The Discovery of Eris

In 2005, Michael Brown of the California Institute of Technology and his team discovered a large body in the outer solar system. It was not the first distant object that had been found in the Kuiper Belt — that region encircling our solar system is composed of hundreds of icy objects. But it was the largest known Kuiper Belt object, just beating out Pluto in terms of size, and so their discovery was heralded as “the tenth planet.”

Brown’s discovery, now named “Eris,” has since been demoted by the International Astronomical Union to a “dwarf planet,” along with the former ninth planet, Pluto. This re-categorization came about partly because scientists think we will discover many planet-sized globes in the Kuiper Belt. Recent discoveries of many unusual extrasolar planets in other solar systems also raised questions about what should and shouldn’t be called a planet.

In part three of a five-part lecture given at NASA’s Jet Propulsion Laboratory, Brown talked about discovering Eris. He also described how scientists manage to take a tiny point of light in the night sky and figure out how big it really is.

Artist’s concept of Eris (the object formerly known as Xena) and its moon. The sun and planets appear in the distance. Credit: R. Hurt, IPAC

“Every time you find indications of an object in the outer solar system, you get a little charge. You go through all this data and there’s nothing there, nothing there, nothing there, and then suddenly there’s something that no one has ever seen before except for you. It’s always a moment of excitement. Every once in awhile, the moments of excitement almost make you fall out of your chair.

One day we found something that was moving really slow, slower by a factor of two anything that we’d seen before, which tells you it’s essentially a factor of two further away. That’s enough to make you fall out of your seat to begin with, because we’d found almost nothing at that distance in the solar system.

We first named our discovery 2003 UB313. That’s kind of a dumb name, so we nicknamed it Xena. But now the official name for this object is Eris.

There are a couple of things you want to learn very quickly when you find one of these things. One of them is, what does its orbit look like? Is it going to fit the circular pattern of planets, or is it going to fit Pluto’s crazy pattern? It turns out this one has a 560-year orbit. To track its orbit you have to look over a relatively significant chunk of its orbit. Three hours is not significant, but we have no patience for tracking it over a long time.

This object was so bright it was easy to find it in everybody else’s old data. Many people had seen it before but they were doing some other type of project, taking a picture for other purposes. We found it in photos dating all the way back to 1950. We’re sure we could find it in Clyde Tombaugh’s original photographic plates, but Lowell Observatory appears to be reluctant to let us look. I can sort of understand why; they don’t want people to think Clyde Tombaugh screwed up by missing it. But it’s not that he screwed up – it would’ve been very faint on his plates. If you knew where to look I think you could see it, but there’s no way he should have noticed it. I’d like to look just for fun, but they don’t answer my emails.

The orbits of the outer planets in the solar system, with the blue and purple dots representing the location of Kuiper Belt objects. Click image for larger view.
Image Credit: NASA/JPL-Caltech.

It turns out that its orbit is even crazier and more elongated than Pluto’s. What’s more, the reason it’s moving so slowly is because it’s now on its furthest point from the sun. So it’s the faintest that it ever is in its orbit. Eris is pretty bright, but if it were closer it would have been much brighter and easier to study. But the good news is if it had been closer Clyde Tombaugh would certainly have found it along with Pluto. Then that would have caused all sorts of mess. If he had found two of them out there with these kinds of orbits, people would have had some inkling that there was something like a Kuiper Belt out there, and that would’ve been interesting. But that would’ve denied us our fun.

Pluto is tilted by 19 degrees compared to the discs of the other planets. Eris is tilted by 45 degrees. Nobody has a good explanation for why that is. We find other objects in the Kuiper Belt that are tilted by a few degrees – 10, 20 or even 30 degrees – but 45 degrees is more than anything else. And to find probably the biggest object in the Kuiper Belt to be tilted like that is quite a surprise. Presumably Neptune is to blame in some way. This is an interesting mystery we’re trying to solve.

One thing everybody wants to know is how big Eris is. When we see planets, asteroids, or other nearby bright objects in the sky, what we’re seeing is sunlight reflected from the surface of that object. You can get a lot of sunlight reflected in two ways: you can either have a relatively small object with a very shiny surface – a snow- or ice-covered surface – that will reflect a lot of sunlight, or you can have a really big object with a darker surface. Either way, you get the same amount of sunlight reflected back, and you can’t tell which it is just by looking at it.

So astronomers have come up with these very clever techniques, using images from things like the Spitzer Space Telescope, to figure out how much heat is coming from the object, and that could indicate how big it is. We’re less clever than most astronomers, so we decided to take pictures with the Hubble Space Telescope. Usually you can’t do that, because the objects are so small they just look like a point of light even with Hubble. But as bright as it was, it had to be bigger than Pluto, so we knew we would be able to see it with Hubble and measure precisely how big it was.

The size is 2400 kilometers, with an uncertainty of about 100 kilometers. Pluto is about 2370, depending on who you ask. So this thing is just barely bigger than Pluto. Finding the largest dwarf planet is not quite as exciting as finding the tenth planet, but I’d just like to point out that Eris is the largest dwarf planet in the known universe.

Photo of Eris taken by the Hubble Space Telescope.

The interesting thing is, when we first discovered it, our best guess was that it was about 25, 30, maybe even 40 percent bigger than Pluto. So Eris is much smaller than we initially guessed. The reason is that it is one of the most reflective objects in the solar system. It reflects 87 percent of the sunlight that hits it. The Earth reflects something like 42 percent. Fresh fallen snow is 80-ish percent. So 87 percent is a lot of reflection.

Why is it so reflective? I think the answer is that this object, when it’s close to the sun, has an atmosphere, just like Pluto does. It’s a thin atmosphere of mostly nitrogen. It also has methane and other things in it, but it’s mostly nitrogen, just like the Earth’s. When it moves further away from the sun, that nitrogen atmosphere freezes onto the surface. If you took the Earth and you moved it to where the dwarf planet is, the Earth’s atmosphere would freeze and you would get a 30-foot section of nitrogen ice on top of the Earth, and it would be really reflective.

This is one of the more interesting atmospheres in the solar system, because it’s the only object that we know that’s big enough to maintain an atmosphere on it, but also has an orbit that’s elongated enough so that atmosphere comes and goes with its 580-year season. It’s going to be fascinating to see that atmosphere reappear as it gets closer to the sun, and I’m looking forward to making those observations in another 280 years.”

Read part 1 of this lecture; “Defining Planets
Part 2: “Hunting Distant Dwarfs

Part 4: “Studying Dwarf Planets
Part 5: “Questioning Dwarfs

Related Web Pages

Speeding by Jupiter
Questioning Pluto
The Ice Dwarf Cometh
Moons Over the Kuiper Belt
A Glimmer of Pluto on the Horizon
Tenth Planet
To Pluto and Beyond