Astronomers have discovered a planet whose existence challenges previous theories.

The star TOI-6894 is located about 240 light-years from Earth in the constellation Leo, but it is similar to many other stars in our galaxy. It is a small red dwarf with a mass that is only 20 percent of that of our Sun. As a result, astronomers did not suspect that it could provide the right conditions for the formation and existence of a large planet. However, it turned out to be the opposite. Around this tiny star orbits a gas giant, called TOI-6894b. The results of the discovery were published in Nature Astronomy.

Scientists on the Trail of Giant Planets

This cosmic pair defies existing theories about planet formation. The unusual system was discovered by a team of scientists as part of a large-scale study of data from the Transiting Exoplanet Survey Satellite (TESS). The aim of the study, led by Edward Bryant, was to search for giant planets orbiting low-mass stars.

"We didn't expect planets like TOI-6894b to form around stars of such low mass. This discovery will be a cornerstone in understanding the extremes of giant planet formation," Bryant said.

Astronomers say they have not yet found a smaller star to support such a large planet. TOI-6894b is a low-density gas giant. Its radius is slightly larger than that of Saturn, but it has only 50 per cent of the planet's mass. Dr Vincent Van Eylen, from the Mullard Space Science Laboratory at University College London, said: "This is an intriguing discovery. We really don't understand how a star of such a low mass can form such a massive planet! This is one of the goals of the search for more exoplanets. By finding planetary systems other than our own solar system, we can test models and better understand how our own system formed."

How could the planet TOI-6894b have formed?

The most common theory of planet formation is the core accretion theory. It assumes that the core of a planet initially forms as a result of the gradual accumulation of material, and as it becomes more massive, it attracts gases that form an atmosphere. Eventually, it is large enough to accrete gas, ultimately becoming a gas giant. Such planets are unlikely to form near low-mass stars, because the amount of gas and dust in the protoplanetary disk around that star (i.e. the amount of material needed to form a planet) is too limited to allow the formation of a sufficiently massive core. However, the existence of TOI-6894b contradicts this theory and suggests that the model described above may not be accurate. As a result, there is a need to develop alternative theories.

“Given the planet’s mass, TOI-6894b could have formed via an intermediate core accretion process, where a protoplanet forms and continuously accretes gas without the core becoming massive enough for runaway gas accretion,” said Edward Bryant. “Alternatively, it could have formed via a gravitationally unstable disk. In some cases, the disk surrounding the star becomes unstable due to the gravitational pull it exerts on itself. These disks can then break apart, and the gas and dust collapse to form the planet,” he added.

However, the team of astronomers found that none of these theories can fully explain the formation of TOI-6894b. The origin of this giant planet is – for now – a mystery. One way to solve it could be through detailed atmospheric analysis. By measuring the distribution of material inside the planet, astronomers can determine the size and structure of the core, which could tell us whether TOI-6894b formed by accretion or an unstable disk.

Gas giant surprises scientists

The planet is also unique in terms of its conditions. Most gas giants are like hot Jupiters, with temperatures of 1,000–2,000 Kelvin. Meanwhile, TOI-6894b has a temperature of just 420 Kelvin.

"Based on TOI-6894b's stellar radiation, we expect its atmosphere to be dominated by methane, which is very rarely identified. The temperatures are low enough that atmospheric observations could even show the presence of ammonia, which would be the first time it has been detected in an exoplanet's atmosphere. TOI-6894b is likely a model exoplanet for studying methane-dominated atmospheres and provides the best 'laboratory' for studying planetary atmospheres containing carbon, nitrogen and oxygen outside the Solar System," said Amaury Triaud, one of the researchers.

Over the next 12 months, the James Webb Space Telescope will observe the atmosphere of the planet TOI-6894b. The results may help determine which theory explains the formation of the gas giant.