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Interplanetary kidnap

Nature volume 441, pages 162163 (11 May 2006) | Download Citation

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Triton, Neptune's largest moon, was probably part of a two-body object similar to the Pluto–Charon system. This tandem might have been ripped apart when it strayed too close to the planet that Triton is now orbiting.

The neptunian moon Triton weighs in at 1.4 times the mass of Pluto, making it the largest irregularly orbiting satellite in the Solar System. So how did this kept giant come to be where it is? On page 192 of this issue1, Agnor and Hamilton advance a capture mechanism that, if correct, could have repercussions for the life stories of other, similar moons.

A cohort of satellites surrounds all four giant planets in the Solar System — Jupiter, Saturn, Uranus and Neptune. These satellites are divided into two distinct groups, regular and irregular, according to their orbital characteristics. Regular satellites are closer to their parent planet, with orbits that are essentially circular and that lie on the planet's equatorial plane. These satellites thus constitute miniature solar systems around their planet. And just as the planets of the Solar System are thought to have formed from a disk of gas and dust (the protoplanetary disk) orbiting the Sun, so the regular satellites are assumed to have formed from a ‘planetesimal disk’ orbiting their planet.

Irregular satellites, in contrast, are more distant from their planet and typically have orbits with larger eccentricities (a measure of deviation from a perfect circle) and/or inclinations. About half the irregular satellites orbit their planet in the retrograde direction; that is, in the opposite direction to the rotation of their planet. Because of these strange orbital characteristics, the general assumption is that these satellites formed on heliocentric orbits, and only later were captured on elliptic orbits around giant planets.

Several mechanisms have been proposed for this capture process. Some invoke the effect of gas-drag exerted by the atmospheres of the planets, which were more extensive when the planets formed some 4.5 billion years ago than they are now, owing to the heat generated by the accretion process. Others posit the abrupt growth of a planet's mass (the ‘pull-down mechanism’) as the culprit. Still others employ gravitational interactions or collisions with the system of regular satellites already established around the planet, or fortuitous encounters with other planetesimals on heliocentric orbits as they passed through the sphere of influence of the planet.

But none of these mechanisms seem appropriate for Triton, Neptune's huge retrograde companion. Its mass — in another word, its inertia — means it was unlikely to have been captured by interactions with the existing satellites or other passing planetesimals. Additionally, Neptune is assumed to have undergone slow growth, and never to have had an extended atmosphere; both the pull-down and gas-drag mechanisms would therefore have been inefficient.

Agnor and Hamilton postulate1 that Triton was originally part of a binary object, for instance similar to the Pluto–Charon system. They show that if this binary had passed sufficiently close to Neptune at low velocity, the different forces acting on its two constituent bodies would have ripped them apart. Each of the constituents would have, relative to Neptune, a velocity that was essentially the vector sum of the velocity of the binary's barycentre (its centre of gravity) and its own orbital velocity relative to this barycentre (Fig. 1). Most of the time, the orbital motion of one of the bodies is opposed to the barycentre's motion, so the net velocity of this body relative to Neptune could easily have been smaller than the escape velocity from Neptune's gravitational field. Thus, it would have become captured in a bound, planetocentric trajectory.

Figure 1: Imminent capture.
Figure 1

A two-body object approaches Neptune, the scenario envisaged by Agnor and Hamilton1 for the capture of Triton. Both constituents of the binary rotate around the trajectory of their mutual centre of mass (barycentre), such that their own motion is almost half of the time with, and almost half of the time against, the motion of the binary as a whole. The net velocity of the constituents relative to the planet is accordingly increased or reduced; if the net velocity of a constituent drops below the escape velocity from a planet's gravitational field, it is captured, entering a new life as an irregular satellite of the planet.

To be a likely explanation of Triton's capture, this model requires that two conditions be met. First, the protoplanetary disk in which Neptune evolved must have contained a very large number of Pluto-sized objects2. This condition cannot be checked directly, but is plausible: the protoplanetary disk is presumed to have had a total mass of about 50 Earth masses3,4, or 5,000 times greater than that of the Kuiper belt5. (This relic of the protoplanetary disk6, where Pluto resides, now orbits the Sun beyond Neptune.) Second, a substantial fraction of the large objects in the protoplanetary disk must have been binary. The likelihood of this is increased by the observation that between 10 and 15% of the objects in the Kuiper belt are two-bodied7. In addition, three of the four largest Kuiper-belt objects — in decreasing order of size, 2003 UB313, Pluto and 2003 EL61 — have satellites8. (The third largest, 2005 FY9, is the odd one out.)

Although Agnor and Hamilton focus exclusively on Triton, it is tempting to conjecture that this mechanism applies to the capture of most of the irregular satellites. It has been pointed out that for all four giant planets, the number of irregular satellites larger than a specific size9 is about the same. This fact argues against the gas-drag and pull-down mechanisms for their capture: because the flux of planetesimals through the giant planets' orbits was about the same7, both mechanisms should have been much more effective for Jupiter and Saturn, which grew rapidly as gas giants, than for Uranus and Neptune, which formed more slowly in a gas-starved environment10.

The only thing that the giant planets have in common is the size of their sphere of gravitational influence, or Hill sphere. This fact, together with the giant planets' similar number of irregular satellites, suggests that some sort of two-body interaction inside the Hill sphere played the dominant role in the capture of such satellites. Additional support for this picture comes from a model proposed by our group11 of the origin of the Late Heavy Bombardment. This model implies that the irregular satellites were captured during this period between 4 billion and 3.8 billion years ago, which was characterized by a large number of collisions of asteroids and comets with the terrestrial planets. This was well after the disappearance of the gas and the growth of the planets.

The problem with two-body interactions is that the encounter in the vicinity of a planet of two planetesimals on independent heliocentric orbits is extremely improbable. The capture of irregular satellites from binary objects brilliantly circumvents this problem as, by definition, the two interacting bodies approach together.

I predict that the model proposed here for the capture of Triton will rapidly become a mainstay for models of the origin of irregular satellites, one of the principal open problems in planetary science. In general, irregular satellites are much less massive than Triton, and the capture mechanism requires that their orbital velocity inside the binary is large enough to cancel out a substantial fraction of the velocity of the binary barycentre. Because this velocity increases with the total mass of the binary, these irregular satellites must originally have been secondary members of binaries with a large primary constituent. Such capture conditions, once thoroughly investigated, will unveil important constraints on the structure of the primordial protoplanetary disk.

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  1. Alessandro Morbidelli is at the Laboratoire Cassiopée, Observatoire de la Côte d'Azur, BP 4229, 06304 Nice Cedex 4, France. morby@obs-nice.fr

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