Title: The destruction of inner planetary systems during high-eccentricity migration of gas giants
Authors: Alexander J. Mustill, Melvyn B. Davies, Anders Johansen
First author’s institution: Lund Observatory, Department of Astronomy & Theoretical Physics, Lund University
Status: Submitted to ApJ
Hot Jupiters are weird and lonely little planets. Well, they’re huge. But otherwise: They’re weird in that they surprised astronomers when we started finding them, giant planets orbiting improbably close in to their stars, as close as 0.015 AU from their stars. (Earth, remember, orbits at 1 AU. Mercury’s at 0.307.) Their presence there clashed with all our ideas about planet formation, modeled on our own solar system.
According to current theories of planet formation, there’s not nearly enough material close to a star to form a planet of Jupiter-ish size. But current theories of planet formation also allow for migration, so there you go—hot Jupiters could have formed farther out from their stars, where raw materials are plentiful, and then later they can migrate in.
Complicating the picture, though, is the fact that hot Jupiters are usually found alone. Small, low-mass planets are common enough in the close orbits that hot Jupiters frequent, except when a hot Jupiter is there.
On the migration end of things, there are, broadly speaking, two possible explanations for hot Jupiters’ positions: Type II migration during planet-formation, moving inward through the gas-rich protoplanetary disk; or, later on, once planets have formed, through gravitational scattering as a giant planet in an eccentric orbit interacts with small planets closer in to the star. But Type II migration doesn’t explain hot Jupiters’ lonely neighborhoods—that migration would happen early enough to leave plenty of planet-forming material undisturbed in its wake. Hot Jupiters’ lack of nearby companions could, of course, be explained by other means, but today’s paper tests the idea that hot Jupiters could be killing two birds with one migrational stone: could late migration via gravitational interactions bring hot Jupiters into their tight orbits while also getting rid of any small, close-in companion planets?
The authors of this paper tested a series of scenarios that are a mix of known circumstance and hypothetical orchestration. They chose four real-world Kepler candidate systems of three low-mass, close-in planets, and added to the mix an imaginary Jupiter on an eccentric orbit with a small pericenter (meaning that on its closest approach to the star, it came very close). For each system-plus-Jupiter, the authors calculated a suite of simulations, each time testing slightly different properties for the giant planet (pericenters ranging from 0.01 AU to 0.25 AU, prograde and retrograde orbits) as well as variations in the small planets’ orbital inclinations.
Even with all of those variations, most of the simulations ended in one of two scenarios as the giant planet migrated in from an eccentric orbit toward a nearly circular one very close to the star: either the low-mass planets were destroyed and the hot Jupiter was left alone, or the giant planet was ejected and one to three low-mass planets were left behind. (In both cases, low-mass planets could be lost by collisions with the star, each other, or the giant planet itself. It’s worth noting that the giant planet’s eccentric orbit often crossed the paths of the orbits of the other planets.) Basically, if a giant planet migrates in to a tight enough orbit to be called a hot Jupiter, it will, in the process, get rid of any other tight-orbit planets. If a giant planet doesn’t eject or destroy those other planets, it won’t become a hot Jupiter.
The mysteries of hot Jupiters are by no means all settled. This is just one proposition, and even at that, it starts with the giant planet in an eccentric orbit, not testing the plausibility of the starting conditions (although they are implied by earlier research). The more we learn about exoplanet systems, the more the diversity of the cosmos and perhaps our own solar system’s weirdness become apparent.