An international team of astronomers discovered a giant exoplanet named TOI-6894b orbiting a red dwarf star called TOI-6894, which possesses only 20% of the mass of the Sun. The findings, published in the journal Nature Astronomy, challenge current theories about planetary formation and raise new questions about how and where giant planets can arise.

The discovery was made using data from NASA's Transiting Exoplanet Survey Satellite (TESS), a space telescope that scans the sky for planets passing in front of their stars. As part of a survey seeking giant planets around low-mass stars, researchers examined more than 91,000 red dwarf stars and found the planet TOI-6894b, which has only half the mass of Saturn.

"We analyzed observations of more than 91,000 low-mass red dwarf stars in the TESS data looking for giant planets," said Dr. Edward Bryant, an astrophysicist and first author of the study, according to Science Alert.

TOI-6894b is a low-density gas giant with a radius slightly greater than that of Saturn but only approximately 50% of Saturn's mass. The exoplanet orbits its host star every 3.37 days, much shorter than Mercury's orbital period. Despite its proximity to its star, TOI-6894b is unusually cold for a gas giant, barely reaching 140 degrees Celsius (420 Kelvin). The low temperature makes it possible to detect not only methane but even ammonia in its atmosphere, which would be a milestone, as these compounds have rarely been observed in exoplanet atmospheres.

"These low temperatures make it possible for us to detect not only methane but even ammonia, which would be a milestone: this compound has never been observed in the atmosphere of an exoplanet," said Francisco J. Pozuelos, a researcher involved in the study.

The discovery of TOI-6894b contradicts the previous premise that only massive stars can form large planets. TOI-6894 is the lowest-mass star known to host a transiting giant planet, underscoring the rarity of such discoveries. The existence of TOI-6894b defies expectations for planets around low-mass stars and challenges current theories about planetary formation.

"Previously we thought that such small stars cannot create or maintain giant planets. Stars like TOI-6894 are the most common type in the Milky Way, so our discovery suggests that there may be many more giant planets than we thought," said Prof. Jamila Chouquar from the University of Liège.

According to the core accretion theory, the formation of gas giants is more difficult around low-mass stars because the protoplanetary disk surrounding them has limited amounts of material. Limited material in the disk prevents the formation of a core massive enough to trigger the process required to form a giant planet. The existence of TOI-6894b suggests that alternative theories may be needed to explain its formation.

"None of the alternative models we considered can comfortably explain the existence of this planet, so the problem of understanding how these systems form is open," stated Dr. Mathilde Timmermans, one of the authors of the publication.

The uniqueness of TOI-6894b makes it a candidate for future observations with the James Webb Space Telescope (JWST). Its low temperature and especially deep transits enhance its potential for atmospheric characterization. Researchers are convinced that TOI-6894b will be a "reference planet" for the study of atmospheres dominated by methane and one of the best natural laboratories to explore atmospheres rich in carbon, nitrogen, and oxygen outside the Solar System.

"The study of its atmosphere is one of the most interesting follow-up investigations. We are eager to see what the JWST observations reveal," said Dr. Andrés Jordán, an astronomer from the Millennium Institute of Astrophysics (MAS) and an academic at Adolfo Ibáñez University in Chile.

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