Sky and Telescope - June 2016 - 20

AUGUSTE COUDER, © MARIE-LAN NGUYEN
/ WIKIMEDIA COMMONS / CCA 2.5

Trojan Asteroids

PITCHED BATTLE Achilles (at center), a key ﬁgure in the Trojan War, is the namesake for the ﬁrst discovered Jupiter Trojan.

E XO PL AN E T TR O JAN S ?
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
observed nothing deﬁnitive yet.

cloud has about 50% more objects than its trailing L5
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 favor 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 L5 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 (S&T: May 2016, p. 11) - ended up a
few hundred astronomical units from the Sun.

The Stuﬀ of Trojans
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

We don't yet know the Trojans' compositions, as they
are relatively small and faint (Hektor, the largest, averages 225 km across). Moreover, their spectra are mostly
featureless.
What we do know is that they're very dark, with
average reﬂectivities of around 5% to 7%. Most Jupiter
Trojans can be classiﬁed as either "dark red" (D-type) or
"less red" (P-type) asteroids. A few have more neutral coloring, akin to the carbon-rich C-type objects seen in the
outer asteroid belt. Intriguingly, a cluster of Trojans in
Jupiter's L 4 cloud have very similar orbits, and most have
the same C-type surface coloration. Dynamicists have
proposed that this group, known as the Eurybates family,
came from a single broken-up parent body.
The reddish D-types that dominate Jupiter's Trojan

0.15

Proper eccentricity

Proper inclination

40°

30°

20°

"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).

0.10

0.05

10°

0°
0°

10°

20°

Libration amplitude

June 2016 sky & telescope

30°

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

0°

10°

20°

Libration amplitude

30°

Sky and Telescope - June 2016

Table of Contents for the Digital Edition of Sky and Telescope - June 2016