Eris Planet Facts

Mike Brown, an astronomer at the California Institute of Technology was the lead discoverer of Eris. He knew that this world was about the same size or larger than Pluto and he realized that he would be hailed as the discoverer of the tenth planet. However, he didn’t agree that Eris, or even Pluto, were big enough objects to actually qualify as planets.

Before Eris’s official announcement in 2005, his wife joked Brown’s discovery meant that some day their daughter could afford college. But Brown maintained his hypothesis: Eris was the biggest known member of the Kuiper belt – a region of icy objects orbiting beyond Neptune.

Today, Brown says that it was tempting to keep the title as the founder of the tenth planet, but as a scientist he couldn’t let that stand. “It’s hard to pretend that was not in my head,” he tells us. “But I felt like there was no way to leave this argument. It made no scientific sense.”

Prior to Eris’s discovery, Brown was a new faculty member at Caltech trying to secure tenure. He chose planet-hunting to make his mark. In 1997, his team began using an ageing 120-centimetre (48-inch) Schmidt telescope at the university’s Palomar Observatory. The telescope was so old that it could only use photographic plates. Each photo would then be scanned into a computer for analysis.

After two years of hunting, Brown determined this method was not working. They resumed the search in 2001 using up-and-coming digital technology. Within months, the team discovered small worlds in the Kuiper belt.

The largest ones, Quaoar and Sedna, were announced to some fanfare. Brown’s tenure was secured. Then came a tiny point of light on 5 January 2005 that would change astronomy forever. It was close: 90 Earth-Sun distances away, the equivalent of the Solar System’s backyard. And it was bright: 18.8 magnitude, easily visible in a professional telescope.

The object was officially labelled 2003 UB313, but Brown nicknamed it Xena in a subtle nod to the mythical Planet X on the outside of our Solar System. He planned to take his time to confirm the discovery, which is standard practice among astronomers. Among other tools, his team used a telescope in Chile. Brown was tracking a number of small worlds with this telescope, including one later named Haumea. Unfortunately for Brown, a public University of Ohio website had records of which objects the telescope was looking at (this was later rectified).

News of Haumea leaked out. According to Brown, a Spanish astronomer falsely claimed its discovery based on the Ohio data. However, the International Astronomical Union (IAU) never made a ruling. Brown nevertheless knew the positions of Xena and another object he was confirming were also in danger of discovery. Hastily, he made an announcement on 29 July 2005. When the IAU realised that Xena was about the same size as Pluto, it decided to call a meeting to vote on the definition of a planet.

Its decision to demote Pluto in 2006 to dwarf planet was controversial among astronomers. As such, Brown suggested the official name of Eris, after the Greek goddess of strife and discord. The IAU accepted it. That dispute has faded in the years since, Brown says, although the public still feels it’s controversial.

According to Brown, Eris is the furthest dwarf planet we have ever spotted orbiting the Sun. It is almost 15 billion kilometres (9 billion miles) away – about three times further than Pluto. Eris takes 560 years to make one trip around the Sun and its orbit is eccentric. Its distance from the Sun varies from 38 Sun-Earth distances to 97 Sun-Earth distances. The dwarf planet is around 2,326 kilometres (1,450 miles) in diameter. Eris also has a moon, Dysnomia. From watching Dysnomia orbit Eris, Brown estimates Eris is 27 per cent more massive than Pluto and has a density of around 2.6 grams per cubic centimetre (1.5 ounces per cubic inch). The high density implies Eris is mostly made of rock.

It is also surprisingly bright, reflecting almost 97 per cent of the light that comes to it. Brown’s team believes it’s because Eris’s atmosphere is frozen, due to its estimated minimum temperature of -240 degrees Celsius (-405 degrees Fahrenheit). The dwarf planet is at its furthest distance from the Sun right now, and will warm up to -215 degrees Celsius (-360 degrees Fahrenheit) as it gets closer. Eris’s atmosphere is likely made up of methane and nitrogen, and astronomers believe that it’s currently just a thin layer of substance lying over the surface.

Brown says there’s still much to learn about Eris. His team is trying to find its pole. This is important because the direction of Eris’s spin determines how much heating the dwarf planet receives as it goes around the Sun. Brown also keeps up on the latest news on the dwarf planets he found.

“There’s a paper that just came out… on the first precise measurement of Makemake,” Brown says, referring to another of his discoveries. “I didn’t have anything to do with [the paper], and it’s hard. I always assumed it was like having your kid grow up and start dating and get married. You don’t always want it to happen, except you know it should happen.”

But Brown is focusing on other things. One of his near-term goals is to restart the search for dwarf planets with more advanced digital technology. “It’s hard to justify doing the entire survey again, except each one of the big bright [objects] is just so interesting that it’s worth going and finding out if they’re there.”

A moon called Dysnomia

Eris’s moon, Dysnomia, was discovered on 10 September 2005. Brown and his team were working at the Keck Observatory in Hawaii when they found the moon.

They nicknamed it Gabrielle (after the character in Xena: Warrior Princess), which stuck until the IAU approved the official name of Dysnomia. In mythology, Dysnomia is the daughter of Eris. The name also closely honours the first two letters in Brown’s wife’s name, Diane.

Keck and Hubble were used several times to figure out Dysnomia’s orbit. It takes about 16 days for the moon to go around Eris. More importantly, Dysnomia’s movements around the dwarf planet allowed astronomers to figure out how massive Eris is: about 27% more than Pluto.

It’s difficult to tell just how big the moon is because of its size and distance, but the astronomers figured Dysnomia is about 200 times fainter than Eris. Brown estimates Dysnomia is about 100km (62 miles) in diameter, but that could range as high as 250km (155 miles). There is no direct evidence showing what Dysnomia might be made of. However, Brown suspects it is mostly made up of frozen water, based on similar objects he has studied.

Dysnomia’s discovery is important because it appears that most of the largest Kuiper belt objects have moons. This implies they all had similar origins, which would give us a window into the early years of our Solar System.