Giant exoplanet orbiting a star that is too small confounds astronomers

An exoplanet a little smaller than Saturn has been found orbiting a star just a third the size of the Sun, raising a confounding question for astronomers: how did such a large planet form around such a small star?

TOI-4860 b is an exoplanet about the size of Saturn, which orbits a red dwarf – one of the smallest star types known in the universe – with a mass just one third that of our Sun.

The planet’s radius is roughly 22% that of its star. That’s almost equivalent to the size difference between the Earth and the Moon

TOI-4860 b orbits  its star at break-neck speed, completing a single circuit in just 1.52 days.

The strange pair is described in two papers, one submitted to the Astronomy & Astrophysics journal and another published in the Monthly Notices of the Royal Astronomical Society.

While there is agreement between the two papers on the planet’s diameter, there is variation in its mass with the preprint paper’s authors saying it is 27% the mass of Jupiter, while the authors of the published work estimating TOI-4860 b to be 67% as heavy as Jupiter.

Either way, the planet is BIG. Too big for conventional methods theorised by astronomers for planet formation.

It is believed that, when stars form by accretion of gas and dust, a disc develops around the young star. This protoplanetary disc clumps together to form planets. Modelling suggests that the amount of material in the disc depends on the size of the star – larger stars have more material orbiting them, while smaller stars have less.

According to this theory, red dwarf stars should not have enough material to form these large planets. And yet TOI-4860 b exists. And it’s not the only one.

Of the nearly 5,500 exoplanets discovered to date, about a dozen gas giants have been found orbiting seemingly impossibly small red dwarfs.

Theories supported by different observations have emerged to try to explain the massive planet, tiny star combo.

In the preprint paper, Dr José Manuel Almenara and colleagues of the French National Center for Scientific Research report evidence of a second, unseen exoplanet orbiting the same red dwarf. That planet, estimated to have an orbital period of 426.9 days and a mass at least 1.66 times that of Jupiter, might have flung TOI-4860 b into its up-close orbit. But it only adds more mass to a system that has too much for our current models to understand.

Professor Amaury Triaud, of the University of Birmingham, UK, and colleagues write in their paper published in the Monthly Notices of the Royal Astronomical Society that both the planet and its star have high levels of heavy elements. But they note that the results were taken under suboptimal conditions and, therefore, are not well constrained.

Both groups of astronomers agree that further research, especially that which analyses the planets’ atmosphere, will be critical in understanding how such giant planets can form around small stars.