Sky and Telescope - May 2018 - 12

COSMIC RELIEF by David Grinspoon

Chipping Away
at Exoplanets
Human ingenuity is helping us unlock the secrets
of unimaginably distant worlds.

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.
allowed 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
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M AY 2 0 1 8 * S K Y & T E L E S C O P E

modeling hinted that two
of these might have lost all
water to a "runaway greenhouse," and the outermost
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 modeling 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 uninhabitable
due 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 column
was going to press, a group of astronomers led by Simon Grimm (University
of Bern, Switzerland) reported much
more precise masses for these planets,
derived by analyzing 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 high-tempo dance between
observers and theorists gives me hope
that in the coming decades, even without actually 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, to be published by Picador this month. Follow him on Twitter
@DrFunkySpoon.

N ASA / JPL- CA LTECH

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 (S&T: June 2017, p. 12). 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' masses and diameters. This
Sky and Telescope - May 2018

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