A (6)

Origin of the Trojans

In theory, the Trojans could have formed in place and
stayed put since the first stages of planet formation.
After all, the Trojan regions would have been relatively
stable early on, and objects could have accumulated
there from the significant amounts of dust present
among the growing planets. But two important
observations suggest this wasn’t the case.
First,mostoftheTrojanscircletheSunalong
orbitsthatarehighlyinclinedrelativetothoseofthe
planets. These large inclinations would have limited
how much material could have been accreted from the
protoplanetary disk.
Second, and more importantly, it’s become clear
thatthemajorplanetsdidn’talwaysoccupythecalm,
nearly circular orbits that we see today. Instead, the
primordial Solar System was a chaotic place, with the
giant planets likely much closer to one another. They
repeatedly ‘pushed each other around,’ resulting in
significant shifts in their orbits. Simulations show
that the Lagrangian regions would have become
unstable during any significant planetary movement
or migration. So the Trojans we see today were likely
capturedaftertheplanetssettledintothemorestable,
widely separated orbits they have now.
Conversely, small objects that approach the Trojan
regions today can’t become captured permanently
becausetodosothey’dneedtolosesomeoftheir
orbital energy — and there’s no easy way to do that.
(This was not the case when the planets first formed
and were still evolving, when the Solar System
structure was vastly different to what it is now.) Instead,

modern-day interlopers might linger for a time, but

they eventually leave the same way they entered.

So how did all those Trojans get captured? Several

mechanisms have been proposed, most operating

only when the disk of material from which the planets

formed still contained significant amounts of dust and

gas with many small objects flying about. At that early

time, friction from gas drag or higher collision rates

could have allowed the capture of Trojans.

However, both of these mechanisms assume

that the capture-prone candidates travelled in low-

inclination orbits around the Sun — in order to make

energy-robbing gas drag more efficient and collision

probabilities more likely. Theorists refer to these kinds

of orbits as dynamically ‘cold’. But the highly inclined

orbits of the Trojans attending both Jupiter and

Neptune suggest that those objects were dynamically

‘hot’ when captured. So the gas-drag and collision

scenarios are not likely the true cause.

Most probably, as proposed in 2005 by Alessandro

Morbidelli (Nice Observatory, France) and colleagues,

the Trojans were pawns in a dramatic interplanetary

NEPTUNE

TROJAN Just

24th magnitude

when discovered

by the author and

Chadwick Trujillo,

2005 TN 53 has

a dynamically

‘hot’ (highly

inclined) orbit that

suggests Neptune

captured it while

migrating outward

to its current orbit

early in Solar

System history.

Ejected

planetesimals

Jupiter Saturn Uranus Neptune Kuiper Belt

“Hot”

population

“Cold”

population

Neptune

Trojans

Jupiter

Trojans

Increasing distancefrom Sun

PLANETS ON THE MOVEThe outer planets occupy stable, well-spaced orbits today. But a radically different view suggests that early in Solar
System history they were bunched much tighter together and closer to the Sun. Gravitational interactions pushed them apart, a dramatic orbital
migration that also led to the capture of the Trojan asteroids.

OUTER-PLANET MIGRATION

S. SHEPPARD & C. TRUJILLO (3)

S&T:

LEAH TISCIONE; SOURCE: A. MORBIDELLI & H. LEVISON