50 Fun Facts About Eris

Eris holds which record among the known dwarf planets?

Most massive. Eris outweighs Pluto by about 27%, yet the two are almost exactly the same width, so Eris must be much the denser world.

Which asteroid-belt world was named a dwarf planet alongside Eris and Pluto in 2006?

Ceres. It had been here before: announced as a new planet in 1801, it was demoted to an asteroid as similar bodies kept turning up, then promoted again in 2006.

Which distant object's discovery prompted the re-check of old images that revealed Eris?

Sedna. Finding a world that far out convinced the team that others might be hiding in pictures they had already taken, and a second look turned up Eris on frames from October 2003.

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

Palomar. Its survey photographed each patch of sky three times over three hours, and anything that shifted against the background stars was flagged as a possible new world.

Why did search software miss Eris in the 2003 images?

It moved too slowly across the sky. To cut false alarms, the software ignored anything drifting under 1.5 arcseconds an hour; Eris surfaced when the team re-ran the search with a lower limit and checked the candidates by eye.

When Eris was announced in 2005, NASA's press release hailed it as what?

The tenth planet. Eris then looked bigger than Pluto, and the claim forced astronomers to define 'planet' for the first time; the vote that followed demoted Pluto instead.

Which test in the 2006 definition of a planet do Eris and Pluto fail?

Clearing its orbital neighbourhood. A planet must dominate its zone, but Eris and Pluto share theirs with swarms of other icy bodies; they pass the other two tests, orbiting the Sun and being round.

Which TV series inspired the nickname Eris carried before it was officially named?

Xena: Warrior Princess. The nickname stuck for nearly two years, and an echo survives: Eris's moon is named after the Greek spirit of lawlessness, and Xena was played by Lucy Lawless.

Eris is named after the Greek goddess of what?

Strife and discord. The discoverers found it fitting: the object had set astronomers quarrelling over what counts as a planet, a row that ended with Pluto's demotion.

In 2008, astronomers coined which name for dwarf planets beyond Neptune, such as Eris?

Plutoids. Only two worlds qualified at first, Pluto and Eris: the term covers dwarf planets orbiting beyond Neptune, so the lone dwarf planet of the asteroid belt was left out.

Which name for Eris topped a New Scientist readers' poll but was ruled out because an asteroid already had it?

Persephone. In myth she is the wife of Pluto's Greek counterpart, which made her the readers' favourite, but asteroid 399 Persephone had held the name since 1895.

Mike Brown filed his first Eris web page under 'planetlila', an echo of which family member's name?

His daughter. Lilah was born in 2005 as the team raced to write up the discovery, and Brown is said to have favoured Lila, a Hindu idea of the cosmos as play, as the planet's name.

What is the name of Eris's moon, discovered in 2005?

Dysnomia. In Greek myth she is a daughter of the goddess Eris; the discoverers admit the name is a mouthful and simply call the moon 'Dy'.

How do astronomers think Eris's moon most likely formed?

From debris of a giant impact. The moon's neat, circular orbit is just what a collision would leave behind, and the same kind of smash-up is thought to have made Earth's own Moon.

A rival claim by astronomers in which country pushed Mike Brown's team to announce Eris in a hurry?

Spain. A Spanish team announced another large world that Brown's group had been quietly tracking, and the group, fearing its remaining finds were exposed too, unveiled Eris within two days.

What did the discovery of Eris's moon let astronomers calculate for the first time?

The dwarf planet's mass. A moon's orbit shows how hard its parent pulls on it, so timing the moon's 16-day circuit let astronomers weigh Eris.

Eris's density, about 2.5 times water's, suggests it is mostly what?

Rock. For all its icy surface, Eris is thought to be mostly rock inside, with the ice only a relatively thin outer shell.

At about 2,300 km across, Eris is closest in size to which of these worlds?

The Moon. Early estimates made Eris a quarter larger than Pluto, but better measurements shrank it to about two-thirds of our Moon's width.

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

By watching it pass in front of a star. Of the many observatories lined up along the predicted track, only three small telescopes at two sites in Chile caught the star blinking out.

Eris's entire surface covers about the same area as which country?

Russia. Unrolled flat, the whole world would cover about 17 million square kilometres: a single country's worth, on a body once thought to be bigger than Pluto.

About how long does Eris take to orbit the Sun?

560 years, roughly. Its path carries it out to some 97 times Earth's distance from the Sun, so a single lap takes more than twice as long as Pluto's.

On average, about how long does sunlight take to reach Eris?

About 9 hours. Sunlight reaches Earth in about eight minutes, but Eris orbits some 68 times farther out on average, and nearly half as far again at its most distant.

Astronomers describe Eris's orbit as tilted about 44 degrees to which reference plane?

The ecliptic. That is the plane the planets keep close to, and so large a world on so steep a tilt hints that bodies like it formed nearer the Sun than where they are found today.

Eris belongs to which population of trans-Neptunian objects?

The scattered disc. Eris's long, lopsided orbit is typical of that far-flung population in every respect but one: few of its members are tilted anything like as steeply.

Which spacecraft has travelled farther from the Sun than Eris, now about 96 times Earth's distance?

Voyager 1. Eris is far but not out of reach: the probe, launched in 1977, sailed past Eris's present distance on its way out of the Sun's protective bubble, which it left in 2012.

What is unusual about Eris's surface for a distant icy world?

It is almost pure white and uniform. That came as a surprise: Pluto, its near twin in size and surface chemistry, is blotchy and distinctly red.

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

Methane. On Earth it is natural gas; on Eris it lies frozen solid, and its signature in the reflected sunlight made the new world look remarkably like Pluto.

When Eris swings closest to the Sun, what are its surface ices expected to turn into?

A thin atmosphere. Even at its warmest Eris reaches only about -217 °C, yet that is enough for its most volatile ices to turn to gas, before freezing back onto the ground as it retreats.

Astronomers confirmed in 2023 that Eris's rotation is locked to what?

Its moon's orbit. Usually the moon is the one that gets locked, as Earth's is; here the dwarf planet itself turns just once every 15.8 days, in step with its moon.

Eris's discoverer Mike Brown later argued that which undiscovered world lurks far beyond Neptune?

Planet Nine. In 2016 Brown and Konstantin Batygin argued that an unseen giant is herding the orbits of distant icy bodies, an ironic sequel for the astronomer known as the man who killed Pluto.

Which spacecraft has observed Eris from afar, from a vantage point deep in the Kuiper belt?

New Horizons. Looking across from beyond Pluto, it saw Eris lit from the side, at angles never available from Earth, which showed how the surface scatters sunlight.

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

Visited by a spacecraft. Pluto, the only dwarf planet that is larger, got its flyby in 2015; Eris, now more than twice as far away, has only ever been watched from a distance.

The telescope that first read Eris's surface chemistry stands on which Hawaiian volcano?

Mauna Kea. From its summit the Gemini telescope caught Eris's reflected sunlight in the near infrared, the part of the spectrum where frozen compounds leave their clearest fingerprints.

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

Makemake. The team unveiled both in a single hurried announcement, right after other astronomers had claimed Haumea, a third large world the same group had been tracking.

In which European capital did astronomers vote in 2006 to class Eris and Pluto as dwarf planets?

Prague. A draft that would have counted Eris as a full planet met heated opposition there, and the meeting ended by voting for a stricter definition that left just eight.

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

A golden apple. In the best-known telling it was marked for the fairest, three goddesses each claimed it, and the contest that followed led in the end to the Trojan War.

In the best-known telling, why was Eris angry at the wedding of Peleus and Thetis?

She was the only god not invited. In the Roman-era telling she came anyway and was turned away at the door; a lost early epic instead had her acting on a plan of Zeus.

Which famous mythological verdict decided who won the prize Eris tossed into a wedding feast?

The Judgement of Paris. The Trojan prince chose Aphrodite, who had promised him Helen, and carrying Helen off to Troy brought the Greek army after her.

Eris's volatile-rich surface resembles that of Pluto and of which moon of Neptune?

Triton. It is thought to be a captured Kuiper belt world itself, which would make it a long-lost cousin of Eris and Pluto and not a home-grown moon.

What is the least powerful equipment able to detect Eris, nearly 100 times farther out than Earth?

A high-end amateur telescope and camera. NASA's 2005 announcement even told stargazers where to point: almost overhead before dawn, in the constellation Cetus.

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

A subsurface ocean of liquid water. Later readings of Eris's surface chemistry pointed to real heat at its centre, which strengthened the case for water kept liquid deep inside.

When it was found, Eris set which record among known objects in the Solar System?

Most distant from the Sun. Eris turned up some 97 times as far out as Earth, yet it was bright enough to have been sitting unnoticed on photographs taken years earlier.

Which of these moons is heavier than Eris?

Europa. Only seven moons in the Solar System outweigh Eris, which in turn outweighs every moon of Uranus and all but one of Saturn's.

An early draft of the 2006 planet definition would have made planets of Eris and which moon?

Charon. It is so large beside Pluto that the pair would have counted as a double planet, giving the Solar System twelve planets; astronomers threw the draft out about a week later.

Which observatory's new laser-guided optics revealed Eris's moon in September 2005?

Keck. A laser fired from the telescope made an artificial star beside Eris, letting the optics cancel the atmosphere's blur until the view from the ground matched one from orbit.

Which space telescope's readings of Eris's surface ices pointed to heat deep inside it?

Webb. It found heavy hydrogen in the ice in proportions that fit a frost cooked up deep inside Eris, not one left over from the cold cloud that formed the Solar System.

In Hesiod's poetry, the goddess Eris is a daughter of which primal power?

Night. Hesiod gives Nyx, the personified night, a brood of children born without a father; Homer tells it differently and calls Eris the sister and companion of the war god Ares.

Mike Brown led the search that found Eris from which California university?

Caltech. Brown called his memoir of the hunt How I Killed Pluto and Why It Had It Coming, because finding Eris forced the vote that stripped Pluto of its planet status.

Which body approved the name Eris in September 2006?

The International Astronomical Union. It names planets and minor bodies under different rules, so it could not act until it had settled which of the two Eris was; hence nearly two years under a nickname.

A 2025 study of a faint 19-hour flicker in Eris's brightness suggested it might be caused by what?

A large unseen inner moon. The flicker matches neither Eris's 15.8-day spin nor its known moon, though the authors themselves grant that the idea has considerable problems.