Artist’s impression of a sub-Neptune or gas dwarf exoplanet
Hydrogen may explain the development and distribution of sub-Neptune exoplanets like this artist’s concept. Credit: Pablo Carlos Budassi, CC BY-SA 4.0

With the discovery of more than 5,000 confirmed exoplanets, astronomers understand more and more about what kinds of planets exist and why. But the data deluge has also thrown into relief the kinds of planets that don’t seem to exist. In particular, there is a steep decrease in the abundance of planets larger than approximately 3 Earth radii, a pattern nicknamed the “radius cliff.”

Current planet formation theories have struggled to explain why those planets can’t grow just a little bit bigger, but new research published in the Planetary Science Journal has shown how high-pressure, high-temperature chemical reactions might put a cap on planet growth.

“It’s exciting that [the researchers] start to probe the chemical interplay between potentially important species at the conditions inside these planets, which had previously only been modeled,” explained William Misener, a Ph.D. candidate investigating super-Earth and sub-Neptune atmospheres at the University of California, Los Angeles, who was not involved in the study.

Experimenting at High Temperatures and High Pressures

Missions like NASA’s Kepler and Transiting Exoplanet Survey Satellite have revealed a curious mystery: Planets 3 times Earth’s size are about 10 times more abundant than planets that are only slightly larger. These planets, called sub-Neptunes, carry most of their bulk in a thick, hydrogen-based atmosphere. The surface-atmosphere interface on those planets exists at much higher pressures and temperatures than those on Earth, and scientists still work to understand how chemical reactions at those extreme conditions can affect a planet at a larger scale.

Previous models have suggested that above a certain pressure at the base of the atmosphere, gases—especially hydrogen—begin to dissolve into magma, putting a strong brake on further growth. But little was known of how such a system would behave chemically. Hydrogen is a strong reducing agent, meaning it readily donates its electron to another element and reacts with it. The team hypothesized that under high pressure and high temperature, hydrogen could release iron out of iron oxides and produce water as a by-product.

In a first-of-its-kind experiment, researchers placed a thin foil of pressed metal oxides into tiny presses called diamond anvil cells. They filled the cells with molecular hydrogen gas to study what happens under the high-temperature, high-pressure conditions expected at the atmosphere–rocky core interface on sub-Neptunes.

The ability to sequester hydrogen could limit growth of an exoplanet’s atmosphere.

High-pressure experiments are never trivial, but they’re even more challenging when hydrogen is involved. As the lightest element, hydrogen has a tendency to diffuse into the diamond anvil itself under high pressures and high temperatures. This can render experiments difficult to interpret or even damage the equipment. To avoid this complication, the new study used pulse heating instead of continuous laser heating.

X-ray diffraction images from the experiment showed that hydrogen not only freed iron from its oxides but also reacted with it, forming an alloy. “The iron oxide is reduced to metal, and that can take up even more hydrogen,” said Harrison Horn, a postdoctoral researcher at Lawrence Livermore National Laboratory and lead author of the study.

That ability to sequester hydrogen, mostly as the iron-hydrogen alloy sinking into the metallic part of the core, could limit growth of an exoplanet’s atmosphere, resulting in the observed radius cliff.

Going After Silicates

This hydrogen sequestration could also allow sub-Neptunes with hydrogen-dominated atmospheres to turn into water-rich super-Earths. Astronomers think that this conversion of one type of planet to the other could explain a different pattern in exoplanet sizes called the “radius valley.”

“This is a new mechanism of endogenous water formation.”

“We normally think of water being on planets because they formed beyond the ice line or had delivery of water-rich materials…but this is a new mechanism of endogenous water formation,” said Horn.

Does hydrogen reacting with iron oxides explain the steep radius cliff completely? Not quite, but there are more ways hydrogen could interact with other materials to keep a sub-Neptune from growing and thus contribute to the radius cliff. The researchers behind the current study have also been using diamond anvil experiments to explore how hydrogen interacts with silicates under extreme pressures and temperatures.

“The silicates would provide an additional species for the hydrogen to reduce, and it will be interesting to see whether that alters where in the planet the hydrogen ends up,” said Misener.

Meanwhile, others have shown that an iron-nickel alloy can store even more hydrogen than iron alone, cementing the case for hydrogen as the likely culprit for the radius cliff.

—Julie Nováková (@Julianne_SF), Science Writer

Citation: Nováková, J. (2023), Hydrogen may push some exoplanets off a cliff, Eos, 104, Published on 10 April 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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