Rocky, Carbon-rich Exoplanets Are More Likely to Orbit Small Stars, James Webb Space Telescope Reveals

Using the James Webb Space Telescope, astronomers have discovered the richest 'menu' of hydrocarbons ever seen in a planet-forming disk. This observation, which involved the protoplanetary disk around a small star, also revealed the first detection of ethane outside the Solar System.

The discovery was made when the James Webb Space Telescope's (JWST) Mid-Infrared Instrument (MIRI) surveyed the object "ISO-ChaI 147" as part of the Mid-Infrared Disk Survey (MINDS). ISO-ChaI 147 is a young star in the Chameleon I star forming region with about 237 stars. This region is located approximately 600 light years away.

These JWST observations of ISO-ChaI 147 imply that the protoplanetary disks of small stars are more efficient at forming smaller, Earth-like planets than at producing much larger, Jupiter-like gas giants. Because low-mass stars are more common than larger stars in the Milky Way, there may be more terrestrial planets in our Milky Way than previously suspected.

The findings also show that the clouds of gas and dust that surround small stars and produce planets are constructed differently - at least chemically - than the clouds around stars about the size of the Sun and larger. The different chemical menu around these relatively small stars could mean that their rocky planets have very different atmospheres than Earth's.

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ISO-ChaI 147's mass is just over 10% that of the Sun, and it is surrounded by a protoplanetary disk with a carbon-rich chemistry containing 13 carbonaceous molecules, including ethane and gasoline. However, the abundance of oxygen-carrying molecules in this disk is very low.

"This is vastly different from the composition we see in disks around solar-type stars, where oxygen-carrying molecules such as water and carbon dioxide dominate," Inga Kamp, team member and researcher at the University of Groningen, said in a statement. .

The MINDS team believes this shows that the material is transported radially through ISO-ChaI 147's protoplanetary disk, affecting the bulk composition of any planets that form within the disk.

What does this mean for the hunt for exoplanets?

Stars are formed when huge clouds of gas and dust develop overly dense patches that eventually collapse under their own gravity. However, this process does not use all that material, resulting in young stars being surrounded by swirling and oblate clouds of gas and dust called protoplanetary disks. When bits of matter condense in this disk, planets form - which is what happened around our young Sun about 4.6 billion years ago.

The amount of material in a protoplanetary disk and the way that gas and dust is distributed puts a limit on the number of planets a star can host, as well as the building blocks that can supply those planets. The JWST ISO-ChaI 147 results indicate that this protoplanetary disk is better suited to producing smaller rocky planets than larger gas giants.

Because the environments in protoplanetary disks determine the conditions in which new planets form, the finding that disks around very low-mass stars evolve differently from those around more massive stars has potential implications for finding rocky planets with Earth-like features. However, small stars can harbor planets that are similar to Earth in many ways but radically different in others.

"Many primary atmospheres of these planets are likely to be dominated by hydrocarbon compounds rather than oxygen-rich gases such as water and carbon dioxide," says Thomas Henning, MINDS team leader and researcher at the Max Planck Institute for Astronomy (MPIA). , as stated in the statement. 'We showed in a previous study that the transport of carbon-rich gas to the zone where terrestrial planets usually form occurs faster and more efficiently in those disks than that of more massive stars.'

The reason for the carbon-oxygen imbalance between protoplanetary disks of stars of different masses is not currently understood. This could be due, for example, to the fact that disks around smaller stars are enriched in carbon, or that they no longer have oxygen.

If the former is true, that would mean that carbon enrichment can occur if solid particles in the disk are stripped of their carbon content. That content would be released as gas. These low-carbon solid particles would form planets with rocky bodies that are low in carbon. Yet the atmospheres of these worlds would be carbon-dominated due to an excess of carbon gas in the environment in which they are born. So these rocky planets around small stars would ultimately be carbon-rich - and very different from Earth.

Research leader Aditya Arabhavi, also from the University of Groningen, added that these findings were made possible by the JWST's unique position about 1.6 million kilometers from Earth.

"These observations are not possible from Earth because the relevant gas emissions are absorbed by the atmosphere," Arabhavi said. "Previously we could only identify the acetylene emission from this object, but the higher sensitivity of JWST and the spectral resolution of its instruments allowed us to detect weak emission from less abundant molecules."