Sub-Neptune Planets Stop Growing By 'Eating' The Sky

In 2014, NASA’s Kepler Space Telescope found 700 new distant planets for scientists to study. However, unlike in earlier surveys wherein the planets spotted were mostly gas giants just like our own Jupiter, these new planets were smaller in size and mostly rock in mass.

What's even more interesting about these planets is that many of them were similar in size with Earth or perhaps bigger, but they seem to have stopped growing before they reach the size of Neptune.

In fact, from the 2,600 planets that Kepler discovered between 2009 to 2018, 1,000 are classified as sub-Neptunes or planets that are less massive than Uranus or Neptune, while Neptune-like planets that are of the same size as Neptune are considerably rarer even if they are only slightly bigger.

“What we have been puzzling over is why planets would tend to stop growing beyond about three times Earth's size,” planetary scientists Edwin Kite of the University of Chicago said.

Magma Oceans ‘Eating’ The Sky

As it happens, many of the planets that are smaller than the drop-off size are believed to have oceans of magma on their surface. However, instead of solidifying, the magma oceans on those planets are kept flowing by their thick, hydrogen-rich atmospheres.

According to the researchers of a new paper published in Astrophysical Journal Letters, it might be possible that the magma oceans begin to essentially “eat” the sky as their atmospheres continue to accumulate hydrogen and build pressure, thereby stalling its growth.

Basically, the idea is that as the gasses accumulate in the atmosphere, the pressure builds in the place where the atmosphere and the magma meet. Because of the high-pressure situation, the hydrogen molecules become so squished together that they begin to repel each other and can no longer be compressed. When this happens, they will have nowhere else to go but down into the magma.

The magma then “eats” or dissolves the atmosphere, steadily at first then more quickly as the pressure continues to rise. And as more hydrogen accumulates in the atmosphere, the more hydrogen the magma eats, therefore stalling the planet’s growth.

Fugacity Crisis

Because the majority of the volume in these planets are in the atmosphere, shrinking the atmosphere also shrinks the planet, and its growth is stalled before it even reaches the size of Neptune.

The authors call this phenomenon the “fugacity crisis” wherein a gas tends to dissolve into an adjacent liquid.

If the theory is, indeed, correct, then planets that are cold enough to have the magma crystallize instead of continuously flowing would then have different profiles because the surface would not absorb as much hydrogen.

“Nothing like these worlds exists in our solar system,” Kite said. “Although our work suggests a solution to one of the puzzles posed by sub-Neptune exoplanets, they still have a lot to teach us!”