NASA reveals the mystery of shrinking exoplanets

A new study could explain the “missing” exoplanets between super-Earths and sub-Neptunian planets.

Some exoplanets appear to be losing their atmospheres and shrinking. In a new study using NASAUsing the retired Kepler space telescope, astronomers have found evidence of a possible cause: that the cores of these planets push their atmospheres apart from the inside out.

Exoplanet size gap

Exoplanets (planets outside our solar system) Comes in a variety of sizesFrom small rocky planets to massive gas giants. In the middle lies a rock Super Earth And larger sub-Neptunes with puffy atmospheres. But there is a conspicuous absence – the “size gap” – of planets between 1.5 and 2 times the size of Earth (or between super-Earths and sub-Neptunian planets) that scientists are working to better understand.

“Scientists have now confirmed the discovery of more than 5,000 exoplanets, but there are fewer planets with a diameter of 1.5 to 2 meters,” said Jesse Christiansen, a research scientist at Caltech/IPAC and science lead for NASA’s Exoplanet Archive. times the diameter of the Earth. Author of the new study in Astronomical magazine. “Exoplanet scientists now have enough data to say that this gap is not just a fluke. There is something going on that prevents planets from reaching and/or staying there.”

This artist’s concept shows what Neptune’s exoplanet TOI-421 b might look like. In a new study, scientists have found new evidence indicating how these types of planets lose their atmospheres. Source: NASA, European Space Agency, Canadian Space Agency, and D. Player(STScI)

Researchers believe that this gap can be explained by some sub-planets losing their atmospheres over time. This loss would occur if the planet did not have enough mass, and thus gravitational force, to hold on to its atmosphere. So sub-Neptunian planets that are not massive enough will shrink to about the size of super-Earths, leaving a gap between the two planet sizes.

But how these planets lose their atmosphere has remained a mystery. Scientists have settled on two possible mechanisms: one is called mass loss as fundamental energy; The other is photoevaporation. The study revealed new evidence supporting the former.

This video explains the differences between the main types of exoplanets, or planets outside our solar system. Credit: NASA/Jet Propulsion Laboratory-California Institute of Technology

solve the puzzle

The loss of mass from the core occurs when radiation from a planet’s hot core pushes the atmosphere away from the planet over time, “and that radiation pushes the atmosphere from below,” Christiansen said.

The other main explanation for a planetary gap is photoevaporation, which occurs when a planet’s atmosphere explodes due to hot radiation from its host star. In this scenario, “the high-energy radiation from the star acts like a hairdryer on an ice cube,” she said.

While photoevaporation is thought to occur within the first 100 million years of the planet’s life, mass loss due to fundamental energy is thought to occur much later – about a billion years after the planet’s life. But with either mechanism, “if you don’t have enough mass, you won’t be able to hold up, you’ll lose your atmosphere and shrink,” Christiansen added.

This chart details the main types of exoplanets. Scientists are working to better understand the “size gap,” or apparent absence, of planets that lie between super-Earths and sub-Neptunian planets. Image source: NASA/JPL-Caltech

Uncovering evidence through observation

In this study, Chittiansen and co-authors used data from NASA’s K2, an extension mission of the Kepler space telescope, to look at the Praesepe and Hyades star clusters, which are between 600 million and 800 million years old. Because planets are generally thought to be the same age as their host star, sub-Neptunian planets in this system will be well past the age at which photoevaporation can occur, but are not old enough to undergo core-energy mass loss.

So, if the team sees that there are a lot of sub-Neptunian planets in Prasepe and Hyades (compared to older stars in other clusters), they can conclude that photoevaporation did not occur. In this case, mass loss due to fundamental energy would be the most likely explanation for what happens to sub-Neptune’s less massive mass over time.

By observing Brycepe and Haades, the researchers found that nearly 100% of the stars in these clusters still contain a substar.Neptune A planet or candidate planet in their orbit. Judging by the size of these planets, researchers believe that they have retained their atmosphere.

This illustration depicts NASA’s exoplanet hunter, the Kepler Space Telescope. The agency announced on October 30, 2018, that Kepler had run out of fuel, and that it would retire within its current and safe orbit, far from Earth. Kepler left a legacy of more than 2,600 exoplanet discoveries. Image credit: NASA/Wendy Stenzel/Daniel Rutter

This differs from the other older stars observed by K2 (stars older than 800 million years), only 25% of which orbit sub-Neptunes. The older age of these stars is closer to the time frame in which fundamental energy mass loss is thought to occur.

From these observations, the team concluded that photoevaporation could not have occurred at Praesepe and Hyades. If that had happened, it would have happened hundreds of millions of years ago, and these planets would have had little, if any, atmosphere left. This makes core-induced mass loss the main explanation for what is likely happening to these planets’ atmospheres.

Ongoing research and the legacy of Kepler

Christiansen’s team spent more than five years building the planet catalog necessary for study. But the research is still far from complete, and current understanding of photoevaporation and/or fundamental energy mass loss could evolve further. The results will likely have to be tested by future studies before anyone can announce that the mystery of this planetary gap has been solved once and for all.

This study was conducted using NASA’s Exoplanet Archive, which is managed by the California Institute of Technology in Pasadena under contract to NASA as part of the Exoplanet Exploration Program, and is located at NASA’s Jet Propulsion Laboratory in Southern California. JPL is a division of the California Institute of Technology.

Reference: “Scaling of K2. VII. “Evidence for a High Rate of Mesozoic Hot Sub-Neptune Creation” by Jesse L. Christiansen, John K. Zinke, Kevin K. Hardigree-Ullman, Rachel B. Fernandez, Philip F. Hopkins, Louisa M. Ripoll, Kirsten M. Polley, Galen J. Bergsten and Saki Burri, November 15, 2023, Astronomical magazine.
doi: 10.3847/1538-3881/acf9f9

NASA’s Kepler mission

On October 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 that 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 development of the Kepler mission. Ball Aerospace & Technologies Corporation operated the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder.