Astronomers have discovered an unusual planetary system consisting of a Jupiter-sized planet orbiting a small star only four times the size of the Solar System’s gas giant. This “forbidden” configuration of a giant planet orbiting a relatively small star could challenge theories about how giant planets form. The exoplanet or “exoplanet” orbits a red dwarf called TOI 5205, which is much cooler and smaller than the Sun. The most common type of stellar body in the Milky Way, these M-dwarf stars’ small size and relatively low temperatures make them redder than the Sun.

Although, on average, this class of stars has more planets around them than any other star type, their formation was previously thought to make it unlikely to have gas giants orbiting them. The discovery of this exoplanet, named TOI 5205b, by astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) challenges this concept. The planet was confirmed and characterized by the team using a variety of ground-based telescopes and instruments.

“Main star TOI-5205 is about four times the size of Jupiter, but somehow managed to form a Jupiter-sized planet, which is pretty amazing!” Group leader and Carnegie Science astronomer Shubham Kanodia, expression

While gas giants have been detected around M dwarf stars before, none have been detected orbiting such a low-mass example of this star class TOI-5205.

Planets form in rotating disks of gas and dust called “protoplanetary disks” that surround young stars. This material is remnants of the same substance that collapsed to give birth to the central star. When dense points collapse under the influence of their own gravity, the cores of planets are born, which collect more material.

Modern models of planet formation show that the birth of a gas giant requires material equivalent to 10 times the mass of the Earth. This first creates a rocky core, which then accumulates gas to form a disk to grow a giant planet. However, this process must happen quickly.