Sky and Telescope - June 2016 - 19

S. SHEPPARD & C. TRUJILLO (3)

from the signiﬁcant amounts of dust present among the
growing planets. But two important observations suggest this wasn't the case.
First, most of the Trojans circle the Sun along orbits
that are highly inclined relative to those of the 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 that
the major planets didn't always occupy the calm, 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 captured after the planets settled into the more
stable, widely separated orbits they have now.
Conversely, small objects that approach the Trojan
regions today can't become captured permanently
because to do so they'd need to lose some of their 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 than 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.

NEPTUNE TROJAN Just 24th magnitude when discovered by the
author and Chadwick Trujillo, 2005 TN53 has a dynamically "hot"
(highly inclined) orbit that suggests Neptune captured it while
migrating outward to its current orbit early in solar system history.

However, both of these mechanisms assume that the
capture-prone candidates traveled 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 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 stabilized and circularized through interactions with the many bodies ﬂying
about the outer solar system. In this way small objects
- even those in "hot" orbits with relatively high inclinations or eccentricities - could suddenly ﬁ nd themselves
trapped in a Trojan region as that planet's orbit changed.
Thus the dynamics of the Trojans seem to conﬁrm

Outer-Planet Migration

Ejected
planetesimals

"Hot"
population
Jupiter
Trojans

Jupiter

Neptune
Trojans

Saturn

Uranus
Neptune
Increasing distance from Sun

"Cold"
population

Kuiper Belt
S&T: LEAH TISCIONE; SOURCE: A. MORBIDELLI & H. LEVISON

PLANETS ON THE MOVE The outer planets occupy stable, well-spaced orbits today. But a radically diﬀ erent 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 led to the capture of the Trojan asteroids.

Sk yandTelescope.com June 2016

http://www.SkyandTelescope.com

Sky and Telescope - June 2016

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