The discovery of a Saturn-sized gas giant orbiting a small red dwarf is urging astronomers to reconsider their theories of planet formation. Typically, astronomers find larger planets around larger stars, but this discovery breaks that connection. The finding puts pressure on the core-accretion theory, the leading explanation for planet formation.
Core accretion theory is the most widely accepted explanation for planetary formation. It describes how planet formation begins with tiny dust grains gathering together and forming planetary cores that grow larger through accretion. It explains much of what we see in our Solar System and others. It can result in rocky planets like ours or gas giants like Jupiter, which started as rocky cores before accreting their hydrogen atmospheres.
Massive stars tend to have massive protoplanetary disks, which means more material is available for planet formation. Low-mass stars are not expected to favour the formation of giant planets. In new research, astronomers from Europe, Chile, and the USA found a giant exoplanet orbiting the small red dwarf TOI-6894.
The research, published in Nature Astronomy, is titled "A transiting giant planet in orbit around a 0.2-solar-mass host star." The lead author is Dr. Edward Bryant from the Department of Physics at the University of Warwick in the UK.
"Planet formation models indicate that the formation of giant planets is substantially harder around low-mass stars due to the scaling of protoplanetary disc masses with stellar mass," the authors write in their article. "The discovery of giant planets orbiting such low-mass stars thus imposes strong constraints on giant planet formation processes."
TOI-6894 is only 20% as massive as the Sun and is about 240 light-years away. Its spectral type is M5V, meaning it's an M-dwarf (red dwarf), the most common type of star in the Milky Way and in the solar neighbourhood. The research team discovered the giant planet, TOI-6894b, during a large-scale effort to find giant planets around low-mass stars in TESS data. The planet has about 53 Earth masses, or about half the mass of Saturn, and its radius is slightly larger than Saturn's.
"I was very excited by this discovery," lead author Bryand said in a press release. "I originally searched through TESS observations of more than 91,000 low-mass red-dwarf stars looking for giant planets."
"Then, using observations taken with one of the world's largest telescopes, ESO's VLT, I discovered TOI-6894b, a giant planet transiting the lowest mass star known to date to host such a planet. We did not expect planets like TOI-6894b to be able to form around stars this low-mass. This discovery will be a cornerstone for understanding the extremes of giant planet formation," Bryand said.
The star is the lowest-mass star ever found with a giant planet orbiting it. TOI-6894 is only 60% as massive as the next lowest star to host a giant planet.
Core accretion models show that the ability to form a giant planet scales with the star's mass. Planets form from material in the protoplanetary disk, which scales with the star's mass. Because of this, astronomers expect to find fewer giant planets around low-mass stars.
Multiple studies confirm this, including this one from 2019 that concluded "The outcome of planet formation depends strongly on the initial disk and stellar properties. Massive planets can grow in circumstances when the initial characteristic disk size is larger, the initial disk accretion rate is higher, the central star is more massive, and the disk metallicity is higher."
To be clear, the core accretion theory doesn't exclude massive planets from forming around low-mass stars; it just makes it less likely. This discovery means the theory needs some adjusting. It also means we need to reassess our understanding of how many giant planets there are in the galaxy.
"Most stars in our Galaxy are actually small stars exactly like this, with low masses and previously thought to not be able to host gas giant planets. So, the fact that this star hosts a giant planet has big implications for the total number of giant planets we estimate exist in our Galaxy," said lead author Bryand.
The core accretion theory says that after a core forms, it eventually becomes massive enough to trigger runaway accretion. During that stage, giant planets form by quickly gobbling up available gas. Bryant says that there are two alternatives.
"Given the planet's mass, TOI-6894b could have formed through an intermediate core-accretion process, in which a protoplanet forms and steadily accretes gas without the core becoming massive enough for runaway gas accretion," Bryant said.
The other explanation is disk instability. It says that planets form when regions of the protoplanetary disk become dense and cool enough to collapse into protoplanetary cores. Most astronomers think that core accretion and disk instability both create planets, depending on disk conditions and location in the disk.
"Alternatively, it could have formed because of a gravitationally unstable disc. In some cases, the disc surrounding the star will become unstable due to the gravitational force it exerts on itself. These discs can then fragment, with the gas and dust collapsing to form a planet."
The researchers say that neither core accretion nor disk instability can completely explain the formation of TOI-6894b.
"As a very low-mass star hosting a transiting giant planet, the TOI-6894 system is a benchmark system for our understanding of giant planet formation and for challenging the current theories, which struggle to explain its presence," they write.
There may be a way to determine which process was behind TOI-6894b's formation. Within the next year, astronomers will use the JWST to probe its atmosphere. Since the planet is relatively cool, only 420 K compared to other hot Jupiters that are between 1,000 and 2000 K, its atmosphere should be easier to measure.
"Based on the stellar irradiation of TOI-6894b, we expect the atmosphere is dominated by methane chemistry, which is very rare to identify," said co-author Professor Amaury Triaud from the University of Birmingham. "Temperatures are low enough that atmospheric observations could even show us ammonia, which would be the first time it is found in an exoplanet atmosphere," Triaud said in a press release.
Atmospheric chemistry is a powerful diagnostic tool because the atmosphere preserves chemical signatures from its formation. The presence of volatiles like methane can indicate where in the disk the planet acquired its atmosphere from.
"The TOI-6894 system may, therefore, be a key exoplanetary system for determining the formation histories of giant planets, especially those with the lowest-mass host stars," the authors conclude.
&Press Release: Discovery of giant planet orbiting tiny star challenges theories on planet formation
Research: A transiting giant planet in orbit around a 0.2-solar-mass host star