Giant planet discovered orbiting a tiny star

A tiny star just one-fifth the mass of our Sun has shocked astronomers by hosting a gas giant planet, TOI-6894b – something long thought to be nearly impossible. The discovery challenges leading theories of planet formation.

 

A planet around a tiny star

TOI-6894 is a small red dwarf, just 20 percent the mass of our Sun. Stars like this are common throughout the galaxy, but they are not usually considered likely to host giant planets. In fact, astronomers have long believed that such low-mass stars do not have the right conditions to form or support large planets.

 

But in a surprising new discovery published in the journal Nature Astronomy , scientists have detected clear signs of a giant planet – dubbed TOI-6894b – orbiting the tiny star.

The planet was found in a large survey of data from NASA's TESS mission, which searches for exoplanets by monitoring the brightness of stars. The project focuses on detecting giant planets around smaller stars and is led by Dr Edward Bryant of the University of Warwick and UCL's Mullard Space Science Laboratory.

 

A landmark discovery

Dr. Edward Bryant said:

I was excited by this discovery. Initially, I searched through TESS's observations of more than 91,000 low-mass red dwarfs looking for giant planets.

Then, using observations made with one of the world's largest telescopes, ESO's Very Large Telescope (VLT), I discovered TOI-6894b, a giant planet transiting the lowest-mass star known to host such a planet. We had no idea that planets like TOI-6894b could form around such low-mass stars. This discovery will be a cornerstone for understanding the extreme limits of giant planet formation.

The lightest star to host a giant planet

The planet (TOI-6894b) is a low-density gas planet with a radius slightly larger than Saturn but only ~50% of Saturn's mass. The star (TOI-6894) is the lowest-mass star known to have a transiting giant planet and is only 60% the size of the next smallest star known to have such a planet.

 

Dr Daniel Bayliss, Associate Professor at the University of Warwick, said: ' Most stars in our Galaxy are actually small stars exactly like this one, which are low in mass and were previously thought to be incapable of hosting gas giant planets. So the fact that this star hosts a giant planet has major implications for the total number of giant planets we estimate to exist in our Galaxy .'

Dr Vincent Van Eylen, from the Mullard Space Laboratory, said: ' It's a fascinating discovery. We really don't understand how such a low-mass star could have formed such a giant planet! This is one of the goals of the search for more exoplanets. By finding planetary systems other than our own, we can test our models and better understand how our own solar system formed. '

The most widely accepted theory of planet formation is called core accretion theory. A planetary core first forms through accretion (the gradual accumulation of matter), and as the core becomes larger, it eventually attracts gases that form an atmosphere. It then becomes large enough to enter a runaway gas accretion process to become a gas giant.

According to this theory, the formation of gas giant planets is less likely around low-mass stars because the amount of gas and dust in the protoplanetary disc around the star (the raw material for planet formation) is too limited to allow a large enough core to form and uncontrolled accretion to occur.

 

However, the existence of TOI-6894b (a giant planet orbiting an extremely low-mass star) shows that this model cannot be completely accurate, and alternative theories are needed.

Given the planet's mass, TOI-6894b may have formed through an intermediate core-accretion process, in which a protoplanet forms and steadily accretes gas without the core becoming large enough to undergo uncontrolled gas accretion.

Alternatively, TOI-6894b may have formed as a result of a gravitationally unstable disc. In some cases, a disc of material surrounding a star becomes unstable due to the gravitational pull it exerts on itself. These discs can then fragment, with gas and dust collapsing to form a planet.

But the team found that neither of these theories can fully explain the formation of TOI-6894b from the available data, leaving the origin of the giant planet an open question.

One way to unravel the mystery of how TOI-6894b formed is to analyze its atmosphere in detail. By measuring the distribution of material inside the planet, astronomers can determine the size and structure of the planet's core, which could tell us whether TOI-6894b formed through accretion or through an unstable disk of material.

This isn't the only interesting thing about TOI-6894b's atmosphere; it's also unusually cold for a gas giant. Most gas giants found by exoplanet hunters are "hot Jupiters," gas giants with temperatures around ~1000-2000 Kelvin. In contrast, TOI-6894b is only 420 Kelvin. The cool temperature, along with other features of the planet, such as its very deep transits, makes it one of the most promising giant planets for astronomers to characterize with a cool atmosphere.

Professor Amaury Triaud, from the University of Birmingham, said: ' Based on the stellar radiation received by TOI-6894b, we predict the atmosphere is dominated by methane chemistry, which is extremely rare .' The temperature is low enough that atmospheric observations could even show us ammonia, which would be the first time it has been found in the atmosphere of an exoplanet.

TOI-6894b has the potential to become a benchmark exoplanet for studying methane-dominated atmospheres, and the best 'laboratory' for studying a planetary atmosphere containing carbon, nitrogen, and oxygen outside the Solar System.

The atmosphere of TOI-6894b is scheduled to be observed by the James Webb Space Telescope (JWST) over the next 12 months. This will allow astronomers to determine which of several possible theories can explain the formation of this unexpected planet.