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
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:
