Giant planet around tiny star: Discovery challenges planet formation theories

Scientists have discovered a giant planet orbiting the smallest known star to host such a companion — a finding that defies current theories of planet formation.

The host star, TOI-6894, is a red dwarf with only 20% the mass of the Sun, typical of the most common stars in our galaxy. Until now, such low-mass stars were not thought capable of forming or retaining giant planets.

Publishing their findings today in Nature Astronomy, an international team of astronomers, including researchers from the Universities of Birmingham and Warwick, and UCL reveal that the unmistakable signature of a giant planet — TOI-6894b — has been detected in orbit around this tiny star.

This system has been discovered as part of a large-scale investigation of TESS (Transiting Exoplanet Survey Satellite) data, looking for giant planets around low-mass stars.

The planet (TOI-6894b) is a low-density gas giant with a radius a little larger than Saturn’s but with only around 50% of that planet’s mass. The star (TOI-6894) is the lowest mass star to have a transiting giant planet discovered to date and is just 60% the size of the next smallest star to host such a planet.

The planetary nature of the signal was then confirmed by an extensive ground-based observation campaign, involving several telescopes — including by a series of 1m robotic telescope, part of the SPECULOOS network, of which the University of Birmingham is a partner.

The University of Birmingham’s role in the SPECULOOS consortium is to lead the confirmation of planet candidates thought to orbit such weak stars. Two PhD students in the SPECULOOS team became interested in gas giant planets transiting small stars.

Dr George Dransfield (then at Birmingham, now a junior research fellow at Madgalen College, Oxford) and Dr Mathilde Timmermans (then a PhD student at the University of Liège , now a researcher at the University of Birmingham) started a dedicated effort called MANGOs to confirm gas-giant candidates transiting M dwarfs. TOI-6894b was identified and rapidly validated as a planet thanks to their efforts.

When the SPECULOOS team announced those results to the scientific community, it became clear another team led by the University of Warwick/UCL had also independently confirmed this particular system. They agreed to collaborate on publishing and announcing this discovery together.

Dr George Dransfield commented: “We were so excited when our observations revealed TOI-6894b was most likely an exoplanet. Its configuration, such a low mass star, for such a massive planet, is unique and a fantastic opportunity to learn more.”

According to current planet formation models, giant planets are rare in orbit around such small stars. This is because their protoplanetary discs — the gas and dust reservoirs from which planets form — are thought to lack the material needed to build massive cores and accrete thick gas envelopes.

Dr Mathilde Timmermans commented: “The existence of TOI-6894b is hard to reconcile with existing planet formation models. None can fully explain how it formed. This shows that our understanding is incomplete, and underscores the need to find more such planets. That’s exactly the goal of MANGOs, a SPECULOOS sub-program led by myself and Dr George Dransfield.”