NASA Data Reveals Possible Reason Some Exoplanets Are Shrinking

A new study could explain the ‘missing’ exoplanets between super-Earths and sub-Neptunes.

Some exoplanets seem to be losing their atmospheres and shrinking. In a new study using NASA’s retired Kepler Space Telescope, astronomers find evidence of a possible cause: The cores of these planets are pushing away their atmospheres from the inside out.

Exoplanets (planets outside our solar system) come in a variety of sizes, from small, rocky planets to colossal gas giants. In the middle lie rocky super-Earths and larger sub-Neptunes with puffy atmospheres. But there’s a conspicuous absence – a “size gap” – of planets that fall between 1.5 to 2 times the size of Earth (or in between super-Earths and sub-Neptunes) that scientists have been working to better understand.

“Scientists have now confirmed the detection of over 5,000 exoplanets, but there are fewer planets than expected with a diameter between 1.5 and 2 times that of Earth,” said Caltech/IPAC research scientist Jessie Christiansen, science lead for the NASA Exoplanet Archive and lead author of the new study in The Astronomical Journal. “Exoplanet scientists have enough data now to say that this gap is not a fluke. There’s something going on that impedes planets from reaching and/or staying at this size.”

Researchers think that this gap could be explained by certain sub-Neptunes losing their atmospheres over time. This loss would happen if the planet doesn’t have enough mass, and therefore gravitational force, to hold onto its atmosphere. So sub-Neptunes that aren’t massive enough would shrink to about the size of super-Earths, leaving the gap between the two sizes of planets.

But exactly how these planets are losing their atmospheres has remained a mystery. Scientists have settled on two likely mechanisms: One is called core-powered mass loss; and the other, photoevaporation. The study has uncovered new evidence supporting the first.

Solving the Mystery

Core-powered mass loss occurs when radiation emitted from a planet’s hot core pushes the atmosphere away from the planet over time, “and that radiation is pushing on the atmosphere from underneath,” Christiansen said.

The other leading explanation for the planetary gap, photoevaporation, happens when a planet’s atmosphere is essentially blown away by the hot radiation of its host star. In this scenario, “the high-energy radiation from the star is acting like a hair dryer on an ice cube,” she said.

While photoevaporation is thought to occur during a planet’s first 100 million years, core-powered mass loss is thought to happen much later – closer to 1 billion years into a planet’s life. But with either mechanism, “if you don’t have enough mass, you can’t hold on, and you lose your atmosphere and shrink down,” Christiansen added.

For this study, Chistiansen and her co-authors used data from NASA’s K2, an extended mission of the Kepler Space Telescope, to look at the star clusters Praesepe and Hyades, which are 600 million to 800 million years old. Because planets are generally thought to be the same age as their host star, the sub-Neptunes in this system would be past the age where photoevaporation could have taken place but not old enough to have experienced core-powered mass loss.

So if the team saw that there were a lot of sub-Neptunes in Praesepe and Hyades (as compared to older stars in other clusters), they could conclude that photoevaporation hadn’t taken place. In that case, core-powered mass loss would be the most likely explanation of what happens to less massive sub-Neptunes over time.

In observing Praesepe and Hyades, the researchers found that nearly 100% of stars in these clusters still have a sub-Neptune planet or planet candidate in their orbit. Judging from the size of these planets, the researchers think they have retained their atmospheres.

This differs from the other, older stars observed by K2 (stars more than 800 million years old), only 25% of which have orbiting sub-Neptunes. The older age of these stars is closer to the timeframe in which core-powered mass loss is thought to take place.

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.

More About the Mission

On Oct. 30, 2018, Kepler ran out of fuel and ended its mission after nine years, during which it discovered more than 2,600 confirmed planets around other stars along with thousands of additional candidates astronomers are working to confirm.

NASA’s Ames Research Center in Silicon Valley, California, manages the Kepler and K2 missions for NASA’s Science Mission Directorate. JPL managed Kepler mission development. Ball Aerospace & Technologies Corporation operated the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

For more information about the Kepler and K2 missions, visit:

https://science.nasa.gov/mission/kepler

News Media Contacts

Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov

Karen Fox / Alise Fisher 
NASA Headquarters, Washington
202-358-1257 / 202-358-2546
karen.c.fox@nasa.gov / alise.m.fisher@nasa.gov

Written by Chelsea Gohd

2023-169   

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Scientists are following neon signs in a search for clues to one planetary system’s future and the past of another – our own solar system. Following up on a peculiar reading by NASA’s previous infrared flagship observatory, the now-retired Spitzer Space Telescope, the agency’s James Webb Space Telescope detected distinct traces of the element neon in the dusty disk surrounding the young Sun-like star SZ Chamaelontis (SZ Cha).

Image: SZ Chamaeleontis Protoplanetary Disk (Artist Concept )

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“How did we get here? It really goes back to that big question, and SZ Cha is the same type of young star, a T-Tauri star, as our Sun was 4.5 billion years ago at the dawn of the solar system,” said astronomer Catherine Espaillat of Boston University, in Massachusetts, who led both the 2008 Spitzer observations and the newly published Webb results. “The raw materials for Earth, and eventually life, were present in the disk of material that surrounded the Sun after it formed, and so studying these other young systems is as close as we can get to going back in time to see how our own story began.”

Scientists use neon as an indicator of how much, and what type, of radiation is hitting and eroding the disk around a star. When Spitzer observed SZ Cha in 2008, it saw an outlier, with neon readings unlike any other young T-Tauri disk. The difference was the detection of neon III, which is typically scarce in protoplanetary disks that are being pummeled by high-energy X-rays. This meant that the high-energy radiation in the SZ Cha disk was coming from ultraviolet (UV) light instead of X-rays. Besides being the lone oddball result in a sample of 50-60 young stellar disks, the UV vs. X-ray difference is significant for the lifetime of the disk and its potential planets.

Image: Neon Gas In Protoplanetary Disk

“Planets are essentially in a race against time to form up in the disk before it evaporates,” explained Thanawuth Thanathibodee of Boston University, another astronomer on the research team. “In computer models of developing systems, extreme ultraviolet radiation allows for 1 million more years of planet formation than if the evaporation is predominately caused by X-rays.”

So, SZ Cha was already quite the puzzle when Espaillat’s team returned to study it with Webb, only to find a new surprise: The unusual neon III signature had all but disappeared, indicating the typical dominance of X-ray radiation.

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“Both the Spitzer and Webb data are excellent, so we knew this had to be something new we were observing in the SZ Cha system – a significant change in conditions in just 15 years,” added co-author Ardjan Sturm of Leiden University, Leiden, Netherlands.

Espaillat’s team is already planning more observations of SZ Cha with Webb, as well as other telescopes, to get to the bottom of its mysteries. “It will be important to study SZ Cha, and other young systems, in multiple wavelengths of light, like X-ray and visible light, to discover the true nature of this variability we’ve found,” said co-author Caeley Pittman of Boston University. “It’s possible that brief, quiet periods dominated by extreme UV radiation are common in many young planetary systems, but we just have not been able to catch them.”

“Once again, the universe is showing us that none of its methods are as simple as we might like to make them. We need to rethink, re-observe, and gather more information. We’ll be following the neon signs,” said Espaillat.

This research has been accepted for publication in Astrophysical Journal Letters.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

Media Contacts

Laura Betz – laura.e.betz@nasa.gov, Rob Gutro– rob.gutro@nasa.gov
NASA’s  Goddard Space Flight Center, , Greenbelt, Md.

Leah Ramsay lramsay@stsci.edu , Christine Pulliam cpulliam@stsci.edu

Space Telescope Science Institute, Baltimore, Md.

Downloads

Download full resolution images for this article from the Space Telescope Science Institute.

Research results have been accepted for publication in Astropyisical Journal Letters.

Related Information

How do Planets Form? https://exoplanets.nasa.gov/faq/43/how-do-planets-form/

Planetary Systems – https://universe.nasa.gov/stars/planetary-systems/

Webb Mission – https://science.nasa.gov/mission/webb/

Webb News – https://science.nasa.gov/mission/webb/latestnews/

Webb Images – https://science.nasa.gov/mission/webb/multimedia/images/

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