Australian Sky & Telescope — November-December 2017

28 AUSTRALIAN SKY & TELESCOPENovember| December 2017

ILLUSTRATION: DETLEV VAN RAVENSWAAY / GETTY IMAGES; EXPANDING SUN: G. DINDERMAN /

S&T

, SOURCE: K.-P. SCHRÖDER (UNIV. OF GUANAJUATO) & R.C. SMITH (UNIV. OF SUSSEX) /

MNRAS

, 2008

much of it now in the atmosphere, our boiling, humid planet

will enter amoist greenhousestage. Over the next billion

years, Earth’s surface will become so seared that not even

heat-loving 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 adry greenhousephase. Eventually Earth will

becomeessentiallyanotherVenus,withsurfacetemperatures

reaching 400°C, hot enough to melt lead.

“You know theKansassong,‘DustintheWind’?”

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:

“Wetendtothinkofourcosmicenvironmentasaconstant,

but it’s actually ever-changing. It’s just changing on timescales

muchlargerthanoneswe’reusedto.”Thatseemslikeamore

orderly model for how the universe works, he says, than one in

which our celestial neighbourhood remains static.

The gas giants

AstheSunmatures,Jupiter,Saturn,NeptuneandUranuswill

alsolosemass,andatratesfasterthantoday.Buteffectswill

be minor relative to the gas giants’ sizes. Any perturbation

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 such as 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 call 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.

Sun as red giant
0.95solar mass rth
5 a.u.

Mars

1.60 a.u.

7.588 billion years from now

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

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.

FATE OF THE SOLAR SYSTEM