Yet Pluto has seasons, it has very thin air and it is not completely covered by snow and ice: it is partly dark red – no one knows why. Parts of Pluto are grey, and, because of the thin clouds, parts are yellow and pink.

Most of Pluto is covered with what looks like large pieces of
, very pretty. That is nitrogen ice. On Earth most of what we breathe is nitrogen!

You cannot see Pluto with the naked eye in Earth’s night sky, even if you know where to look. In fact, even with telescopes, no one would have found it before the invention of photography. It was found almost by accident by comparing pictures of the same piece of the night sky taken days apart.

Pluto has at least three moons: Charon, which is half the size of Pluto and very close, and two much smaller moons discovered in 2005, Nix and Hydra. Because it is so close, Charon looks very large in the sky, though half of Pluto never sees it.

At its closest Pluto is 30 AUs from the sun (one AU is the distance between the earth and the sun), as it was in 1989, and at its farthest it is 50, as it will be in 2113. By then the sun will be three times fainter than it is now.

In 2006 the Americans sent a machine to Pluto, New Horizons. It should fly by Pluto in July 2015 (and start taking pictures that January).

– Abagond, 2006.

Update (July 14th 2015): Two more moons have been discovered: Styx and Kerberos. New Horizons is about to fly by Pluto! Check that post for updates. 

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