EUROPA: NASA / JPL-CALTECH / SETI INSTITUTE; PLUTO: NASA / JHUAPL / SWRIBut thepossibilitythat life could develop and life actually
developing are two different things. We only have one
exampleoflifearising,andwejustdon’tknowhowthings
willtranspireelsewhere.“IthinktheideathatEuropawill
warmupandturnintothisclement,habitablewaterworld—
that’s wrong,” Laughlin says. “It will be even more interesting
than that, in the sense that there are all sorts of weird
chemical pathways.” The same holds for Titan. Laughlin feels
that it’s naïve of us to speculate on what would happen when
Saturn’slargestmoonwarmsup(excepttosaythat,ashetold
me, “it’s gonna smell really bad”). We just don’t know.
Eventually, we may get an idea by studying exoplanets.
Laughlin considers Titan probably the best proxy in our
SolarSystemforcertain‘super-Earths’thathavesimilar
compositions to Titan’s but are balmy. Yet he stresses that
any biosignatures would not necessarily look like ours. “I
think the prospects for giving hints of weird, crazy chemical
disequilibria that have nothing to do with our own biology
butarehintingtowardtrulyalienbiologies—thatmightpan
out,” he says. “It’s important that we keep our expectations
completely open. Seeing blank-slate what’s there is going to be
a really exciting adventure.”Pluto and other Kuiper Belt Objects
The drifting habitable zone will not stop at Saturn, of course.
DuringthehotteststagesoftheSun’sredgiantphases,the
zone will push right out into the Kuiper Belt, as much as 50
astronomical units (a.u.) from the Sun. Alan Stern (Southwest
Research Institute) calls these “delayed gratification habitable
zones”. Triton, Neptune’s largest moon, Pluto and its largest
moon Charon, and large Kuiper Belt objects are all rich in both
water and organic compounds, and thus may become possible
sites of biochemical, if not biological, evolution.
The window of time forcomplexlife to potentially develop
in that distant realm is comparatively short, on the order of
300,000 to 100 million years, versus the roughly 3 billion
years Earth had for such life to arise, Stern says. But at their
enormous distance from the Sun, these bodies will be safe
from wholesale evaporation, they enjoy less harmful stellar
radiation, and they suffer fewer collisions from errant
asteroids and other smaller objects. So maybe they’ll have a
chanceatlife,eventhesophisticatedvariety.
For our Solar System, this is way off in the future, of
course.ButtheMilkyWayhasanestimated1billionred
giantstars.Ificyorganicsarecommon20to50a.u.out
fromthesestars,astheyarefromours,thensuchzones,
Stern says, “may form a niche type of habitable zone that
is likely to be numerically common in the galaxy.” Thus
they might enable us to test the hypothesis that simple life
can evolve on short timescales, or that the slow evolution
of complex life is not necessarily the norm. They might
even allow the potential for intelligent, space-travelling
civilisations to escape the destruction of their home
habitable zones during red giant stretches. Perhaps the
locations of advanced civilisations are even biased toward
such late-term habitable zones, Stern says.Other scenarios
Earth,meanwhile,mightfaceotherdestiniesjustaslethalas
burning to a crisp. Simulations by Jacques Laskar and Mickaël
Gastineau (both at Paris Observatory) have shown that
collisions or even ejections from the Solar System are possible
for the inner planets long before the Sun becomes a red giant.
Altogether, Laskar and Gastineau simulated 2,501 different
solutions to the movement of the Solar System’s planets over
thenext5billionyears.
Most of these calculations showed little change, which
is consistent with our basic understanding that the SolarSNURSERY MYSTERYWill the inevitable warming of Jupiter’s satellite
Europa (seen here in close-up, fractures webbing its ice-covered surface)
spur the evolution of life on the moon — if it hasn’t already evolved there?30 AUSTRALIAN SKY & TELESCOPENovember| December 2017Sun as red giant
0.668 solar mass M
2.
7.59 billion years from now
... and, finally, Earth. Mars will have drifted out far enough to escape the Sun’s grasp, but it will remain a dead world, the last of the inner planets.
FATE OF THE SOLAR SYSTEM