“It’s very hard to
‘cook up’ biochemistry
in the lab. It’s probably
easier just to go to
these places and look”
Jonathan Lunine, Cornell University
EXOMOONS
October 2015 marked the 20th anniversary
of the discovery of 51 Pegasi b, the first planet
to be discovered in orbit around a Sun-like
star. It was a giant planet like Jupiter and was
revealed because of the tiny ‘wobble’ it induced
in its parent star. Since then, thousands more
exoplanets have been discovered around stars
in the Milky Way. Many are like Jupiter and
Saturn, so it seems likely that they would also
have moons.
We may be able to detect such moons
by studying their ‘transits’. During a transit,
a planet crosses the face of its parent star
and blocks out some light. This drop in
brightness can be measured to give the
size of the planet, which is how NASA’s
Kepler space telescope found more than
1,000 planets from 2009-2013. Any moons
around those planets will cause additional,
smaller drops in light, so it should be
possible to detect them with ultra-sensitive
measurements. The SuperWASP (Wide
Angle Search for Planets) has detected at
least one tentative transit signal that could
be an exomoon, and next-generation space
missions such as ESA’s CHEOPS (2017),
NASA’s TESS (2018) and ESA’s Plato (2024)
also stand a chance of detecting exomoons
in this way.
Many of these moons will be found
around giant planets that are much closer
to their parent stars than Jupiter and Saturn.
That means receiving more heat, so oceans
on those moons will not be underground.
Starlight will melt the ice, turning the moon
into a water world where life could flourish.
But the habitat is unlikely to be long-lived:
the weak gravity of a small moon will not
be able to prevent the atmosphere being
eroded by the fierce starlight. The exomoon
will resemble a giant comet, with a gassy
tail stretching into space, and will eventually
become a tiny rocky cinder. Not ideal. But
good while it lasted. ßDR STUART CLARK IS AUTHOR OF SEVERAL BOOKS
INCLUDING THE UNKNOWN UNIVERSE