Astronomers have discovered what they believe to be a moon orbiting a planet orbiting a star other than the Sun for the second time. It's massive, just like the first hypothetical exomoon – bigger than half the planets in the Solar System. It may take some time for confirmation, but the first possible discovery now appears to be part of a trend, rather than a fluke or a crazy one-off.
More than
4,000 planets have now been discovered orbiting other stars, with thousands
more candidates awaiting confirmation. Because the number of moons in our Solar
System much outnumbers the number of planets, an abundance of satellites across
the galaxy makes intuitive sense. Indeed, by sheer numbers, the chances of
finding life close enough to examine with future generations of telescopes may
be better on moons than on planets.
However,
existing approaches are unlikely to identify most moons, thus this is purely
theoretical. However, the Cool Worlds team at Columbia University discovered
signals of a planet-sized object that appeared to be connected with an even
larger planet in 2017. The same team has done it again, as reported in Nature
Astronomy.
Kepler
1708b, a not-quite-Jupiter-sized planet 5,500 light-years away, is thought to
host the exomoon contender. We know Kepler 1708b exists because when the Kepler
Space Telescope passed in front of Kepler 1708, it saw substantial brightness
dips.
Dr. David
Kipping and co-authors discovered that these dips were followed by smaller dips
that were consistent with a moon roughly 12 planetary radii away (between
Europa and Ganymede in the Jovian system).
"It's
a defiant gesture," Dr. Kipping stated in a statement. "We've tried
everything but the kitchen sink to get rid of it".
If real,
Kepler 1708b-i, as the moon is known, has a radius 2.6 times that of Earth,
indicating that it is most likely a gas world rather than a rocky object.
The
thought of a moon this enormous is difficult to comprehend in a system where
the largest moon is Ganymede, which is 60 percent smaller than the Earth and
not much larger than Neptune's own moon Triton. The research speculates that
Kepler 1708b-i was previously its own planet, captured after a near encounter.
Although
such an event is implausible, with adequate mechanisms in place, strange things
are sure to happen now and then, and supermoons may be the only ones we can
locate right now. "The weirdos will usually be the first detections in any
survey," Kipping added.
The first
detections are also likely to be suspect - after all, we don't know what kind
of signal to expect yet. "It could just be a data fluctuation, either
owing to the star or instrumental noise," said University of Washington
Professor Eric Agol, who was not involved in the study. The team, on the other
hand, is more assured.
Confirmation
will necessitate the use of either the ailing Hubble Telescope or the freshly
launched JWST, but acquiring time on either will be difficult. Even if that is
possible, opportunities will only come once every two years, when Kepler-1708b
transits, and we must hope that the star, planet, and moon align in such a way
that detection is possible.
Kipping's
team earlier stated that Kepler 1625b had a moon the size of Neptune, but their
assertion is still debatable. There has also been a previous mention of a
probable exomoon. In that instance, the only information we have is the mass
ratio of the object and the larger body it orbits — the pair may be a moon and
a planet or a planet and a star. Since that question is unlikely to be
answered, Kipping's contenders are the frontrunners to be the first confirmed
moons beyond the influence of our Sun.
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