Rocky, carbon-rich exoplanets are more likely to orbit tiny stars, James Webb Space Telescope reveals

Using the James Webb Space Telescope, astronomers have discovered the richest hydrocarbon “menu” ever seen in a planet-forming disk. This observation, in which the protoplanetary disk surrounded a small star, revealed the first detection of ethane outside the solar system.

The discovery was made when the Mid-Infrared instrument (MIRI) on the James Webb Space Telescope (JWST) investigated the object “ISO-ChaI 147” as part of the Mid-Infrared Disk Survey (MINDS). ISO-ChaI 147 is a young star located in the Chameleon I region of about 237 stars. This region is located approximately 600 light years away.

These JWST observations of ISO-ChaI 147 suggest that protoplanetary disks of tiny stars are more efficient at forming smaller Earth-like planets than they are at forming much larger gas giants like Jupiter. Therefore, since low-mass stars are more common than large stars in the Milky Way, there may be more terrestrial planets in our galaxy than previously suspected.

The results also show that the planet-forming clouds of gas and dust surrounding tiny stars are built differently—at least, chemically—from those around stars about the size of the Sun and larger. The different chemical menu around these relatively small stars may mean that their rocky planets have very different atmospheres than Earth.

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

“This is very different to the composition we see in disks around solar-type stars, where oxygen molecules such as water and carbon dioxide are influenced,” said Inga Kamp, team member and researcher at the University of Groningen , in a statement.

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

Illustration of a red and yellow disk in space, with a diagram of complex molecules in front of it.

Illustration of a red and yellow disk in space, with a diagram of complex molecules in front of it.

What does this mean for exoplanet hunting?

Stars are born when massive clouds of gas and dust develop dense patches that eventually collapse under their own gravity. This process does not use up all that matter, however, resulting in infant stars being surrounded by vortices and clouds of gas and dust called protoplanetary disks. When patches of material in this disk condense, planets form – which is what happened around our infant sun about 4.6 billion years ago.

The amount of material in a protoplanetary disk and the distribution of that gas and dust limit the number of planets a star can host, as well as the building blocks it can provide for those planets. The JWST ISO-ChaI 147 results show that this protoplanetary disk is more suitable for giving birth to smaller rocky planets rather than larger gas giants.

Because the environments in protoplanetary disks determine the conditions under which new planets form, the discovery that disks around very low-mass stars and those around massive stars evolve into rocky planets with Earth-like characteristics has potential implications. to find them. However, tiny stars could host planets that are similar to Earth in many ways, but very different in others.

A few planets growing in a planet-forming disk around a glowing star.A few planets growing in a planet-forming disk around a glowing star.

A few planets growing in a planet-forming disk around a glowing star.

“Many of the primary atmospheres of these planets are likely to be dominated by hydrocarbon compounds and not so much by oxygen-rich gas such as water and carbon dioxide,” said Thomas Henning, MINDS team leader and researcher at the Max Planck Institute for Astronomy (MPIA ). , mentioned in the statement. “We showed in an earlier study that the transport of carbon-rich gas into the zone where terrestrial planets usually form occurs faster and is more efficient in these disks than in the heads of massive stars.”

The reason for the imbalance of carbon and oxygen between protoplanetary disks of stars with different masses is not currently understood. For example, it could be the result of discs around small stars being enriched with carbon, or from being depleted of oxygen.

If the former is true, it would mean that carbon enrichment could occur as solid particles in the disc are stripped of their carbon content. That material would be released as a gas. These solid carbon-deprived particles would form planets with rocky bodies that are low in carbon. However, the atmospheres of these worlds would be carbon dominated due to the excess of carbon gas in the environment in which they were born. So these rocky planets would eventually be around tiny, carbon-rich stars—and very different from Earth.

Research leader Aditya Arabhavi, also from the University of Groningen, said these results were possible because of JWST’s unique location about a million miles (1.6 million kilometers) from Earth.

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


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The MINDS crew now plans to investigate more protoplanetary disks around low-mass stars. This will help us figure out how common global, carbon-rich planet-forming regions like ISO-ChaI 147 are.

“Expanding our study will also enable us to better understand how these molecules can be formed,” said Henning. “There are still some features in the data that have not been identified, requiring further spectroscopy to fully interpret our observations.”

The team’s research was published Thursday (June 6) in the journal Science.

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