Observations of planetary cradles find water and conditions for forming Earth-like planets even in harsh environments

November 30, 2023

Planets like our Earth, including planets with water, could form even in the harshest known star-forming environments, drenched by hard UV light from massive stars. That is a main result of analyses of new observations of such an environment with the James Webb Space Telescope (JWST). The observations are the first of their kind – before the JWST, this kind of detailed observation had not been possible. This is good news for Earth-like planets, and for life in the universe: there is a great variety of environments in which such planets can form.  The results have now been published in the Astrophysical Journal Letters.

Water and carbon-bearing molecules have been discovered in a disk of gas and dust surrounding a young solar-type star, which is located in one of the most extreme environments in our Galaxy. Such disks are where planets form around nascent stars. A team of astronomers led by María C. Ramírez-Tannus at the Max Planck Institute for Astronomy (MPIA) made use of the James Webb Space Telescope to peer into the inner region of the disk, which is where planets similar to our Earth are expected to form: so-called terrestrial planets, with a thin atmosphere covering a planet made of rock.

The disk, which the astronomers call XUE-1, is exposed to intense ultraviolet radiation of nearby hot, massive stars. Yet even in this harsh environment, the observations detected both water and simple organic molecules. Ramírez-Tannus says: “This result is unexpected and exciting! It shows that there are favourable 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

The new observations are the first of their kind. Previous detailed observations of planet-forming disks had been limited to nearby star-formation regions that contain no massive stars. Massive star-forming regions are completely different: there, numerous stars form at roughly the same time, including some of the rare, but extremely powerful very massive stars. During the ‘golden age’ of star formation in the universe, around 10 billion years ago, most star formation took place in such massive clusters. Overall, more than half of all stars in our universe – including our own Sun – were born in massive star-forming regions, together with their planets. Yet nothing was known about the effect of such harsh environments on inner regions of disks, where terrestrial planets are expected to form.

Massive stars are perforce very bright, giving off large amounts of high-energy UV radiation. Their presence causes considerable disruption in their vicinity. It was an open question whether that disruption would routinely interfere with the formation of planets like Earth around stars similar the Sun – which would relegate Earth-like planets to the sidelines in such massive clusters, not impossible to form, but very rare. There were plausible arguments that this could be the case. For instance, UV radiation from the massive stars disperses the gas in the outer disk portions, which inhibits the growth and the inward drift of dust particles that are the building blocks of Earth-like planets (and also of the cores of giant planet like Jupiter or Saturn). This might well stack the odds against the formation of Earth-like planets.

Up to now, observations did not help to answer this question. In the present-day universe, massive star-forming regions are rare, and even the nearest ones are far away. Up until recently, there was no way to observe small disks around sun-like stars in any details. The few planet-forming disks that were close enough to be observed in detail are all located in quiet surroundings, without the intense UV radiation from massive stars, and thus no use in answering the question.

Probing inner disks with JWST

This changed with the advent of the JWST. When the telescope became available for science observations, Ramírez-Tannus and the XUE (eXtreme UV environments) collaboration, successfully applied to observe NGC 6357. At a distance of 5500 light-years from Earth, this is one of the nearest massive star-forming regions. It is also the most promising observational target for answering the inner-disk question: NGC 6357 contains more than ten 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. Diversity is an important factor: The region contains a variety of disks, some of which have been exposed to more, others to less radiation. “If intense radiation hampers the conditions for planet formation in the inner regions of protoplanetary disks, NGC 6357 is where we should see the effect.”, says Arjan Bik from Stockholm University, the co-PI (co-principal investigator) of the XUE collaboration and the second author of the paper.

The observations the astronomers performed record spectra: rainbow-like decompositions of light that allow estimates of the presence of specific molecules in the observed region. To their surprise, Ramírez-Tannus and her colleagues found that, when it comes to the presence (and properties) of key molecules, at least one of the inner disks in NGC 6357, namely XUE-1, 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” says Ramírez-Tannus. “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 extreme conditions like these.

The observations are good news for Earth-like planets, and for life in the universe: Apparently, the inner regions of protoplanetary disks around sun-like stars located in some of the harshest star-forming environments are just as capable of forming Earth-like, rocky planets as their low-mass counterparts. They even provide for an abundance of water, a necessary ingredient for life as we know it. Whether or not this translates to a significantly large number of Earth-like planets born in such environments is not something the researchers can tell from looking at a single disk. The XUE collaboration is taking their observations further: with a JWST survey of 14 additional disks in different parts of NGC 6357 that should go a long way towards settling that important question.

Background information

The MPIA researchers involved are María Claudia Ramírez-Tannus, Thomas Henning, Giulia Perotti, Roy van Boekel and Sierk E. van Terwisga, in collaboration with Arjan Bik (Stockholm University), Lars Cuijpers (Radboud University), Rens Waters (Radboud University and SRON) and additional colleagues.

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