NASA scientists find a clue in the mystery of shrinking planets

An artist’s concept shows the variety of rocky Earth-like planets that could exist in the universe.
NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

NASA scientists are puzzled by a group of planets that appear to be shrinking. Radiation could be the culprit.

There are all kinds of worlds outside our solar system. Distant alien planets, called exoplanets, can be gas giants like Jupiter, rocky globes the size of our planet, or even “super-puffs” the density of cotton candy.

But there is a mysterious gap where there should be planets about 1.5 to twice the width of Earth.

A mysterious void where there should be planets

An illustration shows a variety of possible exoplanets.

Among the more than 5,000 exoplanets discovered by NASA, there are many super-Earths (which are up to 1.6 times the width of our planet) and many sub-Neptunes (about two to four times the diameter of Earth), but almost no planets in between them.

“Exoplanet scientists now have enough data to say that this gap is not a coincidence. There’s something going on that prevents planets from reaching and/or staying at that size,” Jesse Christiansen, a Caltech researcher and NASA Exoplanet Archive scientist, said in a Wednesday press release.

Scientists think this is because some sub-Neptunes are shrinking – losing their atmosphere and accelerating through the size difference until they are as small as a super-Earth.

Illustration of the exoplanet HAT-P-7b, which is 1,000 light-years from Earth.
Mark Garlick / Science Photo Library

Christiansen’s latest research suggests that these worlds are shrinking because radiation from the planets’ cores pushes their atmospheres out into space.

The study, published in The Astronomical Journal on Wednesday, may solve the mystery of the missing exoplanets.

The planets themselves may repel their atmospheres

Collapsing exoplanets may lack the mass (and therefore gravity) to keep their atmospheres close.

However, the exact mechanism for the loss of atmosphere remains unclear.

The new study supports a hypothesis the scientists call “core-driven mass loss,” according to the release.

Basic mass loss is not a trendy new workout plan. That’s when the planet’s core emits radiation that pushes its atmosphere underneath, causing it to separate from the planet over time, according to the publication.

The other hypothesis, called photoevaporation, says that the planet’s atmosphere is dissipated by radiation from its host star.

But photoevaporation is thought to occur when a planet is 100 million years old — and, according to the paper, core-driven mass loss could occur closer to the planet’s one billionth birthday.

To test both hypotheses, Christiansen’s team looked at data from NASA’s retired Kepler space telescope.

They studied star clusters that were over 100 million years old. Since the planets are thought to be about the same age as their host stars, the planets in these clusters would be old enough to undergo photoevaporation, but not old enough to undergo core-driven mass loss.

Scientists found that most of the planets there retained their atmospheres, making core-driven mass loss a more likely cause of eventual atmosphere loss.

“However, recent work suggests an ongoing sequence of mass loss in which both processes are at work,” Christiansen wrote on X, the platform formerly known as Twitter, sharing a link to a Harvard assessment published online in July.

So the mystery is still not solved.

According to Christiansen’s statement in the release, her work isn’t over either — especially because our understanding of exoplanets will evolve over time.

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