NASA's planet-hunting Kepler telescope, which astronomers
hope will find Earth-like planets orbiting other stars, might also find
habitable moons in other solar systems, new research suggests.
Kepler's primary
mission is to monitor thousands of stars looking for characteristic dips in
their brightness as orbiting planets pass in front of them in so-called "transit"
events.
The orbiting observatory, launched
in March, already detected the giant extrasolar
planet HAT-P-7b within its first 10 days of taking data. The planet had
previously been discovered by ground-based telescopes, but the observations
showed Kepler works as expected.
While ground-based observatories, and even some space
telescopes, such as Spitzer and Hubble, can find Jupiter-sized extrasolar
planets, Kepler is the first telescope aimed at detecting alien worlds closer
to the size of our own home planet.
One astronomer suggests that Kepler's capabilities may even
be able to detect so-called "exo-moons."
Modeling moons
David Kipping of University College London has already
devised a method for detecting exomoons but no-one was sure whether it could
really be used with current technology. He and his team have now modeled the
properties of the instruments on Kepler, simulating the expected signal
strength that a habitable moon would generate.
An exomoon's gravity tugs on the planet it orbits, making
the planet wobble during its orbit around its host star. The resulting changes
in the position and velocity of the planet should be detectable by Kepler
through accurate timing of the transits.
The scientists considered a wide range of possible planetary
systems and found that a fluffy Saturn-like planet, which would be low in mass
for its size, gives the best possible chance for detecting a moon, rather than
a denser Jupiter-like
world. This is because planets like Saturn are large – blocking out a lot
of light as they pass in front of their star – but very light, meaning they
will wobble much more than a heavy planet.
If the Saturn-like planet is at the right distance from its
star, then the temperature will allow liquid water to be stable on any
sufficiently large moons in orbit around it. Such water-bearing moons might be
habitable for life.
"For the first time, we have demonstrated that
potentially habitable moons up to hundreds of light years away may be detected
with current instrumentation," Kipping said.
Millions of moons possible
The team found that habitable exomoons down to 0.2 times the
mass of the Earth are readily detectable with Kepler.
"As we ran the simulations, even we were surprised that
moons as small as one-fifth of the Earth's mass could be spotted," Kipping
said.
While it is not known if habitable exomoons are common in
the galaxy, the observatory could potentially look for Earth-mass habitable
moons around 25,000 stars up to 500 light-years away from the sun. In the whole
sky, there should be millions of stars which could be surveyed for habitable
exomoons with present
technology.
"It seems probable that many thousands, possibly
millions, of habitable exomoons exist in the Galaxy and now we can start to
look for them," Kipping said.
The team's findings will be detailed later this month in the
journal Monthly Notices of the Royal Astronomical Society.
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