Tucson Standard

A recent study reveals that a combination of cosmic processes influences the formation and characteristics of sub-Neptunes, one of the most common types of planets found outside our solar system. Researchers from the University of Arizona, utilizing data from NASA's Transiting Exoplanet Survey Satellite (TESS), have explored young sub-Neptunes—planets larger than Earth but smaller than Neptune—that orbit close to their stars. The study provides insights into how these planets may migrate inward or lose their atmospheres in early stages.

Rachel Fernandes, a U of A alumnus and current President's Postdoctoral Fellow at Penn State University, led the research team. She stated, "The majority of the 5,500 or so exoplanets discovered to date have a very close orbit to their stars, closer than Mercury to our sun, which we call ‘close-in’ planets." Many are gaseous sub-Neptunes, absent from our solar system. Fernandes added that while gas giants like Jupiter and Saturn formed farther from the sun, it remains unclear how many close-in sub-Neptunes survive near their stars despite intense stellar radiation.

Galen Bergsten, a graduate student at the U of A Lunar and Planetary Laboratory and co-author of the study, emphasized that "our work provides one of the first glimpses into young planet populations," offering insights previously unavailable.

The researchers focused on planets around young stars using TESS data. Fernandes explained that comparing exoplanet sizes around different-aged stars can reveal planet formation processes: "If planets commonly form at specific sizes and locations, we should see a similar frequency of those sizes across different ages."

Observing young stars is challenging due to high levels of noise caused by intense radiation bursts. To address this issue, Fernandes mentioned developing a computational tool called Pterodactyls over six years to detect young planets in TESS data.

Using Pterodactyls with TESS data allowed researchers to identify planets with orbital periods under 12 days. They examined planet sizes and how they were affected by host star radiation. Their survey focused on planets between 1.8 and 10 times Earth's radius.

The findings showed that close-in sub-Neptune frequency changes over time; fewer are found around stars aged 10 million to 100 million years compared to those aged 100 million to 1 billion years. However, older systems show even less frequency.

Study co-author Ilaria Pascucci noted that "the occurrence rate wasn't uniformly high in the past," suggesting different physical processes shape planetary populations at various stages.

Fernandes suggested that many sub-Neptunes might have formed farther away before migrating inward over time. Atmospheric mass loss could explain why fewer sub-Neptunes are observed as they age.

Fernandes concluded by stating that combining individual planet studies with population studies would enhance understanding: "Future missions might enable us to find smaller planets around young stars...helping us better understand how our solar system...came to be."

This research received funding from NASA, Chile’s National Fund for Scientific and Technological Development, and the U.S. National Science Foundation. Additional support came from Penn State University's Center for Exoplanets and Habitable Worlds and Extraterrestrial Intelligence Center.