Forming Icy Super-Earths in the Cold

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]