Dwarf star with exceptionally giant planet discovered

Astronomers discover largest extrasolar planet around a red dwarf

Against all theory: Astronomers have discovered an unusually large planet around a red dwarf. The hot gas giant is the size of Jupiter, the largest exoplanet ever seen around a dwarf star. The strange thing about it: According to popular theory, such small stars should not be able to produce such planetary giants. Why this gas giant was born and whether other red dwarfs can have such large planets, is still open.

Astronomers have discovered the biggest exoplanets around a red dwarf. © University of Warwick / Mark Garlick

Typically, planets are created in the same cloud of dust and gas that produced their central star. The amount of material in such accretion disk determines how big the star can become and how much is left for its planets. If the material is sufficient for only one dwarf star, then, according to established theory, only small rocky planets form in its orbit.

Gas giant with Mini-Star

But astronomers around Daniel Bayliss of the University of Warwick have now discovered an exoplanet that torpedoes this theory. They discovered the planet observing a red dwarf about 600 light-years away with the twelve coupled telescopes of the Next-Generation Transit Survey (NGTS) at the Paranal in Chile. The dwarf star is only half the size of the sun.

The surprise: In orbit around this red dwarf circles a big gas giant. According to astronomers' estimates, the planet named NGTS-1b is at least as large as Jupiter and has only about 20 percent less mass. NGTS-1b is thus only the third gas giant, which was observed around a red dwarf - and by far the largest, orbiting its dwarf star very closely needing only 2.6 days for a round trip.

Surprise for astronomers

The discovery of MGTS-1b was a complete surprise to us, says Bayliss. Because so far we thought that so massive planets around such small stars can not occur.

In fact, many of the red dwarfs known to date are surrounded by smaller, Earth-like planets and planetary systems, including our nearest neighbor, Proxima Centauri, and the diminutive star TRAPPIST-1, only 40 light-years away.

But the discovery of the enourmous gas giant NGTS-1 now demonstrates that this is not the norm. Apparently, contrary to previous assumptions and models, red dwarfs can also produce planetary giants. Given that around 75 percent of all stars in the universe belong to these dwarf stars, this may throw a whole new light on planetary distribution in cosmos.

Our job now is to figure out how common such gas giants are around red dwarfs in our galaxy, says Bayliss.

A matter of age?

But how can such a giant planet emerge, if the material in the primeval cloud obviously did not even suffice for a larger star? After all, according to popular theory, a star takes on the lion's share of the material at its formation, with only a few percent remaining for planets and other objects in its orbit. This was also the case in our solar system: the sun swallowed 99 percent of the original cloud.

The astronomers suspect that the existence of the gas giant around NGTS-1 might be related to the age of the dwarf star. This is supported by its slow rotation and missing outbursts.

This may suggest that the formation of gas giants around such dwarf stars may have been possible earlier in the history of our galaxy, said Bayliss and his colleagues.

Source: Cornell University, 2017 arXiv