Webb discovers that rocky planets can form in more extreme environments

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Using the joint NASA, European Space Agency, and Canadian Space Agency James Webb Space Telescope, a team of scientists has discovered water and other important molecules within the inner, rocky-planet-forming regions of a protoplanetary disk located around a star. However, the star itself is located in one of the most extreme environments in our galaxy — a region wherein massive stars are formed.

Typically, rocky planet formation is found in the protoplanetary disks of stars located in regions where low-mass stars are formed. However, the new results from Webb indicate that rocky planet formation can occur in a broad range of environments.

The observations were the first to be made as part of Webb’s new eXtreme UV Environments (XUE) program. XUE aims to focus on characterizing planet-forming disks in a variety of different environments throughout our galaxy, specifically in regions where massive stars form. These regions play host to environments similar to the environments in which most planetary systems formed. Furthermore, understanding how local environments affect planetary formation will give scientists insights into why exoplanets are as diverse as they are.

XUE will target a total of 15 disks in three distinct areas of the Lobster Nebula (NGC 6357), which is a large emission nebula located in the constellation Scorpius, approximately 5,500 light-years away from Earth. The nebula is among the youngest and closest stellar nurseries to Earth and hosts some of the most massive stars in all of our galaxy.

Image of the Lobster Nebula (NGC 6357), taken using data from NASA’s Chandra and Spitzer telescopes, the ROSAT telescope, and the UK’s SuperCosmos Sky Survey. (Credit: NASA/CXC/PSU/L. Townsley et al./UKIRT/NASA/JPL-Caltech)

However, why are scientists specifically interested in how massive stars affect planetary formation?

Massive stars are significantly hotter than low-mass stars like our Sun. Due to their increased temperatures, the stars emit more ultraviolet (UV) radiation into their surrounding environments.  If the stars have a protoplanetary disk surrounding them, this UV radiation will disperse the gas within the disk, leading the life expectancy of the disks to shorten to as young as just a few million years. For years, scientists have theorized about how the more hostile conditions of massive stars would affect planetary formation, and with Webb, they’re finally starting to get answers.

“Webb is the only telescope with the spatial resolution and sensitivity to study planet-forming discs in regions where massive stars are formed,” said lead author María Claudia Ramírez-Tannus of the Max Planck Institute for Astronomy in Germany.

To specifically characterize the physical and chemical properties of the rocky-planet-forming regions of protoplanetary disks within the Lobster Nebula, Ramirez-Tannus et al. used Webb’s Medium Resolution Spectrometer, which is a part of the Mid-InfraRed Instrument on Webb. This first set of results focused on a protoplanetary disk named XUE 1 in the star cluster Pismis 24.

Data from Webb’s Mid InfraRed Instrument showing the first XUE results. Note the different peaks that indicate the presence of different elements and compounds. (Credit: NASA/ESA/CSA/STScI/J. Olmsted (STScI)/M. C Ramírez-Tannus (Max Planck Institute for Astronomy))

“Only the MIRI wavelength range and spectral resolution allow us to probe the molecular inventory and physical conditions of the warm gas and dust where rocky planets form,” said coauthor Arjan Bik of Stockholm University in Sweden.

XUE 1 is located next to several massive stars within the Lobster Nebula, which means the disk has likely been exposed to very high amounts of UV radiation throughout its life, which could lead to the disappearance of molecules necessary for the formation of rocky planets. To the shock of Ramirez-Tannus et al., though, Webb’s observations identified a wide range of molecules that are known to make up rocky planet formation, such as small, partially crystalline silicate dust grains.

“We find that the inner disc around XUE 1 is remarkably similar to those in nearby star-forming regions. We’ve detected water and other molecules like carbon monoxide, carbon dioxide, hydrogen cyanide, and acetylene. However, the emission found was weaker than some models predicted. This might imply a small outer disc radius,” said co-author Rens Waters of Radboud University in the Netherlands.

“We were surprised and excited because this is the first time that these molecules have been detected under such extreme conditions,” added Lars Cuijpers of Radboud University.

This first set of results from Webb is already showing scientists that rocky planet formation within the disks of massive stars is similar to the formation of rocky planets in disks around low-mass stars. More observations are going to be needed to confirm theories and hypotheses. Still, this first set of results indicates that rocky planets can form in a wide range of environments and protoplanetary disks, not just within the disks of low-mass stars.

“XUE1 shows us that the conditions to form rocky planets are there, so the next step is to check how common that is. We will observe other discs in the same region to determine the frequency with which these conditions can be observed,” Ramírez-Tannus said.

Ramírez-Tannus et al.’s results were published in The Astrophysical Journal on Nov. 30, 2023.

(Lead image: Artist’s impression of a protoplanetary disk. Credit: ESO/L. Calçada)

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