Sub-Neptunes — extrasolar planets with radii between 2.7 and 3 times that of Earth — are much more numerous than Neptune-sized and larger planets. A new study proposes that this drop-off is so abrupt because atmospheres of sub-Neptunes readily dissolve into magma oceans on their surface once the planets reach about 3 times the size of Earth.
An artist’s impression of a super-Earth and two sub-Neptunes in the TOI-270 planetary system. Image credit: Sci-News.com.
“This is a cliff edge in the data, and it’s quite dramatic. What we have been puzzling over is why planets would tend to stop growing beyond about 3 times Earth’s size,” said Dr. Edwin Kite, a planetary scientist at the University of Chicago.
Sub-Neptunes are thought to have oceans of magma on their surfaces, which are kept hot by a thick blanket of hydrogen-rich atmosphere.
“So far, almost all models we have ignore this magma, treating it as chemically inert, but liquid rock is almost as runny as water and very reactive,” Dr. Kite said.
The question Dr. Kite and his colleagues considered was whether, as the planets acquired more hydrogen, the ocean might begin to dissolve the atmosphere.
In this scenario, as a sub-Neptune acquires more gas, it piles up in the atmosphere, and the pressure at the bottom where the atmosphere meets the magma starts to build.
At first, the magma takes up the added gas at a steady rate, but as the pressure rises, the hydrogen starts to dissolve much more readily into the magma.
“Not only that, but the little bit of the added gas that stays in the atmosphere raises the atmospheric pressure, and thus an even greater fraction of later-arriving gas will dissolve into the magma,” Dr. Kite explained.
Thus the planet’s growth stalls out before it reaches the size of Neptune.
The study authors call this the ‘fugacity crisis,’ after the term that measures how much more readily a gas dissolves into a mixture than what would be expected based on pressure.
“The theory fits well with existing observations,” Dr. Kite noted.
“There are also several markers that astronomers could look for in future.”
“For example, if the theory is correct, planets with magma oceans that are cold enough to have crystallized on the surface should display different profiles, since this would prevent the ocean from absorbing so much hydrogen.”
The study was published in the Astrophysical Journal Letters.
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Edwin S. Kite et al. 2019. Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis. ApJL 887, L33; doi: 10.3847/2041-8213/ab59d9
