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