Possible Explanation for Shrinking Exoplanets Unveiled by NASA Data

November 15, 2023

This article has gone through the rigorous editorial policy and process of Science X. Editors have fact-checked the article, made sure it came from a trustworthy source, and completed proofreading for credibility.

Chelsea Gohd of NASA reviewed the article.

A recent study might shed light on why there are 'missing' exoplanets falling between the size of super-Earths and sub-Neptunes.

It seems that some exoplanets are eroding and diminishing. A novel study using the now-retired Kepler Space Telescope from NASA provides evidence suggesting their planet's cores might be responsible by repelling their atmospheres from the inside.

The study was released in The Astronomical Journal.

Exoplanets, which are planets outside our solar system, exhibit a wide array of sizes. They range from small, rocky planets to mammoth gas giants. However, an apparent 'size gap' exists for planets which should sit between 1.5 to 2 times the size of Earth, a puzzle that scientists are keen to decipher.

"Over 5,000 exoplanets have been confirmed by scientists, but there is a shortage of planets with a diameter 1.5 and 2 times that of Earth," stated Jessie Christiansen, the chief research scientist at Caltech/IPAC and the study's main author. "The data gathered by exoplanet scientists confirms that this gap is not random. An unidentified factor prevents these planets from achieving or maintaining this size."

The research postulates that this gap might be due to certain sub-Neptunes gradually losing their atmospheres over time. This would take place should a planet lack the gravitational force needed to retain its atmosphere due to insufficient mass. Consequently, undermassive sub-Neptunes would erode to approximately the size of super-Earths, leaving the size gap.

The actual process these planets are using to lose their atmospheres has remained elusive. Scientists are torn between two theories: core-powered mass loss, and photoevaporation. The study found new evidence pointing towards the former.

Core-powered mass loss happens due to radiation from a planet's hot core gradually expelling the atmosphere. 'The radiation causes the atmosphere to disperse from underneath,' Christiansen explained.

The other theory, photoevaporation, occurs when a planet's atmosphere is effectively stripped away by its parent star's hot radiation. In this case, 'The star's high-energy radiation acts like a hairdryer on an ice block,' she stated.

While it is likely that photoevaporation takes place during the first 100 million years of a planet's life, core-powered mass loss is believed to occur much later – roughly 1 billion years. But whether through either mechanism, 'if you lack sufficient mass, you cannot maintain control, and you end up losing your atmosphere and shrinking,' Christiansen stated.

In this study, Christiansen and fellow authors examined star clusters Praesepe and Hyades using data from K2, an extension of the Kepler Space Telescope. These clusters are thought to be 600 million to 800 million years old, indicating that the sub-Neptunes within this system would be beyond the age at which photoevaporation typically occurs but not old enough to have undergone core-powered mass loss.

If the researchers noticed a plethora of sub-Neptunes in these clusters compared to older star clusters, they could deduce that photoevaporation had not yet taken place. Hence, core-powered mass loss would be the most probable explanation for the fate of lower-mass sub-Neptunes.

After examining Praesepe and Hyades, the researchers discovered that nearly all stars in these clusters still have a sub-Neptune or sub-Neptune candidate orbiting them. From the size of these planets, the researchers hypothesize that they have maintained their atmospheres. This contrasts with only 25% of older stars, over 800 million years old, observed by K2 that have sub-Neptunes in orbit, reflecting a timeline more suited to core-powered mass loss theory.

From these observations, the team concluded that photoevaporation could not have taken place in Praesepe and Hyades. If it had, it would have occurred hundreds of millions of years earlier, and these planets would have little—if any—atmosphere left. This leaves core-powered mass loss as the leading explanation for what likely happens to the atmospheres of these planets.

Christiansen's team spent more than five years building the planet candidate catalog necessary for the study. But the research is far from complete, she said, and it is possible that the current understanding of photoevaporation and/or core-powered mass loss could evolve. The findings will likely be put to the test by future studies before anyone can declare the mystery of this planetary gap solved once and for all.

This study was conducted using the NASA Exoplanet Archive, which is operated by Caltech in Pasadena under contract with NASA as part of the Exoplanet Exploration Program, which is located at NASA's Jet Propulsion Laboratory in Southern California. JPL is a division of Caltech.

Provided by NASA