The Economist - USA (2019-12-21)

38 Planetary shenanigans The EconomistDecember 21st 2019

2 outer planets’ final distances from the sun,

the eccentricities of their orbits and their

inclinations. The effects of the migrating

giants on the planetesimals in the disk at

the time also explained why tnos have the

orbits they do, and why the orbits of the

Trojan asteroids associated with Jupiter are

highly inclined. “The Trojans, I think, were

the smoking gun,” says Dr Bottke. Before,

no-one had an account of why they were as

they were; after the Nice model, they did.

Nice as it was, the Nice model left some-

thing to be desired. The starting position of

the planets was highly specified, and did

not look like the arrangements seen in

planetary systems observed in the act of

formation around other stars. “So we de-

cided to redo everything,” says Dr Morbi-

delli. Nice II was published in 2011, and was

yet more dramatic. It showed Jupiter

throwing one of the other giant planets out

of the solar system entirely, which seemed

like a problem. But David Nesvorny, one of

the authors, suggested that it might not be.

Dr Nesvorny found that a five-planet

version of Nice II reproduced various char-

acteristics of the modern solar system ten

times more often than a four-planet ver-

sion of the model did. And the idea that Ju-

piter might have thrown a smaller sibling

into outer darkness was not unprecedent-

ed; models of the more dramatic migra-

tions needed to produce hot Jupiters

showed exoplanets being flung from their

stars with reckless abandon. The galaxy

may contain more such nomads than it

does stars.

Some wondered, though, if the prodigal

had stuck around, like a banished dog that

stays in sight of the campfire. In 2016 Mi-

chael Brown and Konstantin Batygin, at the

California Institute of Technology, pro-

posed that the orbits of some very oddly be-

haved tnos might be explained by a ninth

planet a bit smaller than Neptune that had

been flung between 10 and 40 times farther from the sun. Dr Mor-

bidelli is having none of it. But if Dr Brown is right and such a plan-

et does exist, it will have to have come from somewhere.

There is another problem that the migrants might explain. Ve-

nus is 90% as big as Earth; Mars only 10%. But models of the way

that rocky planets form from a disk of dust which successfully pro-

duce a couple the size of Earth and Venus have a strong tendency to

produce a third big planet farther out, rather than a runt.

One possibility is that, beyond the Earth, there was a dearth of

raw material. In 2011 Kevin Walsh of swri, with Dr Morbidelli and

others, proposed that giant-planet migration was again to blame.

They suggested that very early on in the history of the solar system,

before the orbital ballet of the Nice models, interactions between

Jupiter and the Sun-centred disk in which it was embedded drove

the giant planet towards the Sun. This is the same process thought

to be responsible for pushing the hot Jupiters in other systems so

close to their stars.

In this system, though, around the time when Jupiter got to

where Mars orbits today, it changed its mind and headed back

out—a manoeuvre the scientists dubbed the “Grand Tack”. They

think it came about because Saturn was following Jupiter inwards.

Inward-moving planets create gaps in the disk of dust

through which they travel. When the gap created by

Saturn reached Jupiter, it changed the way the planet

interacted with the disk, leading Jupiter to swing back

outwards—and take Saturn with it.

This was a lucky escape for the inner solar system;

had Jupiter plunged through it, Earth and Venus might

never have formed. But by getting as close as it did, Ju-

piter cleared out the disk around the orbit of Mars—

thus stunting its development. Had it never strayed

sunwards, the inner solar system might boast another

planet as large as the Earth—one which might have

done a better job of holding on to an atmosphere and

even an ocean than cold little Mars did. The Grand Tack

may have spared the Earth; but it may also have cost it a

habitable, perhaps even inhabited, neighbour.

Fanciful, perhaps. But as Dr Morbidelli says, “It’s

clear that the history of planetary systems is very dy-

namic.” And where there is dynamism, there is luck

and chance. Before there was a celestial clockwork,

there was, it seems, a celestial roulette wheel.

*

The

Sun 264 8 10 12

Sun 264 8 10 12

Present-day distance from Earth to the Sun (1 AU)

Jupiter

Jupiter'sinteractionswiththediskofmaterialcausedit tomigrateinwards,towardstheSun.

Thisscatteredsmallbodiesinthediskintoneworbits,oroutofthesolarsystemaltogether

Early in the solar system's history, the young Sun was circled by giant planets embedded in

a disk of material. The giant planets cleared gaps in this disk as they orbited

A similarprocesshappenedwithSaturn, whichmigratedinwards,alsoscatteringmaterial

initspath.EventuallyitsgapinthediskjoinedupwiththatcreatedbyJupiter

ThischangedthedynamicsofJupiter'smovement,reversingthedirectionofits

migration,andcausingit to“tack”awayfromtheSun,andtakingSaturnwithit

Therockyplanets(Mercury,Venus,EarthandMars)thenformedfromthe

remainingdebris,withtheleftoversformingtheasteroidbelt

Saturn

Saturn

Rocky

planets

Uranus

Uranus

Neptune

Neptune

Once

upona

time

70,000

years

later

100,000

years

later

5 0,000,

s

l ter

1.5m

years

later

Jupiter

Inner material Outer belt

Source: Alessandro Morbidelli

Asteroid belt