Unraveling the Enigma of Diminishing Exoplanets: Insights from NASA’s Research

by Klaus Müller
Exoplanet Atmospheric Loss

NASA’s investigation into the phenomenon of shrinking exoplanets has yielded new insights. Analysis of data from the Kepler Space Telescope indicates that some exoplanets experience a reduction in size due to atmospheric loss. This process is believed to be driven by internal radiation emanating from the planets’ cores, offering a fresh perspective on the previously noted size discrepancy among exoplanets. This finding is pivotal in understanding the significant atmospheric loss mechanism, which differs from the earlier hypothesized concept of photoevaporation.

The recent research provides a plausible explanation for the absence of exoplanets that are intermediary in size between super-Earths and sub-Neptunes.

Evidence points to certain exoplanets undergoing atmospheric depletion, leading to a decrease in their size. Utilizing data from the now-decommissioned Kepler Space Telescope, astronomers have uncovered potential reasons behind this phenomenon. It appears that the radiation from the cores of these planets is pushing their atmospheres outward, causing them to shrink.

The Exoplanet Size Discrepancy

Exoplanets, which are planets outside our solar system, vary significantly in size. This range includes smaller, rocky planets and larger, gaseous giants. Between these two extremes are rocky super-Earths and the more voluminous sub-Neptunes, which possess extensive atmospheres. However, there is a notable absence of planets in the size range of 1.5 to 2 times that of Earth, a gap that has puzzled scientists.

Over 5,000 exoplanets have been identified, yet there is a scarcity of planets with diameters 1.5 to 2 times that of Earth, according to Jessie Christiansen, a research scientist at Caltech/IPAC and lead author of the study published in The Astronomical Journal. This gap is now recognized as a significant phenomenon rather than a random occurrence, indicating an underlying process that prevents planets from achieving or maintaining this specific size.

An artist’s conception illustrates the appearance of the sub-Neptune exoplanet TOI-421 b. New findings suggest how planets of this type could be losing their atmospheres. Credit goes to NASA, ESA, CSA, and D. Player (STScI) for the visualization.

The hypothesis is that this size gap may be due to the atmospheric loss experienced by certain sub-Neptunes. Planets lacking sufficient mass, and consequently gravitational strength, to retain their atmospheres might shrink to the size of super-Earths, hence the gap between these two categories.

The precise mechanism of this atmospheric loss has been elusive. Two main theories have been proposed: core-powered mass loss and photoevaporation. The recent study lends support to the former.

Deciphering the Puzzle

Core-powered mass loss occurs as radiation from a planet’s heated core gradually expels the atmosphere. Christiansen explains this as the core’s radiation exerting pressure on the atmosphere from beneath.

Conversely, photoevaporation involves the planet’s atmosphere being stripped away by intense radiation from its host star, akin to a hair dryer’s effect on an ice cube. While photoevaporation is believed to occur in the initial 100 million years of a planet’s existence, core-powered mass loss likely happens much later, around 1 billion years into the planet’s lifecycle. In both cases, a planet’s inability to retain sufficient mass results in atmospheric loss and subsequent shrinkage.

An infographic presents the main types of exoplanets. Scientists continue to explore the “size gap,” the noticeable absence of planets sized between super-Earths and sub-Neptunes. This graphic is credited to NASA/JPL-Caltech.

Observational Evidence

Christiansen and her team utilized data from NASA’s K2 mission, an extension of the Kepler Space Telescope, to observe the star clusters Praesepe and Hyades, aged between 600 million and 800 million years. Given that planets typically share their host star’s age, the sub-Neptunes in these systems would be beyond the photoevaporation stage but not old enough for core-powered mass loss.

Observations in Praesepe and Hyades showed that nearly all stars still host a sub-Neptune planet or candidate. This suggests these planets have retained their atmospheres, unlike those around older stars observed by K2, where only 25% host sub-Neptunes, aligning with the timeframe for core-powered mass loss.

These findings indicate that photoevaporation did not occur in Praesepe and Hyades. If it had, it would have happened much earlier, leaving these planets with little or no atmosphere. This points to core-powered mass loss as the primary explanation for the atmospheric depletion of these planets.

Ongoing Research and Kepler’s Legacy

The study, spanning over five years, involved building a catalog of planet candidates. However, the research is ongoing, and our understanding of photoevaporation and core-powered mass loss may evolve. Future studies will likely test these findings further before the mystery of the

Frequently Asked Questions (FAQs) about Exoplanet Atmospheric Loss

What is the main finding from the Kepler Space Telescope data regarding exoplanets?

Researchers analyzing data from the Kepler Space Telescope discovered that some exoplanets are shrinking due to atmospheric loss. This process is believed to be caused by radiation from the planets’ cores, providing new insights into the observed size gap between super-Earths and sub-Neptunes.

How does core-powered mass loss contribute to exoplanet shrinkage?

Core-powered mass loss occurs when a planet’s core emits radiation, gradually pushing away the atmosphere over time. This phenomenon is one of the proposed explanations for the atmospheric loss leading to the shrinkage of exoplanets.

What is the significance of the exoplanet size gap?

The exoplanet size gap refers to the observed lack of planets with sizes between 1.5 to 2 times that of Earth. This gap is significant as it suggests an underlying process that impedes planets from reaching or maintaining this specific size.

How does the study differentiate between core-powered mass loss and photoevaporation?

The study differentiates these two processes based on the timing and mechanism of atmospheric loss. While photoevaporation, caused by a host star’s radiation, is thought to occur early in a planet’s life, core-powered mass loss, driven by the planet’s own core radiation, is believed to happen much later, around 1 billion years into a planet’s lifecycle.

What future research is anticipated in the study of exoplanet atmospheric loss?

Ongoing research aims to further understand and test the concepts of photoevaporation and core-powered mass loss. Future studies will likely delve deeper into these processes to provide a more comprehensive understanding of the exoplanet size gap and atmospheric loss phenomena.

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AstroNerd November 18, 2023 - 6:46 pm

the core-powered mass loss theory is fascinating, makes you wonder what else we don’t know about these exoplanets. Kepler has truly been a game changer in space research.

Sarah_101 November 19, 2023 - 4:42 am

I’ve been following Kepler’s discoveries for a while and this is some next level stuff. How do they even figure out all this about planets so far away?

Mike Johnson November 19, 2023 - 6:25 am

Wow this article is really interesting, didn’t know that planets could shrink like this. NASA’s research is just amazing, always bringing new things to light.

EmilyR November 19, 2023 - 3:38 pm

this is why I love astronomy, theres always something new and mind-blowing. But gotta say, the article could’ve been a bit clearer on the details about photoevaporation and core-powered mass loss.


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