Earth-like planets, including planets with water, could form even in the harshest known planet-forming environments, according to a new study by an international team that includes Penn State astronomers.
Using the James Webb Space Telescope (JWST), the team observed disks of gas and dust where planets form within a rich stellar nursery about 5,500 light years away hosting thousands of young stars with diverse masses. They specifically studied disks located around stars of similar mass to Earth’s sun exposed to intense ultraviolet (UV) radiation from their more massive stellar counterparts. A paper describing one of these disks was published today (Nov. 30) in Astrophysical Journal Letters.
"We are examining 15 planet-forming disks around young solar-mass stars influenced by strong UV radiation emitted by numerous nearby massive stars," said Konstantin Getman, research professor of astronomy and astrophysics in the Penn State Eberly College of Science and a member of the XUE (eXtreme UV environments) collaboration conducting the research. "This emission has the potential to alter the planet-forming process. Within the inner regions of a highly irradiated disk called XUE-1, we discovered molecules containing water and carbon. The presence of these molecules indicates that Earth-like planets may form in a diverse range of environments."
The team used the JWST to peer into the inner region of the disk, where Earth-like "terrestrial" planets — rocky planets with a thin atmosphere — are expected to form. The disk surrounds a young star, with a mass similar to that of Earth’s sun, located in a star-forming nebula known as NGC 6357. Disks in this region are exposed to the intense UV radiation of nearby hot, massive stars within the nebula. Yet even in this harsh environment, the researchers detected both water and simple organic molecules.
"This result is unexpected and exciting!" said María C. Ramírez-Tannus, a postdoctoral researcher at the Max Planck Institute for Astronomy and leader of the research team. "It shows that there are favorable conditions to form Earth-like planets and the ingredients for life even in the harshest environments in our galaxy."
Unprecedented detail in massive star-forming regions
Previous detailed observations of planet-forming disks had been limited to nearby star-forming regions that do not contain massive stars, commonly referred to as low-mass star-forming regions. However, more than half of all stars in our universe — including the sun — as well as the planets that orbit them were born in massive star-forming regions. Yet nothing was known about the effect of such harsh environments on the inner regions of disks, where terrestrial planets are expected to form.
"Our Milky Way galaxy has massive star-forming regions, but none are conveniently close to our sun, so it is challenging to observe the details of planet-forming disks around solar-like stars in hostile ultraviolet environments," said Eric Feigelson, distinguished senior scholar and professor of astronomy and astrophysics and of statistics at Penn State and a member of the XUE team. "Thanks to the powerful instruments onboard JWST, we were able to take the first detailed look at the inner regions of these disks."
Massive stars give off large amounts of high-energy UV radiation, which causes considerable disruption in their vicinity. According to the researchers, it was an open question whether that disruption would interfere with the formation of Earth-like planets around stars similar to the sun. For example, it is thought that UV radiation from the massive stars disperses the gas in the outer part of the disk, which inhibits the growth and inward drift of dust particles that are the building blocks of Earth-like planets and also of the cores of giant planets like Jupiter or Saturn.
At 5,500 light-years from Earth, the NGC 6357 nebula is one of the nearest massive star-forming regions and a promising candidate to determine if these regions interfere with the creation of Earth-like planets. It contains more than 10 luminous high-mass stars, ensuring that some of the planet-forming disks visible in the region have been exposed to intense UV radiation for most of their existence. The region contains a variety of disks, with varying exposure to radiation.
JWST recorded the disks' spectra, a breakdown of light emitted at each wavelength in the electromagnetic spectrum that allows estimates of the presence of specific molecules. To their surprise, the team found that, when it comes to the presence — and properties — of key molecules, at least one of the inner disks in NGC 6357 is not fundamentally different from its counterparts in low-mass star-formation regions.
Silicates, water and other molecules in a harsh environment
"We found an abundance of water, carbon monoxide, carbon dioxide, hydrogen cyanide and acetylene in the innermost regions of XUE-1," Ramírez-Tannus said. “This provides valuable clues about the likely composition of the initial atmosphere of the resulting terrestrial planets."
The researchers also found silicate dust in similar amounts as in low-mass star-formation regions. This is the first time that such molecules have been detected under such extreme conditions.
"Our results suggest that the formation of Earth-like, rocky planets is not restricted to regions without massive stars; rather, it can also occur in regions where massive stars are present," Getman said. "The harsh environments near massive stars also contain water, so it's possible that planets in these regions could even support life."
Because the researchers studied one disk, it is unclear if Earth-like planets are more frequently born near massive star-forming regions or near low-mass star-forming regions. To help answer this question, the XUE collaboration is planning to study 14 additional disks in different parts of NGC 6357 using JWST.
In addition to Getman, Feigelson and Ramírez-Tannus, the research team includes Arjan Bik at Stockholm University; Lars Cuijpers and Rens Waters at Radboud University; Christine Göppl and Thomas Preibish at the Ludwig Maximilian University of Munich; Thomas Henning, Giulia Perotti, Roy van Boekel and Sierk E. van Terwisga from the Max Planck Institute for Astronomy; Inga Kamp at the Kapteyn Astronomical Institute; Germán Chaparro, Pablo Cuartas-Restrepo and Sebastián Hernández at the University of Antioquia; Alex de Koter at the University of Amsterdam; Sierra Grant at the Max Planck Institute for Extraterrestrial Physics; Thomas Hartowrth at the Queen Mary University of London; Michael Kuhn at the University of Hertfordshire; Matthew Pavich at the California State Polytechnic University; Megan Reiter at Rice University; Veronica Roccatagliata at the University of Pisa; Elena Sabbi at the Space Telescope Science Institute; Benoît Tabone and Andrew Winter at the French National Centre for Scientific Research; and Anna McLeod at Durham University.
Editor’s note: A version of this story was originally published by the Max Planck Institute for Astronomy.