Sky and Telescope - October 2017 - 26

Fate of the Solar System

of it now in the atmosphere, our broiling, humid planet will
enter a moist greenhouse stage. Over the next billion years,
Earth's surface will become so seared that not even heatloving bacteria will survive. By about 3 billion years from
today, according to cloud-free climate models run by James
Kasting (Penn State University), the planet will have entered
a dry greenhouse phase. Eventually Earth become essentially
another Venus, with surface temperatures reaching 400°C
(750°F), hot enough to melt lead.
"You know the Kansas song, 'Dust in the Wind'?" Konstantin Batygin (Caltech) asked me rhetorically. "You know,
'Nothing lasts forever but the Earth and sky'? It's clear those
guys had not carefully considered the red giant branch of the
Sun's evolution." Batygin for one ﬁnds the notion that our
planet has an expiration date philosophically satisfying: "We
tend to think of our cosmic environment as a constant, but
it's actually ever-changing. It's just changing on timescales
much larger than ones we're used to." That seems like a more
orderly model for how the universe works, he says, than one
in which our celestial neighborhood remains static.

The Gas Giants
As the Sun matures, Jupiter, Saturn, Neptune, and Uranus
will also lose mass, and at rates faster than today. But effects
will be minor relative to the gas giants' sizes. Any pertur-

bation of their orbits will likely be small as well, says Jack
Lissauer (NASA Ames Research Center). In fact, working with
Martin Duncan (Queen's University, Ontario, Canada), Lissauer determined through detailed simulations that the orbits
of the giant planets will likely remain stable for more than 10
billion years, and possibly far longer than that.
What most interests astronomers is what will happen to
some of their moons. As the red giant Sun mushrooms in
size and our solar system's habitable zone - deﬁned as where
liquid water can exist on a body's surface - moves outward,
currently frozen worlds like Jupiter's Europa or Saturn's
Titan will warm up. The question is: Will they warm up long
enough to perhaps evolve life (if they haven't already)?
These distant moons will certainly heat up sufﬁciently to
partly melt. On Titan, for instance, Ralph Lorenz (University of Arizona) and colleagues envision that, about 6 billion
years from now, a window of several hundred million years
will open when liquid water-ammonia could form oceans
on Titan's surface and react with the abundant organic
compounds there. While water-ammonia would be toxic to
Earth's organisms, it might provide a viable solvent for prebiotic and possibly biotic chemistry on Titan and elsewhere.
(They dub it a "primordial gazpacho.") Such conditions on
Titan might last 500 million years, which is longer than it
took life to develop on Earth, they note.

7.588 billion years from now
Sun as red giant
0.95 solar mass

Earth
1.05 a.u.

. . . into a red giant over the next 7.5 billion years, will first engulf Mercury and Venus . . .

O C T O B E R 2 0 17 * S K Y & T E L E S C O P E

Mars
1.60 a.u.

ILLUSTR ATION: DE TLE V VA N R AV ENSWA AY / G E T T Y IM AG ES; E X PA NDING SUN: G. DINDER M A N / S&T,
SOURCE: K .-P. SCHRÖDER (UNIV. OF GUA N A JUATO) & R.C. SMITH (UNIV. OF SUSSE X ) / MNR AS, 20 0 8

SCORCHED EARTH About 3 billion years from now, our planet will enter the dry greenhouse,
with temperatures approaching those on the surface of Venus today. No life will be possible.

Sky and Telescope - October 2017

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