Why some distant exoplanets are mysteriously shrinking

Have you ever wondered about planets outside our solar system, called exoplanets? They come in all sizes, from small, rocky specimens like Earth to giant gas giants.

But there is a strange mystery in the world of astronomy: a clear gap in the size of these exoplanets, particularly between the medium-sized rocky planets (super-Earths) and the larger ones with large atmospheres (sub-Neptunes).

Scientists are puzzled as to why there aren’t many planets in this middle region.

A recent study using data from NASA’s decommissioned Kepler Space Telescope may have just found a clue to this mystery.

The study, published in The Astronomical Journalsuggests that the missing link in this size spectrum may be due to some planets actually shrinking over time!

Here’s the thing: Scientists have identified over 5,000 exoplanets, but there is a strange lack of planets 1.5 to 2 times the size of Earth. The study, led by Jessie Christiansen, a research scientist at Caltech/IPAC and scientific director of NASA’s Exoplanet Archive, offers a possible explanation.

It appears that some sub-Neptunes are losing their atmosphere, causing them to shrink to the size of super-Earths and creating this strangely large gap.

Now you may be wondering: How do these planets lose their atmosphere? The researchers propose two main theories. The first is called “nuclear-driven mass loss.” This happens when radiation from the planet’s hot core pushes the atmosphere away, similar to a balloon deflating from the inside.

The other theory is “photoevaporation,” in which the planet’s atmosphere is blown away by the intense radiation from its parent star, much like a hairdryer melts an ice cube.

To test these ideas, Christiansen and her team used data from NASA’s K2 mission to observe two star clusters, Praesepe and Hyades, that are about 600 to 800 million years old.

The idea was to find out whether the sub-Neptunes around these younger stars had lost their atmosphere or not.

If these planets still had their atmospheres, this would suggest that photoevaporation has not occurred (since it is said to occur in the first 100 million years of a planet’s life), so nuclear-driven mass loss would be the likely cause.

Their findings were fascinating. Almost all of the stars in these younger star clusters still had sub-Neptune planets, suggesting that these planets had not yet lost their atmosphere. This differed from what they saw in older stars (more than 800 million years old), where only about 25% had sub-Neptunes.

This pattern suggests that the younger planets had not yet suffered significant atmospheric loss, suggesting that nuclear-driven mass loss is the likely explanation for the shrinking of less massive sub-Neptunes.

However, this research is just the beginning. Christiansen’s team worked for over five years to compile the data needed for this study. But there is still much to learn, and future studies will further explore and test these findings.

This ongoing research is part of NASA’s Exoplanet Exploration Program, operated by Caltech in Pasadena and managed by NASA’s Jet Propulsion Laboratory in Southern California.

So next time you look up at the stars, remember that there is a whole universe of planets out there and some of them even change size! The universe is full of mysteries and we are just beginning to unravel them.

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Source: NASA.