The fundamental building blocks for planet formation can exist even in environments with extreme ultraviolet radiation, according to a new study by an international collaboration led by Penn State astronomers. The study leveraged the unparalleled capabilities of NASA's James Webb Space Telescope (JWST) and sophisticated thermochemical modeling to investigate a protoplanetary disk - the dust and gas surrounding a new star that can eventually give rise to planets and other celestial bodies - in one of the most extreme environments in the galaxy.
A paper describing the study appeared May 20 in The Astrophysical Journal.
"Astronomers have long sought to understand how planets form within the swirling disks of gas and dust that encircle young stars," said Bayron Portilla-Revelo, a postdoctoral researcher in astronomy and astrophysics in the Eberly College of Science at Penn State and lead author of the study. "These structures - referred to as protoplanetary disks - are the birthplaces of extrasolar systems, like our own solar system, which formed 4.5 billion years ago. Protoplanetary disks often form in proximity to massive stars that emit substantial amounts of ultraviolet (UV) radiation, potentially disrupting the disks and affecting their capability to form planets. While significant progress has been made by studying protoplanetary disks in nearby star-forming regions, these regions lack the intense UV radiation present in more massive and common stellar nurseries."
UV radiation refers to non-visible light with more energy than visible light. On Earth, this can damage cells, ranging from a mild sunburn to skin cancer. In space, without a planet's atmospheric filters, UV radiation is far more intense. The focus of the study was a young, solar-mass star known as XUE 1, located approximately 5,500 light-years away from our sun, within a region called the Lobster Nebula, also known as NGC 6357. This region is renowned for harboring over 20 massive stars, two of which are among the most massive known in our galaxy and are extreme UV emitters. In the same region, the team observed a dozen lower-mass young stars with protoplanetary disks subjected to intense ultraviolet radiation.
Combining JWST observations with sophisticated astrochemical models, the researchers identified the composition of tiny dust grains in the protoplanetary disk around XUE 1 that will eventually grow to form rocky planets. They found that the disk contains sufficient solid material to potentially form at least 10 planets, each with a mass comparable to that of Mercury. The authors also determined the spatial distribution in the disk of a variety of previously detected molecules, including water vapor, carbon monoxide, carbon dioxide, hydrogen cyanide and acetylene.
"These molecules are expected to contribute to the formation of the atmospheres of emerging planets," said Konstantin Getman, research professor in the Department of Astronomy and Astrophysics at Penn State and co-author of the study. "The detection of such reservoirs of dust and gas suggests that the fundamental building blocks for planet formation can exist even in environments with extreme ultraviolet radiation."
Moreover, based on the absence of certain molecules that serve as tracers of UV irradiation in the light detected by JWST, the team inferred that the protoplanetary disk is compact and devoid of gas in its outskirts. It extends only about 10 astronomical units - a measure based on the average distance between the Earth and sun - from the host star, roughly the distance from the sun to Saturn. This compactness is likely a result of the external UV radiation eroding the outer regions of the disk, according to the research team.
"These findings support the idea that planets form around stars even when the natal disk is exposed to strong external radiation," said Eric Feigelson, distinguished senior scholar and professor of astronomy and astrophysics and of statistics at Penn State. "This helps explain why astronomers are finding that planetary systems are very common around other stars."
The study of XUE 1 represents a pivotal step in understanding the impact of external radiation on protoplanetary disks, the researchers said. It lays the groundwork for future observational campaigns with both space- and ground-based telescopes aimed at building a more comprehensive picture of planet formation across different cosmic environments. This research underscores the transformative capabilities of NASA's James Webb satellite observatory in probing the intricacies of planet formation and highlights the resilience of protoplanetary disks in the face of formidable environmental challenges, according to Portilla-Revelo.
In addition to Portilla-Revelo, Getman and Feigelson, the research team includes Maria Claudia Ramírez-Tannus and Thomas Henning at the Max-Planck Institut für Astronomie in Heidelberg, Germany; Thomas J. Haworth at Queen Mary University of London; Rens Waters at Radboud University and SRON Netherlands Institute for Space Research in the Netherlands; Arjan Bik and Jenny Frediani at Stockholm University in Sweden; Inga Kamp at University of Groningen in the Netherlands; Sierk E. van Terwisga at the Austrian Academy of Sciences; Andrew J. Winter at the Université Côte d'Azur in Nice, France, and the Max-Planck Institut für Astronomie in Heidelberg, Germany; Veronica Roccatagliata at the Universitàdi Bologna and INAF-Osservatorio Astrofisico di Arcetri in Italy; Thomas Preibisch at Ludwig-Maximilians-Universität in Germany; Elena Sabbi at the Gemini Observatory in Tucson, Arizona; Peter Zeidler at the Space Telescope Science Institute in Baltimore, Maryland; and Michael A. Kuhn at the University of Hertfordshire in the United Kingdom.
NASA funded the research, with additional support from the Center for Exoplanets and Habitable Worlds at Penn State, the Deutsche Forschungsgemeinschaft, the international Gemini Observatory - a program of NSF NOIRLab, which is managed by the Association of Universities for Research in Astronomy under a cooperative agreement with the U.S. National Science Foundation, the Royal Society Dorothy Hodgkin Fellowship and UKRI guaranteed funding for a Horizon Europe ERC consolidator grant, the Swedish National Space Agency, the German Aerospace Center, the German Federal Ministry for Economic Affairs and Energy, the European Union's Horizon 2020 research and innovation program, and the European Research Council via the ERC Synergy Grant "ECOGAL."
