Shrinking Exoplanets: NASA Data Offer a Possible Clue

Within the realm of exoplanets, ranging from compact rocky planets to expansive gas giants, a distinctive range exists, encompassing rocky super-Earths and more substantial sub-Neptunes with voluminous atmospheres. However, a noticeable void, often termed a “Size gap,” is apparent for planets measuring 1.5 to 2 times the size of Earth, representing the intermediate space between super-Earths and sub-Neptunes. Scientists have been dedicated to unraveling the mysteries behind this particular planetary size range.

Jessie Christiansen, a research scientist at Caltech/IPAC and the science lead for the NASA Exoplanet Archive, acknowledged that despite the confirmation of over 5,000 exoplanets, there’s an observed scarcity of planets with a diameter between 1.5 and 2 times that of Earth. Christiansen emphasized that the amassed data indicates this gap is not an incidental occurrence, indicating a systemic phenomenon that hinders planets from attaining or maintaining sizes within this specific range.

Scientists propose that the observed gap may result from the gradual loss of atmospheres in certain sub-Neptunes. This atmospheric depletion is anticipated when a planet lacks sufficient mass and gravitational force to retain its atmosphere. Consequently, sub-Neptunes with inadequate mass could contract to the size of super-Earths, contributing to the observed void between the two planetary size categories.

The precise mechanisms behind the atmospheric loss of these planets have posed a longstanding mystery. Scientists have primarily considered two potential processes: core-powered mass loss and photoevaporation. The latest study has presented fresh evidence bolstering the case for core-powered mass loss as a significant factor in this phenomenon.

Unlocking the enigma

Jessie Christiansen explains that core-powered mass loss involves the gradual expulsion of a planet’s atmosphere over time due to radiation emitted from its hot core, exerting pressure on the atmosphere from beneath. In contrast, the alternative explanation, photoevaporation, occurs when a planet’s atmosphere is essentially stripped away by the intense radiation from its host star, likened to a hair dryer melting an ice cube.

The distinction between photoevaporation and core-powered mass loss lies in their occurrence timelines. Photoevaporation is believed to occur within the first 100 million years of a planet’s existence, while core-powered mass loss is thought to take place later, approaching 1 billion years into a planet’s life. In both cases, the common outcome is that insufficient mass leads to the inability to retain the atmosphere, resulting in the planet’s contraction.

In this research, Jessie Christiansen and her co-authors utilized data from NASA’s K2, an extended mission of the Kepler Space Telescope. They focused on the star clusters Praesepe and Hyades, with ages ranging from 600 million to 800 million years. Given that planets are typically considered to be of the same age as their host star, the sub-Neptunes in this system would have surpassed the age at which photoevaporation could occur but haven’t yet reached the age for core-powered mass loss to take place.

In the analysis, the researchers could infer the occurrence of photoevaporation by examining the prevalence of sub-Neptunes in Praesepe and Hyades compared to older stars in different clusters. A higher abundance of sub-Neptunes in these clusters would suggest that photoevaporation hadn’t occurred. In such a scenario, core-powered mass loss emerges as the more probable explanation for the fate of less massive sub-Neptunes over time.

Upon observing Praesepe and Hyades, the scientists discovered that nearly all stars in these clusters still hosted a sub-Neptune planet or planet candidate in their orbit. Based on the size of these planets, the researchers hypothesize that they have successfully retained their atmospheres.

In contrast to older stars observed by K2, where only 25% have sub-Neptunes in orbit, the star clusters Praesepe and Hyades, aged between 600 million to 800 million years, exhibited a significantly higher percentage, with nearly 100% of stars still hosting sub-Neptune planets or planet candidates. The disparity in sub-Neptune prevalence suggests that core-powered mass loss, rather than photoevaporation, is a more plausible explanation for the fate of less massive sub-Neptunes in these clusters.

The team’s analysis led to the conclusion that photoevaporation did not occur in Praesepe and Hyades. Had it taken place, it would have transpired hundreds of millions of years earlier, resulting in these planets having minimal to no remaining atmosphere. As a result, the predominant explanation for the fate of these planets’ atmospheres is considered to be core-powered mass loss.

The research, requiring over five years to compile the essential planet candidate catalog, is just the beginning. Jessie Christiansen emphasizes that the understanding of photoevaporation and/or core-powered mass loss could undergo refinement. These findings are poised for scrutiny in future studies, signaling that the enigma of the planetary gap may not be conclusively resolved until subjected to further examination.

The investigation utilized the NASA Exoplanet Archive, managed by Caltech in Pasadena through a contractual agreement with NASA as an integral component of the Exoplanet Exploration Program. This program is based at NASA’s Jet Propulsion Laboratory in Southern California, a division of Caltech.

Resources

1. ^NEWSPAPER Gohd, C. & NASA. (2023, November 15). NASA data reveal possible reason some exoplanets are shrinking. Phys.org. [Phys.org]
2. ^JOURNAL Christiansen, J. L., Zink, J. K., Hardegree-Ullman, K. K., Fernandes, R. B., Hopkins, P. F., Rebull, L. M., Boley, K. M., Bergsten, G. J., & Bhure, S. (2023). Scaling K2. VII. Evidence For a High Occurrence Rate of Hot Sub-Neptunes at Intermediate Ages. The Astronomical Journal, 166(6), 248. [The Astronomical Journal]