Australian Sky & Telescope — November-December 2017

32 AUSTRALIAN SKY & TELESCOPE November | December 2017

ORBITS: G. DINDERMAN /

S&T

, SOURCE: J. LASKAR /

PROGRESS IN MATHEMATICAL PHYSICS

2013; EXPANDING SUN: G. DINDERMAN /

S&T

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

MNRAS

, 2008

Systemhasn’tevolvedmuchin4.5billionyears.Butin1%of

cases, Mercury’s orbital eccentricity increases considerably.

Thiscausestroubleinmorethan200oftheFrenchteam’s

simulations. In 29 cases, Earth and Mars collide, for instance,

andin18othersEarthandVenusdo.Batygin,inaseparate

study with Laughlin, turned up similar probabilities for errant

behaviour.Theirexperimentsyieldedoneevolutioninwhich

MercuryfallsontotheSunabout1.2billionyearsfromnow,

andanotherinwhichMercuryandVenuscollideabout862

million years forward.

Laskarfoundtwoaspectsofhisfindingssurprising.The

first is that the likelihood that such catastrophic events

couldoccurwasrelativelyhighandnotjustamathematical

curiositywithavanishinglylowprobability.Thesecond

wasthatifhereliedonlyonNewtonianmechanicsinhis

simulations, and not on general relativity, the probability of

such collisions within 5 billion years rose to 60%. “So the

oddsgofroma1%chanceofhavingabigproblemforthe

terrestrialplanetstoabouta60%chance,”Laughlinsays,

referringtoLaskar’scalculation.“It’skindofironicthatthe

failureofNewton’slawthroughgeneralrelativityiswhat

actuallystabilisestheSolarSystemandallowstheEarthto

be here.”

ThechancethatMercurywillenterthekindofsecular

resonancethat would result in a radically altered orbit is low,

however. “It has to be relatively low, because we’re still here,”

Batyginsays.Ifitwerehigh,theSolarSystemlikelywould

haverearrangeditselfalreadyinthe4.5billionyearssince

theplanetsassumedtheircurrentorbits.ButhisandLaskar’s

findings do provide food for thought. Understanding how

thatinstabilitygetsturnedon—ifit ever does — is essential

to understanding how planets evolve.

Earth might also get banished from the Solar System or

even kidnapped. In a study that investigated the likelihood

within the next 3.5 billion years of our Solar System’s planets

scattering due to the perturbing influence of passing stars,

Laughlin and Adams put the chance of Earth being expelled

from the Solar System at 1 in 400,000, or captured by a

passing star at 1 in 2 million. While our Solar System may

seemisolated,intheimmensityoftimestarsdodriftnearby.

A billion threads

In the end, all we know is that the Solar System has been

stable for a long time and may remain so for a long time yet,

but if and when it becomes unstable, anything could happen.

“You might find planets scattering all over the place,” says

Smith. “Which planets would do what, is anybody’s guess.”

Batygin, again, takes a philosophical stance. “The Solar

System, despite its seeming regularity and immutability, is

actually an unpredictable beast.” He likens it to a double

pendulum, an ostensibly simple system that can exhibit

unpredictable behavior. Change an initial condition even

slightly, and the long-term evolution of the double pendulum —

or the Solar System — can change incalculably. “We have to stop

thinking of the evolution of the Solar System as one thread that

will take us somewhere,” he says. “We have to start thinking

about it as a statistical ensemble of a billion threads, all of which

are pinched together by today. The Solar System could take any

one of them, and we can’t predict exactly which.”

■ PETER TYSON is editor of the US edition of this magazine.

A

C

B

D

SCOLLISION COURSES These four examples of the long-term

evolution of the inner-planets’ orbits show Mercury (white), Venus (green),

Earth (blue) and Mars (red). In (a), roughly our current time, all is well. But

in 1% of cases, Mercury’s orbit can deform enough for it to collide with

either the Sun or Venus (b). One simulation had the eccentricity of Mars’

orbit increase enough to allow a close encounter or collision with Earth

(c). This eccentricity leads to a destabilisation of the inner planets such

that Venus and Earth could collide (d).

SLAKES ON PLUTO? As the Sun matures, all that nitrogen, carbon

monoxide and methane ice in Sputnik Planum may become liquid.

FATE OF THE SOLAR SYSTEM