A combination of cosmic processes shapes the formation of one of the most common types of planets outside of our solar system, a new study finds. 

The research team, which included University of Arizona planetary scientists, used data from NASA's Transiting Exoplanet Survey Satellite, or TESS, to study young sub-Neptunes – planets bigger than Earth but smaller than Neptune – that orbit close to their stars. The work provides insights into how these planets might migrate inward or lose their atmosphere during their early stages.

A paper describing the research was published Monday in the Astronomical Journal. The findings offer clues about the properties of sub-Neptunes and help address long-standing questions about their origins.

"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," said Rachel Fernandes, a U of A alumnus who led the research team and is now a President's Postdoctoral Fellow in the Department of Astronomy and Astrophysics at Penn State University. "Many of these are gaseous sub-Neptunes, a type of planet absent from our own solar system. While our gas giants like Jupiter and Saturn formed farther from the sun, it’s unclear how so many close-in sub-Neptunes managed to survive near their stars, where they are bombarded by intense stellar radiation."

The project began when Fernandes was a graduate student at the U of A  Lunar and Planetary Laboratory.

"Our work provides one of the first glimpses into young planet populations, about which we haven't had a lot of insights so far," said study co-author Galen Bergsten, a graduate student at the U of A Lunar and Planetary Laboratory.

The research gives a peek into what was happening very early on for these planets, said Bergesten, who handled the statistics and modeling for the study.

To better understand how sub-Neptunes form and evolve, the researchers turned to planets around young stars, which only recently became observable thanks to TESS.

"Comparing the frequency of exoplanets of certain sizes around stars of different ages can tell us a lot about the processes that shape planet formation," Fernandes said. "If planets commonly form at specific sizes and locations, we should see a similar frequency of those sizes across different ages. If we don't, it suggests that certain processes are changing these planets over time."

Observing planets around young stars, however, has traditionally been difficult. Young stars emit bursts of intense radiation, rotate quickly and are highly active, creating high levels of "noise" that make it challenging to observe planets around them. 

"These stellar tantrums cause a lot of noise in the data, so we spent the last six years developing a computational tool called Pterodactyls to see through that noise and actually detect young planets in TESS data," Fernandes said.

The research team used Pterodactyls to evaluate TESS data and identify planets with orbital periods of 12 days or less – for reference, much less than Mercury's 88-day orbit – with the goal of examining the planet sizes, as well as how the planets were shaped by the radiation from their host stars. Because the team's survey window was 27 days, the researchers were able to see two full orbits from potential planets. They focused on planets between a radius of 1.8 and 10 times the size of Earth, allowing the team to see if the frequency of sub-Neptunes is similar or different in young systems versus older systems previously observed with TESS and NASA's retired Kepler Space Telescope. 

The researchers found that the frequency of close-in sub-Neptunes changes over time, with fewer sub-Neptunes around stars between 10 million and 100 million years of age compared to those between 100 million and 1 billion years of age. However, the frequency of close-in sub-Neptunes is much less in older, more stable systems. 

"I found particularly striking that the occurrence rate wasn't uniformly high in the past. Instead, it started off lower, then increased, only to drop significantly when stars are billions of years in age – strongly suggesting that different physical processes shape planetary populations at different stages," said study co-author Ilaria Pascucci, a professor at the U of A Lunar and Planetary Laboratory.

It's possible that many sub-Neptunes originally formed farther away from their stars and slowly migrated inward over time, so we see more of them at this orbital period in the intermediate age. In later years, it's possible that planets are more commonly shrinking when radiation from the star essentially blows away its atmosphere, a process called atmospheric mass loss, that could explain the lower frequency of sub-Neptunes. But it's likely a combination of cosmic processes shaping these patterns over time rather than one dominant force, Fernandes said.

"Combining studies of individual planets with the population studies like we conducted here would give us a much better picture of planet formation around young stars," Fernandes said. "The more solar systems and planets we discover, the more we realize that our solar system isn't really the template; it’s an exception. Future missions might enable us to find smaller planets around young stars and give us a better picture of how planetary systems form and evolve with time, helping us better understand how our solar system, as we know it today, came to be."

Funding from NASA, Chile’s National Fund for Scientific and Technological Development, and the U.S. National Science Foundation supported this research. Additional support was provided by the Penn State Center for Exoplanets and Habitable Worlds and the Penn State Extraterrestrial Intelligence Center. Computations for this research were performed with Penn State University’s Institute for Computational and Data Sciences’ Roar supercomputer.