Sedna, 2012 VP113 and several other objects belong to an
intriguing population of small, icy worlds that orbit the Sun far beyond Pluto.
These objects never come closer to the Sun than Neptune and have semimajor axis
(i.e. the “average” distance from the Sun) greater than 150 AU. They are
believed to be just a tiny fraction of a vast population of similar objects
lurking in the dark far from the Sun. Observations of the orbits of these
objects reveal a clustering of their arguments of perihelion around 0°. Such a
distribution is statistically unlikely and suggests the presence of one or more
super-Earths at 200 to 300 AU from the Sun “shepherding” the orbits of these
objects.
Figure 1: Artist’s impression of an object orbiting far from
the Sun.
However, the presence of one or more super-Earths at large
distances from the Sun is not supported by the current understanding of planet
formation which cannot account for the in-situ formation of such massive
objects so far from a Sun-like star. Nonetheless, a recent study suggests it
might be possible for one or more super-Earths to form in-situ at distances of
around 125 to 250 AU from a Sun-like star. Unlike a typical planet which may take
several million years to coalescence from a disk of protoplanetary material, it
takes one to several billion years for icy material far from a Sun-like star to
form super-Earths.
A disk of material with ~15 Earth-masses at 125 to 250 AU
from a Sun-like star can potentially form super-Earths in-situ. When the disk
material consists of planetesimals with initial radii of 1 cm to 1 m, super-Earths
can form in 1 to 3 billion years at 125 AU and in 2 to 5 billion years at 250
AU. Larger planetesimals with initial radii of 10 m to 10 km take longer to form
super-Earths; ~4 billion years or less at 125 AU and ~10 billion years or more
at 250 AU (i.e. longer than the age of the Solar System). Discovering a
super-Earth at ~200 AU from the Sun would imply the presence of a very large
reservoir of material far from the Sun and would also change the current
understanding of the layout of the Solar System.
Figure 2: Growth of the largest object at 125 AU for planetesimals
with initial radii of 10 cm to 10 km. Kenyon & Bromley (2015).
Figure 3: Growth of the largest object at 250 AU for planetesimals
with initial radii of 1 cm to 10 m. Planetesimals 10 m or more cannot grow into
super-Earths over the age of the Solar System. Kenyon & Bromley (2015).
References:
- S. J. Kenyon, B. C. Bromley (2015), “Formation of
Super-Earth Mass Planets at 125-250 AU from a Solar-type Star”, arXiv:1501.05659
[astro-ph.EP]
- C. de la Fuente Marcos, R. de la Fuente Marcos (2014), “Extreme
trans-Neptunian objects and the Kozai mechanism: signalling the presence of
trans-Plutonian planets”, arXiv:1406.0715 [astro-ph.EP]