Saturday, October 25, 2014

Scattering of Super-Earths

In the core-accretion model, the formation of gas giant planets begins with the rapid coalescence of solids to form massive cores. When a massive core reaches ~10 times the mass of Earth, it accretes gas in a runaway fashion and eventually becomes a full-fledged gas giant planet. During the formation of gas giant planets around a young star, multiple massive cores can form. These massive cores can be gravitationally scattered out to large distances by other massive cores and by newly-formed gas giant planets. Typically, over 80 percent of massive cores between 1 to 15 times the mass of Earth get scattered out to the peripheries of their natal planetary systems.

The scattering process hurls the massive core, also known as a scattered planet, onto an eccentric orbit which takes the planet out to a large distance from its host star. Subsequently, the planet can interact with the gaseous disk around its nascent host star. Planet-disk interactions can damp the planet’s orbital eccentricity, causing the planet to settle into a more circular orbit in the outer regions of its planetary system. However, the outcome of such a planet-disk interaction depends on the planet’s mass and both the characteristics and evolution of the gaseous disk.

A scattered planet needs to be at least as massive as the Earth in order for its orbital eccentricity to be damped. In fact, orbital circularization is most effective for massive planets as they interact most strongly with the gaseous disk. The orbits of scattered Earth-mass or smaller planets tend to remain eccentric. Gaseous disks that are more massive and longer-lived can damp eccentricities more effectively than those which are less massive and short-lived. Additionally, gaseous disks that decay with an expanding inner cavity can circularise orbits at larger distances than those which dissipate homogeneously.

Typically, a gaseous disk around a young star has a lifespan of a few million years before it decays completely. Gaseous disks that dissipate homogeneously are only effective in circularising the orbits of more massive super-Earths and Neptune-mass planets. The orbits of these planets are circularised at smaller orbital distances (i.e. within 50 AU for Sun-like stars). For smaller planets, planet-disk interactions are less effective and these planets tend to remain in moderately eccentric orbits that continue to take them out to larger orbital distances (i.e. ~200 AU for Sun-like stars). Gaseous disks that decay with an expanding inner cavity can circularise the orbits of super-Earths at orbital distances beyond 100 AU. Smaller planets can remain at larger orbital distance, although their orbital eccentricities cannot be damped completely.

The orbital parameters of a few distant trans-Neptunian objects such as Sedna and 2012 VP113 suggests it is plausible a super-Earth with 2 to 10 times the mass of Earth is lurking in a low-eccentricity orbit between 200 to 300 AU from the Sun. If this planet exists, it is unlikely to have formed where it currently is and also unlikely to have migrated from inside 30 AU to its present orbit. However, the existence of such a planet in the far outer reaches of the Solar System is conceivable if it was scattered from inside 30 AU and subsequently interacted with a massive gaseous disk around the young Sun. Such a gaseous disk would have to dissipate from the inside out in order for planet-disk interactions to “circularise” the planet’s orbit to where the planet is currently predicted to reside.

Bromley & Kenyon (2014), “The Fate of Scattered Planets”, arXiv:1410.2816 [astro-ph.EP]