Sky and Telescope - July 2017 - 14
COSMIC RELIEF by David Grinspoon
Life Outside the Habitable Zone
Over its history, a planet might host oceans long enough to support life.
life? We often deﬁne the habitable zone
as the distance from a star where water
can remain liquid on a planet's surface.
Starward of the inner edge we imagine
planets in a Venus-like state - broiling
hot and dry. Beyond the outer edge, we
picture Mars-like worlds, freeze-dried
and oceanically deprived.
Since stars become brighter as they
evolve, the inner and outer edges of
the zone will both move outward, away
from the heat, as hot worlds boil and
frozen worlds melt. So we often talk
about the "continuously habitable zone"
hosting planets that remain in this zone
over a star's lifetime.
But when we study planetary evolution we learn this simple guideline is
inadequate, because without knowing
a planet's history, its position doesn't
really tell us if it's habitable. What if a
planet can be conducive for life, even if
its oceans are not stable?
Two recent results illustrate this
ambiguity. Some colleagues and I have
modeled the history of Venus with an
eye toward understanding the inner
edge of habitability. We used a 3D
climate model to simulate billions of
years of evolution under the warming
Sun and, when we included the complex
interplay of clouds, topography, planetary rotation, and atmospheric motions,
we learned something unexpected: As
the Sun warms and the oceans evapo-
It's not hard to imagine life following oases of habitability,
surviving by hopping from one promising world to the next.
rate, the clouds arrange themselves to
keep the planet cool and slow down
the loss of oceans. Thus, although the
oceans of Venus became unstable early
in its history, the process of actually
losing them to space likely took billions
of years. Venus, while outside the habitable zone, might have been habitable for
much of its lifetime.
More recently, a group of researchers at the University of Washington has
considered the evolution of icy moons,
like those of Jupiter and Saturn. As
planets migrate inward toward a star
- as we've learned young giant plan-
p An artist's concept of the TRAPPIST-1 system seen from one of its seven known planets, several of which lie within the star's habitable zone.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
ets are wont to do - their icy moons
will melt. How long will their oceans
last? That depends on size. Oceans
on a larger moon, such as Ganymede,
might become stable and last as long as
its host planet is in the habitable zone.
For smaller moons like Europa, surface
oceans would not be stable but would
instead fully evaporate, though this process could take well over a billion years.
These results made me realize that a
planet does not have to be a stable resident of the habitable zone to host life.
We describe a chemical as being
"metastable" if in its current conditions
it will eventually decompose but the rate
of this decomposition is quite slow. Similarly, I think we have to consider worlds
that are "metahabitable." If oceans can
persist for hundreds of millions or even
billions of years, this might be plenty of
time for life to ﬂourish on worlds that
are not in a stable habitable state.
We don't yet know how long an ocean
takes to come alive, but Earth's history
hints at less than 100 million years.
Since we suspect that microbial life can
travel from planet to planet, hitchhiking
on meteorites, then a few worlds with
metastable oceans may sufﬁce to cultivate and maintain life in a planetary
system. Especially now that we know of
systems like Trappist-1, around which
many planets orbit very close together
(S&T: June 2017, p. 12), it's not hard to
imagine life following oases of temporary habitability, surviving by hopping
from one promising world to the next.
¢ DAVID GRINSPOON is an astrobiologist at the Planetary Science Institute.
His new book, Earth in Human Hands,
came out in December.
ESO / N. BA RT M A NN / SPACEENGINE.ORG
WHAT WOULD ALLOW a planet to host