50 Fun Facts About Eris

Eris holds which record among the dwarf planets?

It outweighs Pluto by about 27%, though Pluto is a hair wider.

Who led the team that discovered Eris?

Chad Trujillo and David Rabinowitz were his co-discoverers; Brown later wrote a book called How I Killed Pluto.

In what year was Eris discovered on archived images?

The images had been taken in October 2003 but its slow motion had been filtered out by the search software.

Which observatory's Samuel Oschin telescope took the images in which Eris was found?

The 1.2-metre Schmidt camera in California had already found Quaoar, Orcus and Sedna for the same team.

Why did the search software initially miss Eris in the 2003 images?

The code ignored anything slower than 1.5 arcseconds per hour; Sedna's discovery prompted a re-check by eye.

Because it seemed bigger than Pluto, NASA initially described Eris as what?

That claim forced the IAU to define 'planet' for the first time in 2006, and both bodies became dwarf planets.

What nickname did the discovery team use for Eris before it was formally named?

They had saved it for the first body they found bigger than Pluto; the moon became 'Gabrielle' after the sidekick.

Which TV series inspired the informal nickname for Eris?

Brown said they liked that it started with an X, as in Planet X, and that it 'sounds mythological'.

Eris is named after the Greek goddess of what?

Brown said the object had 'caused strife and discord' too, by triggering the fight over Pluto's status.

What was Eris's provisional designation before it got its name?

It kept the awkward label for an unusually long time while the IAU decided what a planet was.

Which name, winner of a New Scientist poll, could not be used because an asteroid already had it?

399 Persephone had taken it long before; 'Xena' came fourth in the same poll.

Brown initially wanted to call the object 'Lila', a Hindu concept pronounced like whose name?

He had put the name in a web address, then left it there by accident during the Haumea controversy.

What is the name of Eris's confirmed moon?

She was the goddess of lawlessness and daughter of Eris in Greek myth.

Eris's moon is named after a daughter of Eris who was the goddess of what?

Brown also liked its similarity to his wife's name, Diane; and Xena was played by Lucy Lawless.

The moon's name carries an accidental nod to which actress, who played Xena?

'Dysnomia' means lawlessness; Brown insists the pun was unintentional.

What did the discovery of the moon allow astronomers to calculate for the first time?

It came out 27% greater than Pluto's, settling the mass argument for good.

Eris is about how much more massive than Pluto?

Its density of 2.52 g/cm3 shows it must be mostly rock.

What is Eris's measured diameter?

A November 2010 stellar occultation showed it was slightly smaller than Pluto after all.

How was Eris's true size finally pinned down in 2010?

It was one of the most distant stellar occultations ever observed from Earth.

Which 2015 mission confirmed that Eris's old rival is slightly larger in volume after all?

For nearly a decade Eris had been thought the ninth-largest object orbiting the Sun.

How long does Eris take to orbit the Sun?

It reached aphelion around 1977 and will not be at perihelion again until December 2257.

Eris's orbit ranges from 38 AU in to how far out from the Sun?

At perihelion it actually comes inside Pluto's orbit, but it never nears Neptune.

Eris's orbit is tilted to the plane of the planets by roughly how much?

That steep tilt is why nobody found it earlier: surveys concentrate on the ecliptic.

Eris belongs to which population of trans-Neptunian objects?

Its eccentric, inclined orbit is the signature of a body flung outward by Neptune long ago.

In February 2016, how far was Eris from the Sun?

More than three times Neptune's distance; it was the most distant known natural object until 2018.

Eris's surface is unusual for a distant icy world in what way?

Its albedo of 0.96 makes it one of the most reflective bodies known, probably from freshly condensed methane frost.

Which ice did spectroscopy at Gemini North reveal on Eris's surface in 2005?

That made it similar to Pluto and Triton, then the only distant bodies known to have surface methane.

Roughly what is Eris's surface temperature range?

When it swings closer to the Sun some of its ices may sublime into a temporary atmosphere.

Eris and its moon form the second known example of what kind of system, after Pluto and Charon?

Eris's day equals Dysnomia's 15.78-day orbit, and each always shows the other the same face.

How long is a day on Eris?

It is locked to its moon's orbit; earlier estimates ranged from tens of hours to several days.

Which spacecraft observed Eris from afar in May 2020, from a vantage point inside the Kuiper belt?

At 112 AU it was farther from Eris than Earth was, but it saw the surface at angles impossible from home.

Eris is the largest known object in the Solar System that has never been what?

A flyby with a Jupiter gravity assist would take almost 25 years.

How long would a flyby mission to Eris take, according to 2010s studies?

Launch windows in 2032 or 2044 would find Eris about 90 AU out on arrival.

The discovery was announced on 29 July 2005, the same day as which other dwarf planet?

Haumea had been announced two days earlier, amid a controversy that hastened the Eris announcement.

NASA has used which symbol, borrowed from Discordianism, for the dwarf planet?

Discordianism is a tongue-in-cheek religion devoted to the goddess of chaos.

In Greek myth, what did the goddess Eris throw among the guests at the wedding of Peleus and Thetis?

Hera, Athena and Aphrodite each claimed it, leading to the Judgement of Paris and the Trojan War.

Why was the goddess Eris angry at the wedding of Peleus and Thetis?

The 'Sleeping Beauty' fairy who curses a princess after being left off the guest list echoes the same story.

The quarrel Eris started was settled by which famous mythological verdict?

Paris chose Aphrodite, won Helen, and started the Trojan War.

As of 2026, Eris lies in which constellation?

Its tilted orbit takes it through few zodiac constellations; it enters Pisces in 2036 and Aries in 2065.

Eris's apparent magnitude of about 18.7 means it can be detected with what?

It is faint, but not out of reach for a well-equipped amateur.

Models suggest Eris might hide what beneath its surface, warmed by radioactive decay?

The ocean, if real, would sit at the boundary between the icy mantle and rocky core.

In roughly 800 years, how does Eris's position change relative to its famous rival?

Its perihelion of 37.9 AU lies inside Pluto's orbit.

Where does Eris rank among the heaviest known objects orbiting the Sun directly?

Counting moons too, it is sixteenth-most massive in the Solar System.

The 2006 IAU planet definition left how many planets in the Solar System?

The same number as before Pluto's discovery in 1930.

Which telescope system with laser-guide-star adaptive optics found Eris's moon in September 2005?

The team surveyed the four brightest trans-Neptunian objects: Pluto, Makemake, Haumea and Eris.

The James Webb Space Telescope in 2022 detected which variety of ice on Eris?

Its low deuterium content hints its methane was made inside Eris rather than inherited from comets.

What is Eris's minor-planet number in the IAU catalogue?

The IAU added it to the Minor Planet Catalogue after its August 2006 planet definition made both Eris and Pluto dwarf planets.

Which two astronomers co-discovered Eris alongside Mike Brown?

They rushed out the announcement in July 2005 after a Spanish team controversially revealed Haumea, another large object Brown's group had been tracking.

What geometric albedo does Eris have, making it one of the most reflective bodies in the Solar System?

Fresh methane frost condensing evenly across the surface as the eccentric orbit swings closer to and farther from the Sun is thought to keep it so bright.

A 2025 study proposed a large unseen inner moon of Eris to explain what recurring signal?

Gaia data revealed the tiny periodic flicker, which the rotation of neither Eris nor Dysnomia can account for; the hypothetical moon would be roughly 1,000 km across.