Researchers have identified a massive gas planet orbiting a small red dwarf star, a unique pairing that has left astrophysicists puzzled.
The star, named TOI-6894, resembles many others in the Galaxy. It’s a red dwarf, characterized by its small size, low brightness, and mass—only 20% that of our Sun. Scientists have traditionally believed that such stars lack the necessary conditions to support the formation of giant planets.
However, an international team of astronomers has detected the unmistakable signature of a gas giant orbiting TOI-6894, as detailed in a study published today in the journal Nature Astronomy.
TOI-6894 hosts the smallest star known to have a giant planet in orbit around it. The planet, designated TOI-6894b, has a radius marginally larger than Saturn but is only half its mass, completing a full orbit around its star every 3.36 days.
To uncover this find, researchers utilized photometric data from the Transiting Exoplanet Survey Satellite (TESS) during their quest for giant exoplanets around dwarf stars. The existence of TOI-6894b was later verified with ground-based telescopes, particularly the Very Large Telescope (VLT) in Chile.
“Most stars in our Galaxy are small like this one, with minimal mass, and until now, we didn’t consider that they could host giant gaseous planets,” explained Warwick University professor Daniel Bayliss, one of the study authors. This discovery suggests a need to reassess estimates regarding the prevalence of giant planets in the galaxy.
“This is a fascinating finding. We still don’t fully understand how a low-mass star can develop a giant planet! This is one of our research goals in exoplanets: by discovering systems different from our own, we can run simulations to gain insights into our own planetary formation,” remarked Vincent Van Eylen, a researcher at the Mallard Space Science Laboratory in Britain.
The prevailing theory for planetary formation is accretion. This process begins in a protoplanetary disk—a swirl of gas and dust encircling a young star. A new planet forms as materials gradually accumulate. As it grows, it gathers gases to develop a thick atmosphere, eventually becoming a gaseous planet.
However, this theory posits that the formation of giant planets is challenging around low-mass stars due to insufficient gas and dust in the protoplanetary disk.
An alternative hypothesis cites gravitational instability. The protoplanetary disk can become gravitationally unstable and fragment, with gas and dust then clumping together to form a planet.
Current data, however, do not support the notion that TOI-6894b formed via this mechanism. A detailed analysis of its atmosphere might provide insights into the planet’s origins, offering valuable information about its core’s size and structure.
This atmosphere is of interest for additional reasons: most giant, gaseous exoplanets discovered so far are classified as “hot,” similar to Jupiter, with temperatures between 1000 and 2000 Kelvin (726 to 1726 degrees Celsius). In contrast, TOI-6894b is relatively “cold,” with temperatures not exceeding 420 Kelvin (146.85°C). Researchers also suspect that its atmosphere is largely composed of methane.
The James Webb Space Telescope will study the planet’s atmosphere over the next year.
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