Planet-forming disk around small star offers big surprises

According to recent research, it has been found that smaller stars, known as very low mass stars (VLMS), are more abundant in the Milky Way compared to stars the size of our sun. These VLMS host a larger number of rocky planets, which are of particular interest to scientists in the search for potential habitable worlds.

The study has shed light on the formation of planets around these small stars by examining the chemistry of the planet-forming disks composed of gas and dust that surround them. Interestingly, these disks differ significantly from those observed around sun-like stars.

One example of such research involves the James Webb Space Telescope, which has collected data on a protoplanetary disk surrounding a newly formed star called ISO-ChaI 147. This star has a mass approximately 11% that of the sun, a diameter about 43% of the sun’s, and a luminosity around 2% of the sun’s.

ISO-ChaI 147 is located approximately 625 light years away from Earth. To put this distance into perspective, a light year is equivalent to the distance light travels in one year, which is approximately 5.9 trillion miles (9.5 trillion km).
Webb’s observations revealed that the gas in the protoplanetary disk surrounding this particular star is carbon-rich and lacks any traces of water vapor. This is in contrast to the oxygen-rich gas and abundant water vapor found in the disks around newly formed stars similar to our sun.

The question arises: could this difference impact the potential habitability of rocky planets that form around these smaller stars?

According to lead author and astrophysicist Aditya Arabhavi from the University of Groningen, Netherlands, “For a planet to be habitable, it typically requires liquid water and a suitable atmosphere.” Arabhavi’s study, published in the journal Science, explores the implications of the unique environment in Very Low Mass Stars (VLMS) disks on planetary composition and habitability.

Arabhavi acknowledges that while the distinct conditions in VLMS disks could strongly influence the composition of planets and their potential habitability, it is challenging to determine whether this makes them less likely to be habitable.
Stars are born when dense clusters of gas and dust in space collapse due to their own gravity. As a star forms at the center of these clusters, the remaining material forms a spinning disk around it. This disk fuels the growth of the star and can also lead to the formation of planets.

The disks surrounding larger stars, similar in size to our sun, contain more material compared to those around smaller stars. This abundance of material increases the likelihood of gas giant planets, such as Jupiter and Saturn, forming around these larger stars.

According to astrophysicist Thomas Henning, very low mass stars are the most common in our Milky Way galaxy. These stars are particularly interesting in the search for rocky planets because they are easier to detect. Additionally, the disks around these stars have enough material to form rocky planets, but not enough to form gas giants.
Scientists have already discovered rocky planets orbiting small stars, some of which are located in the “habitable zone” where conditions for life may exist. Henning, a scientist, believes that if these planets are in the zone where liquid water can exist, they are likely to remain habitable.

One such star, ISO-ChaI 147, is still in its early stages and may eventually become a red dwarf, a type of very small star. Compared to Jupiter, ISO-ChaI 147’s diameter is only about four times larger, while our sun’s diameter is approximately ten times larger than Jupiter’s.

In 2022, the Webb telescope detected ethane in ISO-ChaI 147’s protoplanetary disk, marking the first time this compound has been observed outside of our solar system. Additionally, the telescope discovered other carbon-rich gases that had not been previously observed in such a disk.
Arabhavi expressed gratitude for the valuable data provided by Webb, which was previously unattainable. While previous studies using the retired Spitzer Space Telescope suggested differences between planet-forming disks around very low-mass stars (VLMS) and sun-like stars, it was not known that these differences extend beyond slight variations. In fact, the gas composition of these disks is unlike any observed in sun-like stars thus far.

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