An article published in “The Astrophysical Journal Letters” offers an explanation of the abundance of sub-Neptune planets discovered compared to gas giants. A team of researchers led by Edwin Kite of the University of Chicago studied the characteristics of these planets offering as an explanation what they called fugacity crisis in reference to the term that measures how much more easily a gas dissolves into a mixture than it would be expected based on pressure. In the case of the sub-Neptune planets, the their atmosphere’s gas dissolve in the ocean of magma that probably covers the surface of their rocky core.
In recent years the number of known exoplanets has risen to some thousands allowing to create some statistics based on their size and mass. In the early years of the search for exoplanets, gas giants were the most abundant because they were easier to locate, but the situation changed a lot thanks to planet hunters such as NASA’s Kepler space telescope. Over the years, sub-Neptune planets, with a size slightly larger than the Earth, have become the most common and astronomers started wondering why certain planets seem to stop growing at at about three times the Earth’s size.
Edwin Kite is a planetary scientist who studies the surface and atmospheric characteristics of other planets at the University of Chicago’s Department of Geophysical Sciences and tried to apply his experience to the problem of the abundance of sub-Neptune exoplanets. Together with Bruce Fegley Jr., Laura Schaefer and Eric Ford he examined the possible evolution of exoplanets with a solid core whose surface is covered with magma, the likely situation of most of the planets close to what appears to be an upper limit of the size of about three times the Earth’s.
The bottom image (Courtesy Kite et al. All rights reserved) shows a graph illustrating exoplanets’ evolution possibilities. In the upper part a situation is shown in which there’s no fugacity crisis, where an entire and impermeable planetary core allows to commonly have gas giant planets. In the lower part, a fugacity crisis situation is shown where there’s an interaction between atmosphere and magma ocean on the surface of the rocky core with gas absorption.
According to Edwin Kite’s team’s reconstruction, as planets grow, magma starts absorbing part of the gas as a result of the increase in atmospheric pressure. Initially, absorption is steady but with increasing pressure hydrogen starts dissolving much more quickly into the magma.
This model shows a match with the data available on the discovered exoplanets. For further verifications, Edwin Kite stated that it will be possible to look for certain characteristics to understand which ones actually have a core with a magma ocean and which ones have a core that cooled down. There’s still a lot to discover but slowly these studies are helping to understand the processes of planetary evolution.
