28 AUSTRALIAN SKY & TELESCOPE JULY 2016
Picking planetary partners
Max Wolf discovered 588 Achilles, the first asteroid
known to share Jupiter’s orbit, in 1906. Observers
have since found about 6,300 Jupiter Trojans — so
many, in fact, that the entire population is likely only
slightly smaller than that of the main asteroid belt and
includes almost a half million bodies larger than 1 km
in diameter.
Jupiter’s Trojans are named after mythological
characters from the Trojan War. The leading (L 4 )
region is the ‘Greek camp’; objects there are named
after participants on the Greek side of the war. Those
in the trailing (L 5 ) region are named after participants
from the ‘Trojan camp’ of the conflict. Each group has
one object named for a combatant from the opposing
side: 617 Patroclus is in the L 5 swarm, and 624 Hektor
is in the L 4 swarm.
Saturn and Uranus have no known stable Trojans
(though at least two of Saturn’s moons have them).
This is because the Lagrangian zones of Saturn and
Uranus are highly perturbed by the gravity of the
other giant planets that sandwich these worlds’ orbits.
Unlike the ‘embedded’ locations of Saturn and
Uranus, Neptune is a ‘bookend’ giant planet, as is
Jupiter, making its Lagrangian regions more stable.
Numerical simulations suggest 20% to 50% of
Neptune’s original Trojan population should have
survived for the age of the Solar System after any
significant planetary movement. Observers discovered
the first object in Neptune’s L 4 location in 2001 and
the first one at L 5 in 2008. The known Neptune Trojan
population, currently at 12, is low only because these
bodies are much harder to see at their extreme distance
from the Sun. We’ve started to name Neptune’s Trojans
after the Amazons, an all-female warrior tribe that
sided with the Trojans during the war.
As for the terrestrial planets, Earth lacks any
known Trojans from primordial times, probably
because gravitational perturbations from Venus, Mars
and Jupiter would have destabilised their orbits over
the age of the Solar System. However, our planet does
have one known Trojan in its L 4 region, a small body
called 2010 TK 7. This was likely an asteroid in an
Earth-crossing orbit that just happened to pass near
Earth’s L 4 region a few thousand years ago and became
temporarily trapped there. Chances are good that 2010
TK 7 will likely escape its confinement within another
few thousand years.
Mars also has a few known Trojans at both
its L 4 and L 5 points. Those locations appear to be
dynamically stable over the age of the Solar System, so
these objects could be primordial and related in some
way to the Jupiter and Neptune Trojans.2 astronomical unitsHill
sphereSun JupiterL 5L 4 THE FOLLOWERS
To date, observers have
identified nearly 6,300 objects
in the two Trojan ‘camps’ that
accompany Jupiter. Curiously,
the leading ‘Greek’ group is
roughly 50% more populous
than the trailing ‘Trojan’ group.
The dashed circle around
Jupiter shows itsHill sphere,
which is the limit of stability of
any of the planet’s moons.NeptuneUranusSaturnSun JupiterUNEQUAL DISTRIBUTION
JupiterandNeptunehavecaptured
many stable Trojan asteroids. But
Saturn and Uranus have none, likely
because their Lagrangian zones
are subject to the gravitational
perturbationsoftheirmassive
planetary neighbours and thus are
not stable.
A LUNAR CONNECTION?
If a very large object struck our planet early in the Solar System’s history
to form the Moon, could it have been an Earth Trojan? Geochemists think
such an impactor would have needed a composition quite similar to
Earth’s and a not-too-fast encounter velocity. An object that initially shared
Earth’s orbit could satisfy both of these conditions.NeptuneSunL 4 L 5NEPTUNE’S RETINUE
Although only 12 Neptune Trojans
are known, the planet’s Lagrangian
regions are likely even more densely
populated than Jupiter’s. The L 5
region appears sparser because this
region is currently aligned with the
galactic plane, making it harder to
detect objects there.Trojan Asteroids
S&T: LEAH TISCIONE; SOURCE: SCOTT SHEPPARD (3)