Gas giants, like other planets, form from protoplanetary disks of material around protostars. According to core accretion theory, they first form a core of rock, ice and other heavy solids, attracting an outer layer of gas once this core is sufficiently massive (about 15 to 20 times that of Earth). However, low-mass stars have low-mass disks that, models predict, would not provide enough material to form a gas giant in this way, or at least not quickly enough before the disk breaks up. In new research, astronomers have measured the occurrence rate for giant planets orbiting low-mass stars and demonstrated this occurrence rate to be non-zero.
An artist’s impression of a gas-giant exoplanet and its parent star. Image credit: Sci.News.
In their research, Dr. Ed Bryant, an astronomer at University College London and the University of Warwick, and colleagues looked at 91,306 low-mass stars, using observations from NASA’s Transiting Exoplanet Survey Satellite (TESS), and in 15 cases found dips in the brightness of the light corresponding to a gas giant passing in front of the star.
Five out of the 15 potential giant exoplanets have since been confirmed as planets using independent methods.
One of these confirmed planets orbits a star that is a fifth of the mass of the Sun — which would not be possible according to planet formation models.
“Low-mass stars are better at forming giant planets than we thought,” Dr. Bryant said.
“Our results raise serious questions for planet formation models.”
“In particular, our detection of gas giants orbiting stars as low as 20% of the mass of the Sun poses a conflict with current theory.”
“The fact that, although rare, gas giants do exist around low-mass stars is an unexpected finding and means that models of planet formation will need to be revised,” added Dr. Vincent Van Eylen, an astronomer at University College London.
One possible interpretation is that gas giants do not form through core accretion but through gravitational instability, where the disk surrounding a star fragments into planet-sized clumps of dust and gas.
If this is the case, low-mass stars could host very large gas giants, two or three times the mass of Jupiter.
However, this is considered unlikely, as the disks around low-mass stars do not appear to be massive enough to fragment in this way.
Another explanation is that astronomers have underestimated how massive a star’s disk can be, meaning small stars could form giant planets via core accretion after all.
This could either be because we have incorrectly calculated the mass of disks we can observe through telescopes, or because disks have a greater mass at the start of a star’s life, when they are very challenging to observe (as they are embedded in clouds of dust), compared to later in a star’s life when we can observe them.
“It’s possible we don’t understand the masses of these protoplanetary disks as well as we thought we did,” said Dr. Dan Bayliss, an astronomer at the University of Warwick.
“Powerful new instruments such as the NASA/ESA/CSA James Webb Space Telescope will be able to study the properties of these disks in more detail.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
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Edward M. Bryant et al. The occurrence rate of giant planets orbiting low-mass stars with TESS. MNRAS, published online March 3, 2023; doi: 10.1093/mnras/stad626
