Eris
What is Eris?
Dwarf planets· Diameter 2,326 km. A dwarf planet slightly smaller than Pluto but heavier. Its discovery in 2005 started the debate over what a planet is, which led to Pluto's reclassification.
Orbit and size
| Diameter | 2,326 km |
|---|---|
| Distance | 38~97 AU |
| Year | 558 yr |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
How Dysnomia revealed the mass of Eris
The mass of a distant body cannot be weighed on a scale, so astronomers use a moon that orbits it. Knowing how far away the moon circles and how fast, they can calculate how heavy the central body is. In 2005 the Keck telescope in Hawaii found Eris's moon Dysnomia, which goes around Eris every 15.8 days.
The result showed Eris to be about 27 percent heavier than Pluto. Yet when Eris passed in front of a distant star in 2010, its measured width came out slightly smaller than Pluto's. Smaller but heavier means more tightly packed: Eris is about 2.5 times as dense as water, so it seems to contain more rock than Pluto.
Dysnomia is about 600 km across and very dark, reflecting only about 5 percent of sunlight. A study published in 2023 found that Eris spins exactly once for each orbit of Dysnomia. So, like Pluto and Charon, the two always show each other the same face.
The dwarf planet in depth
Eris is a dwarf planet announced in 2005 by Michael Brown's team. At 2,326 km across it is a little smaller than Pluto (2,377 km), yet it is about 27 percent more massive. Its discovery forced astronomers to choose: accept Eris as a tenth planet, or define the word 'planet' anew. In 2006 the International Astronomical Union chose a new definition, and the category of dwarf planet was born.
It is named after the Greek goddess of strife and discord. She threw a golden apple into a feast of the gods and so sowed the seeds of the Trojan War, a fitting name for a body that caused a great argument. Its moon, Dysnomia, is named after Eris's daughter, the goddess of lawlessness.
Eris follows a long orbit between 38 and 97 AU from the Sun, taking 558 years per lap. It is now about 96 AU away, so sunlight takes more than 13 hours to reach it. It is so cold there that its atmosphere is thought to lie frozen on the ground as frost. This methane frost makes Eris one of the most reflective bodies in the solar system.
Frequently asked
- How big is Eris?
- The diameter of Eris is 2,326 km.
- How far is Eris from Earth?
- The distance of Eris is 38~97 AU.
- In one line, what is Eris?
- Dwarf planets — A dwarf planet slightly smaller than Pluto but heavier.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.