Eris Facts

If you’re not sure why astronomers suddenly decided that Pluto wasn’t a real planet after all, read on! It’s all because of Eris. The discovery of this tiny, distant world sparked a debate about what was a planet, and the result was the creation of a whole new category – dwarf planets.


So Eris is quite an important object, and astronomers have already learned quite a lot about it. It’s too far away to see much for yourself though, so we’ve collected some Eris facts to help you out.

Eris Overview

Mass: 1.66×1022 kg

Equatorial Diameter: 2,326km

Equatorial Circumference: 7,307km

Known Moons: 1

Notable Moons: Dysnomia

Orbit Period: 558.04 years

Surface Temperature: -243°C to -217°C

Eris Facts

How far is Eris from the Sun?

It varies, because Eris has a highly elliptical orbit, but even at its closest the distance is a huge 37.911 AU. That’s 5,671,404,875km, or 3,544,628,046 miles. At the far end of Eris’s circuit that increases to 97.651 AU (14,608,381,670km, or 9,130,238,543 miles). From out there it takes more than a day for light from Eris to reach Earth.

Can Eris be seen from Earth?

If you have a good telescope it’s possible, but not easy. Although Eris is quite bright it’s far too small and far away to be seen with the naked eye or binoculars, and only larger amateur telescopes have any chance of picking it up.

 When was Eris discovered?

Eris was first identified in January 2005 by a team from California. The same team also discovered Haumea and Makemake, by taking repeated images of the Kuiper Belt then searching for objects that had moved between shots.

In fact, they’d taken pictures of Eris earlier, in 2003, but the software they used to find movements didn’t pick it up because it was moving across the sky too slowly. That’s not because it’s actually moving slowly – its average speed round its orbit is almost 3.5km per second – but because it was so far away.


Then the team found another dwarf planet that was moving just above the threshold of the software, and realised they could be missing things. When they lowered the threshold Eris was detected.

What is Eris made up of?

Astronomers believe that Eris is mostly made of rock, with a thick layer of ice on top. The size of Eris has been calculated from Hubble space telescope images, and its mass can be worked out by studying the orbit of its moon Dysnomia.

These two facts show that it’s denser than Pluto, so despite being slightly smaller it weighs around 27% more. That means it must have a higher ratio of rock to ice.

Rock contains small amounts radioactive elements, and when enough rock is gathered together that can produce quite a lot of heat. Eris’s mass suggests that the core might generate enough heat for an ocean of liquid water to exist under the ice. That would make it the most distant known object with liquid water.

 Could there be life on Eris?

It’s possible. The surface of Eris is covered in methane ice, and at its closest the world comes close enough to the Sun for that to evaporate. The fact it’s still there suggests that there’s an internal source of methane that steadily replenishes the surface, and biological activity could do that.

There are other possibilities though, and it doesn’t seem likely that there’s life on Eris. It’s just too small and too cold.


How long is a year on Eris?

Because Eris’s orbit is so far out and elliptical it takes a long time to go round it. A single round trip takes just over 558 years. On the other hand we don’t know how long a day lasts on Eris. The easiest way to work out how fast a distant object rotates is to measure how its brightness changes over time.

The amount of reflected light is affected by darker and lighter patches on the surface, so it varies as these patches turn to face the telescope. If the light brightens every 20 hours it’s safe to assume that the planet is spinning every 20 hours.

But Eris seems to be covered in a uniform layer of methane ice, so its brightness doesn’t change. The only way to find out how fast it spins is to send a probe there.

How long would it take to reach Eris?

A while. A spacecraft could reach Eris in 24.66 years if it used Jupiter’s gravity to accelerate it. But there aren’t many windows of time when Earth, Jupiter and Eris line up in the right way to do that. The next possible launch date will be in 2032, with another following in 2044.

 More Eris facts

  • When Eris was discovered it was described as the tenth planet – but it was soon demoted to the new category of dwarf planets.
  • The team that found it initially called it Xena, after the fictional warrior princess. Eris’s moon, Dysnomia, was named as a tribute to actress Lucy Lawless who plays Xena – Dysnomia was the Greek goddess of lawlessness.
  • Eris is the ninth most massive, and tenth largest, object orbiting the Sun.