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A recent study spearheaded by Penn State researchers has revealed new insights into the formation of sub-Neptunes, a prevalent type of planet found beyond our solar system. Utilizing data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the team investigated young sub-Neptunes—planets that are larger than Earth but smaller than Neptune and which orbit closely around their host stars. Their findings shed light on the potential mechanisms by which these planets might migrate inward or undergo atmospheric loss during their formative years.
The research was published in the Astronomical Journal on March 17. The results provide valuable information regarding the characteristics of sub-Neptunes and contribute to ongoing discussions about their origins, as highlighted by the researchers.
“A significant number of the approximately 5,500 exoplanets identified to date are in close orbits to their stars, some even closer than Mercury’s orbit around the sun, which we categorize as ‘close-in’ planets,” said Rachel Fernandes, a President’s Postdoctoral Fellow in Penn State’s Department of Astronomy and Astrophysics and the study’s lead author. “Among these exoplanets are many gaseous sub-Neptunes, a planetary type not found in our solar system. While gas giants like Jupiter and Saturn developed further from the sun, the survival of numerous close-in sub-Neptunes amidst intense stellar radiation remains a mystery.”
To delve into the formation and evolution of sub-Neptunes, the researchers focused on planets surrounding younger stars, a class of celestial bodies that have only recently become easier to observe, thanks to TESS.
“By comparing the occurrence rates of specific exoplanet sizes around stars of varying ages, we can glean significant insights into the factors influencing planet formation,” Fernandes explained. “If certain planet sizes and locations consistently yield similar frequencies across different stellar ages, it suggests stable formation patterns. Conversely, discrepancies would indicate ongoing changes affecting these planets over time.”
Traditionally, studying young stars has presented challenges due to their propensity to emit intense radiation and interact vigorously with their surroundings, generating high levels of data noise that obscure planet detection.
“In the early stages of their lives, young stars can be quite volatile, releasing substantial radiation,” Fernandes noted. “These ‘stellar tantrums’ complicate data analysis, prompting us to develop a computational tool named Pterodactyls over the past six years to filter through the noise and detect young planets in TESS data.”
The research team utilized Pterodactyls to analyze TESS data, focusing on planets with orbital periods of 12 days or less—significantly shorter than Mercury’s 88-day orbit. This allowed them to observe two full orbits of potential planets, concentrating on those with a radius ranging from 1.8 to 10 times that of Earth. The goal was to assess how the radiation from their host stars impacts their physical characteristics and the frequencies of sub-Neptunes in both young and older systems previously observed with TESS and NASA’s retired Kepler Space Telescope.
The analysis revealed that the prevalence of close-in sub-Neptunes fluctuates over time. Specifically, there are fewer of these planets around stars aged between 10 and 100 million years compared to those around stars that are between 100 million and 1 billion years old. Moreover, the frequency of close-in sub-Neptunes diminishes significantly in older, more stable star systems.
“We propose that a variety of mechanisms are influencing the abundance we observe in close-in planets of this size,” Fernandes remarked. “Many sub-Neptunes may have originated at greater distances from their stars, gradually migrating inward. This could explain the higher frequency associated with intermediate-aged stars. Over time, however, atmospheric mass loss—induced by stellar radiation effectively stripping away the atmosphere—may contribute to the decline of close-in sub-Neptunes in older systems. It is likely a synergistic effect of multiple cosmic processes rather than a singular dominant factor.”
The research team plans to extend their observational campaigns with TESS to include planets with longer orbital periods. Future missions, such as the European Space Agency’s PLATO, are anticipated to enhance their ability to detect smaller planets comparable to Mercury, Venus, Earth, and Mars. Analyzing these smaller and more distant planets could provide a refined understanding of planet formation.
Additionally, utilization of NASA’s James Webb Space Telescope may allow for detailed examination of the density and composition of individual planets, revealing further insights into their formation environments.
“Integrating studies focused on specific planets with broader population studies like ours can significantly enhance our understanding of planet formation around young stars,” Fernandes stated. “The more we discover about various solar systems and planets, the clearer it becomes that our solar system is not a universal standard; it appears to be more of an outlier. Upcoming missions may help us identify smaller planets orbiting young stars, offering deeper insights into how planetary systems evolve over time and contributing to our knowledge of the origins of our solar system.”
This research team, alongside Fernandes, includes experts such as Rebekah Dawson, who was the Shaffer Career Development Professor in Science and is now a physical scientist at NASA, as well as Galen J. Bergsten, Ilaria Pascucci, Kevin K. Hardegree-Ullman, Tommi T. Koskinen, and Katia Cunha from the University of Arizona. Other contributors include Gijs Mulders from Pontifical Catholic University of Chile, and several scientists from the California Institute of Technology, University of Cambridge, Princeton University, Carnegie Institution for Science, Columbia University, Stony Brook University, the University of Southern Queensland in Australia, and the University of North Georgia.
The research received funding from various institutions, including NASA’s “Alien Earths” grant, Chile’s National Fund for Scientific and Technological Development, and the U.S. National Science Foundation. Additional support was granted by the Penn State Center for Exoplanets and Habitable Worlds and the Penn State Extraterrestrial Intelligence Center, with significant computational resources provided by Penn State’s Institute for Computational and Data Sciences.
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