Researchers have detected the first
"exomoon" candidate -- a moon orbiting a planet that lies outside our
solar system. Image credit: NASA/JPL-Caltech. Full image and caption
Titan, Europa, Io and Phobos are just a few members of our solar
system's pantheon of moons. Are there are other moons out there,
orbiting planets beyond our sun?
NASA-funded researchers have spotted the first signs of an "exomoon,"
and though they say it's impossible to confirm its presence, the
finding is a tantalizing first step toward locating others. The
discovery was made by watching a chance encounter of objects in our
galaxy, which can be witnessed only once.
"We won't have a chance to observe the exomoon candidate again," said
David Bennett of the University of Notre Dame, Ind., lead author of a
new paper on the findings appearing in the Astrophysical Journal. "But
we can expect more unexpected finds like this."
The international study is led by the joint Japan-New
Zealand-American Microlensing Observations in Astrophysics (MOA) and the
Probing Lensing Anomalies NETwork (PLANET) programs, using telescopes
in New Zealand and Tasmania. Their technique, called gravitational
microlensing, takes advantage of chance alignments between stars. When a
foreground star passes between us and a more distant star, the closer
star can act like a magnifying glass to focus and brighten the light of
the more distant one. These brightening events usually last about a
month.
If the foreground star -- or what astronomers refer to as the lens --
has a planet circling around it, the planet will act as a second lens
to brighten or dim the light even more. By carefully scrutinizing these
brightening events, astronomers can figure out the mass of the
foreground star relative to its planet.
In some cases, however, the foreground object could be a
free-floating planet, not a star. Researchers might then be able to
measure the mass of the planet relative to its orbiting companion: a
moon. While astronomers are actively looking for exomoons -- for
example, using data from NASA's Kepler mission - so far, they have not
found any.
In the new study, the nature of the foreground, lensing object is not
clear. The ratio of the larger body to its smaller companion is 2,000
to 1. That means the pair could be either a small, faint star circled by
a planet about 18 times the mass of Earth -- or a planet more massive
than Jupiter coupled with a moon weighing less than Earth.
The problem is that astronomers have no way of telling which of these two scenarios is correct.
"One possibility is for the lensing system to be a planet and its
moon, which if true, would be a spectacular discovery of a totally new
type of system," said Wes Traub, the chief scientist for NASA's
Exoplanet Exploration Program office at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., who was not involved in the study. "The
researchers' models point to the moon solution, but if you simply look
at what scenario is more likely in nature, the star solution wins."
The answer to the mystery lies in learning the distance to the
circling duo. A lower-mass pair closer to Earth will produce the same
kind of brightening event as a more massive pair located farther away.
But once a brightening event is over, it's very difficult to take
additional measurements of the lensing system and determine the
distance. The true identity of the exomoon candidate and its companion, a
system dubbed MOA-2011-BLG-262, will remain unknown.
In the future, however, it may be possible to obtain these distance
measurements during lensing events. For example, NASA's Spitzer and
Kepler space telescopes, both of which revolve around the sun in
Earth-trailing orbits, are far enough away from Earth to be great tools
for the parallax-distance technique.
The basic principle of parallax can be explained by holding your
finger out, closing one eye after the other, and watching your finger
jump back and forth. A distant star, when viewed from two telescopes
spaced really far apart, will also appear to move. When combined with a
lensing event, the parallax effect alters how a telescope will view the
resulting magnification of starlight. Though the technique works best
using one telescope on Earth and one in space, such as Spitzer or
Kepler, two ground-based telescopes on different sides of our planet can
also be used.
Meanwhile, surveys like MOA and the Polish Optical Gravitational
Experiment Lensing Experiment, or OGLE, are turning up more and more
planets. These microlensing surveys have discovered dozens of exoplanets
so far, in orbit around stars and free-floating. A previous NASA-funded
study, also led by the MOA team, was the first to find strong evidence
for planets the size of Jupiter roaming alone in space, presumably after
they were kicked out of forming planetary systems. (See http://www.jpl.nasa.gov/news/news.php?release=2011-147).
The new exomoon candidate, if real, would orbit one such
free-floating planet. The planet may have been ejected from the dusty
confines of a young planetary system, while keeping its companion moon
in tow.
The ground-based telescopes used in the study are the Mount John
University Observatory in New Zealand and the Mount Canopus Observatory
in Tasmania.
Additional observations were obtained with the W.M. Keck Observatory
in Mauna Kea, Hawaii; European Southern Observatory's VISTA telescope in
Chile; the Optical Gravitational Lens Experiment (OGLE) using the Las
Campanas Observatory in Chile; the Microlensing Follow-Up Network
(MicroFUN) using the Cerro Tololo Interamerican Observatory in Chile;
and the Robonet Collaboration using the Faulkes Telescope South in
Siding Spring, Australia.
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
whitney.clavin@jpl.nasa.gov
