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30 AUSTRALIAN SKY & TELESCOPE JULY 2016

tug of war. As the giant planets shifted and migrated

from where they formed to their current arrangement,

they scattered countless smaller objects. As planetary

formation ceased, their orbits slowly stabilised and

circularised through interactions with the many

bodies flying about in the outer Solar System. In this

way small objects — even those in ‘hot’ orbits with

relatively high inclinations or eccentricities — could

suddenly find themselves trapped in a Trojan region as

that planet’s orbit changed.

Thus the dynamics of the Trojans seem to confirm

that the giant planets did migrate significantly

early in the Solar System’s history. Moreover, this

capture scenario implies that the Trojan objects likely

originated in the outer Solar System, not the asteroid

belt. That’s because the giant planets most likely

pushed each other outward into a distant belt of small

objects that had not yet collected into a planet due to

the large volume of space and low collision velocities

out there.

(As an interesting aside, it’s also possible that many

asteroids now occupying the outer main belt, including

the dwarf planet 1 Ceres, might have originated much

farther out in the Solar System and then were flung

closer to the Sun as the planets moved around.)

But outer planet migration leaves one perplexing

observation unresolved: Jupiter’s leading (L 4 ) Trojan

cloud has about 50% more objects than its trailing L 5

cloud does. That’s not the case for Neptune, whose

Trojan clouds appear to have comparable numbers of

objects (based on the few currently known).

This discrepancy is challenging to explain with any

of the capture scenarios mentioned so far, all of which

statistically should not favour one of Jupiter’s Trojan

clouds over the other. David Nesvorný (Southwest

Research Institute) and colleagues have suggested that

this difference in populations could be the consequence

of a third ‘ice giant’ — a sibling of Uranus and Neptune

— passing through Jupiter’s L 5 region and scattering

away many of the bodies that had accumulated there.

This massive body would also have altered Jupiter’s orbit

significantly before the giant planet ejected it entirely

from the Solar System.

This type of planet-planet interaction likely occurred

often in the Solar System’s early history, and it might

explain how a massive, eccentrically orbiting ninth planet

— if it exists (AS&T: May/June 2016, p. 8) — ended up a

few hundred astronomical units from the Sun.

The stuff of Trojans

We don’t yet know the Trojans’ compositions, as they

are relatively small and faint (Hektor, the largest,

EXOPLANET TROJANS?

Trojan objects likely exist in the orbits of exoplanets as well. Astronomers

have searched for signs of these ‘exo-Trojans’ by looking for transit events

ahead and behind of known transiting exoplanets, but they’ve not observed

anything definitive yet.

0 10 20 30

0.10

0.05

0

0.15

Proper eccentricity

Libration amplitude

0 10 20 30

20

10

0

30

40

Proper inclination

Libration amplitude

‘HOT’ ORBITS Computer

simulations of Trojans captured

by Jupiter during a hypothetical

early migration of the outer

planets (large dots) yield the

same wide variety of orbits

exhibited by known Trojan

objects (small dots). S&T:

LEAH TISCIONE; SOURCE: A. MORBIDELLI ET AL. / NATURE (2)

PITCHED

BATTLEAchilles

(centre),akey

figureinthe

Trojan War, is the

namesake for the

first discovered

Jupiter Trojan.

AUGUSTECOUDER,©MARIE-LANNGUYEN/WIKIMEDIACOMMONS/CCA 2.5

Trojan Asteroids