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The moon of the 10th planet

Dysnomia, the moon of Eris

On September 10th 2005, astronomers at the Keck Observatory on Mauna Kea took a look at Eris with a new instrument that allows them to see details as precise as those seen from the Hubble Space Telescope. The images  quickly revealed that it has a faint moon in orbit around it!

The team that found the satellite included the original discoverers of Eris plus the large engineering team at the Keck Observatory who made the observations possible. Technical details about the discovery can be found in the scientific paper recently published.

The discovery of Dysnomia, the moon of Eris,  from the W.M. Keck Observatory. Eris appears in the center,
while the moon is the small dot at the 3 o'clock position. Credit: M.E. Brown, W.M. Keck Observatory

How was the moon found?
  Eris is so far away that the moon appears quite close to it. So close, in fact, that all of the telescopes that have looked at the planet up until now have completely missed it. The telescopes miss it because the earth's atmosphere blurs the view of the planet as the light makes its way to the ground. In the past few years a technique known as Adaptive Optics has allowed astronomers to measure and then partially correct for the blurring caused by the atmosphere. One limitation of Adaptive Optics has been that you must be looking at a very bright object to be able to measure the blurring, otherwise you can make no corrections. The Keck Observatory has recently commissioned a new cutting-edge addition to its Adaptive Optics system. The new addition uses a laser beam launched from the side of the telescope to make an artificial star in the sky right next to the target. This laser star then allows Adaptive Optics to be used even when the target is not particularly bright. Just one year ago, observations such as these would have required the Hubble Space Telescope. Today they can be made from observatories on earth. T

Artist's concept of Eris and Dysnomia. The sun and other planets appear in the distance. Credit: R. Hurt, IPAC

Why is the discovery of the moon important?
While we know that Eris is larger than Pluto, that doesn't neccessarily mean it is more massive than Pluto. For example, a snowball could be bigger than a rock, but still be much less massive. Pluto appears to be a combination of ice and rock. If Eris were purely made out of ice, it could be a good deal less massive than Pluto. Alternatively, if it is mostly rock, it could be much more massive than Pluto. The one way to find out the mass of an object like Eris is to hope to find a moon around it. Finding a moon, and then determining the distance that  the moon is from the planet and how long it takes the moon to circle the planet allows us to precisely measure the mass of the body. A more massive body will pull on the moon tightly and it will circle the body more quickly. A less massive body will allow the moon to have a slow lazy orbit around the planet.

From a series of seven observations using the Keck Telescope and the Hubble Space Telescope, we have now measured the complete orbit of Dysnomia going around Eris. Dysnomia takes almost 16 days to go around Eris. Using  equations for used by Isaac Newton to figure out the mass of Jupiter, we can now tell that Eris has a mass 27% higher than that of Pluto (with an uncertainty of only 2%). The full orbit can be seen below.

Our best images came from the Hubble Space Telescope, and clearly showed (much to our suprise) that Dysnomia is the only moon around. These images, shown below, are so good that you also can see some of the artifacts caused by the Hubble Space Telescope itself. In particualr, the spikes coming out of Eris, the spotty "ring" around Eris, and the slight elongation to the right of Eris are all expected patterns caused by the telescope itself, rather than from anything going around Eris.

A second reason that discovering the moon is important is that understanding how moons form provides insight into the history of the solar system. It is quite surprising the 3 of the 4 largest objects in the Kuiper belt (Eris, Pluto, and 2003 EL61) all have moons. Such a large fraction of objects with moons suggests that some very common mechanism must be responsible. In the scientific paper describing the discovery we suggest the hypothesis that the moons of Eris and 2003 EL61 were both formed from a collision between Kuiper belt objects, much like it is thought the the Earth's moon was formed from a collision between the Earth and an object about the size of Mars. Understanding the orbit of the moon around the planet will help to show if this hypothesis is feasible.

Images of the four largest Kuiper belt objects from the Keck Observatory Laser Guide Star Adaptive Optics system. Satellites are seen
around all except for 2005 FY9. Without the adaptive optics system the images of the Kuiper belt objects would he
smeared out so much by the earth's atmosphere that the satellites would not be visible. Credit: M. Brown/Keck Observatory

Why is the moon called Dysnomia?
Dysnomia is the mythological daughter of Eris. Eris is, of course, the Greek goddess of discord and strife. Dysnomia is the daemon spirit of lawlessness.

What is the moon made out of?
It was long thought that all moons were made out of green cheese, but that theory has been generally discredited. Currently we have no direct evidence to tell us what the moon is made out of, but we have some educated guesses. We do know that the moon of 2003 EL61 (the third largest object in the Kuiper belt, after Eris and Pluto) appears to be a ball of almost pure frozen water. We know this because we have been able to look at the sunlight reflected off the moon at infrared wavelengths, and the pattern of the light reflected shows us that there is frozen water and nothing else. From the limited information that we have, Dysnomia appears like it might be similar to the moon of 2003 EL61 (2003 EL61 was code named Santa, by the way, so the satellite is, of course, Rudolph). We are planning to use the Hubble Space Telescope later this year to study Dysnomia in more detail.

How big is the moon?
Right now we are not certain how big the moon is, but we can make some guesses based on how much light it reflects. We know that it is about 500 times fainter than Eris, suggesting that it is perhaps 22 times smaller in diameter than Eris. Eris is about 2400 km in diameter, so Dysnomia is perhaps 100 km in diameter. It is possible, however, that Dysnomia has a darker surface than Eris's very frosty highly reflective surface. In this case,  Dysnomia could plausibly be as large as about 250 km. To scale, the Pluto-Eris system looks like this: