http://www.skyandtelescope.com.au 65by David Grinspoon COSMIC RELIEFGetting to know our
exoplanet neighbours
Astronomers are slowly unlocking the secrets of unimaginably distant worlds.
of astronomers led by Simon Grimm
(University of Bern, Switzerland)
reported much more precise masses
for these planets, derived by analysing
tiny changes in the timing of transits
induced as the planets tweak one
another’s orbits. Now, geophysicists
can apply these improved constraints
to their interior models, and we’ll get
closer to understanding what these
planets are really like.
The dance between observers and
theorists gives me hope that in the
coming decades, even without going
there, we’ll make huge strides toward
finding out what lies behind all those
enticing exoplanet doorways.■ Astrobiologist DAVID GRINSPOON
is coauthor with Alan Stern of Chasing
New Horizons. Follow him on Twitter
@DrFunkySpoon.IT WASN’T LONG AGO that it was
astonishing simply to know that planets
beyond our own Solar System really do
exist. Now we know they’re everywhere,
orbiting nearly every star.
Yet, in a way, exoplanets seem like
a cosmic tease. Given their enormous
distance and dimness beside the
blindingly radiant stars they hug so
tightly, it will be hard to learn enough
about them to satisfy the profound
questions they raise. It’s like coming
upon a palace full of doorways we can’t
open, though we know behind them lie
clues to the mystery of our existence.
One of the most extraordinary
systems we’ve discovered so far is
Trappist-1. There, seven roughly Earth-
size planets orbit close enough to a
dim red star that several may be in the
habitable zone, with liquid water stable
on the surface. Or not. It’s awfully
hard to say, because what we know is so
rudimentary.
We’ve had only a rough idea of the
worlds’massesanddiameters.Thisenabled us to estimate what mix of
metal, rock and ice they’re made of.
And we know their orbital periods and
distances from the star, along with
constraints on the ellipticities of their
orbits. From this, simple calculations
of surface temperature showed that
three orbit within the habitable zone.
Further modelling hinted that two
of these might have lost all water
to a ‘runaway greenhouse,’ and theoutermost may be frozen over.
Given the little data we have, can
we ever determine conditions on these
worlds? Recently, planetary geophysicist
Amy Barr Mlinar (Planetary Science
Institute) and colleagues published
some clever modelling they did that
factors in the effects of tidal heating.
Tidal heating is insignificant among
our Solar System’s inner planets. But
it’s potent among Jupiter’s big moons,
which repetitively pull on one another
and yank their orbits into very slight
ellipses. This non-circular motion leads
to enough tidal forcing from Jupiter to
melt their insides, creating volcanism
on Io and a water ocean inside Europa.
In some ways the Trappist-1 system
resembles more a system of Jovian
moons than our Solar System. These
planets torque one another’s orbits and
tug at each other’s insides, depositing
heat. Barr Mlinar’s results suggested
that, because of this heating, two of
these Trappist-1 worlds — including one
of those previously ruled uninhabitabledue to a runaway greenhouse — might
indeed support surface liquid-water
oceans. Who knows? Maybe something
is swimming in those waters.
This study is far from the last word.
Barr Mlinar et al. described how the
great uncertainties in these planets’
masses and densities made it hard to
reach definitive conclusions.
Yet, amazingly, just as this
NASA / JPL-CALTECH column was going to press, a group
It’s like coming upon a palace full of doorways
we can’t open, though we know behind them
lie clues to the mystery of our existence.