Eris
Dwarf planet · Scattered disc
Eris is almost the same size as Pluto but more massive. It is currently about three times further away than Pluto, on a long, tilted orbit. Its discovery started the great debate about what a planet is.
- ~2,326 kmYou could line up 5 of them across Earth.
- ~559 yearsYear
- DysnomiaMoon
Take a closer look
Weighed by its moon
By timing how its moon Dysnomia circles it, astronomers measured that Eris is about 27% more massive than Pluto, even though the two are nearly the same width. That means Eris must be packed with more rock. Its discovery made scientists ask if it should be the tenth planet.
Shiny as fresh snow
Eris is one of the most reflective objects in the Solar System, bouncing back about 96% of the sunlight that hits it – whiter than fresh snow. Scientists think its thin air freezes into a fresh coat of frost on the ground while it is far from the Sun.
A tilted path
Most planets circle the Sun on a flat plane, like marbles rolling on a table. Eris’ orbit is tipped by about 44°, and it swings from about 38 to 98 times Earth’s distance from the Sun. Being so far away and so far off that flat plane, where most searches looked, helped it stay hidden until 2005.
Size it up
You could line up 5 of them across Earth. Both are drawn to the same scale.
How we found out
- 2005
Discovered by Mike Brown, Chad Trujillo and David Rabinowitz in images taken in 2003. Its nickname was “Xena”.
- 2006
Because Eris is about as big as Pluto, astronomers have to define the word “planet” – and Pluto and Eris both become dwarf planets.
- 2006
Named after the Greek goddess of strife and discord – fitting for an object that caused so much argument!
Are there more dwarf planets we haven’t found yet?
Almost certainly! The outer Solar System is huge and dark. Some astronomers even search for an undiscovered big planet far beyond Neptune.
See it yourself
Only with large telescopes.
From the encyclopedia
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