New 'Jupiter-sized' exoplanet challenges ideas about how planets form

A team of Australian astronomers has discovered strange exoplanet orbiting very close to a small cool star 500 light years away that is challenging ideas about how planets form.

Researcher George Zhou from the Australian National University said that they have found a small star, with a giant planet the size of Jupiter, orbiting very closely, adding that it must have formed further out and migrated in, but their theories can't explain how this happened.

In the past two decades more than 1,800 extrasolar planets (or exoplanets) have been discovered outside our solar system orbiting around other stars.

The host star of the latest exoplanet, HATS-6, is classed as an M-dwarf, which is one of the most numerous types of stars in galaxy. Although they are common, M-dwarf stars are not well understood. Because they are cool they are also dim, making them difficult to study.

HATS-6 emits only one twentieth of the light of our sun. The giveaway that the faint star had a planet circling it was a dip in its brightness caused as the planet passed in front of the star, observed by small robotic telescopes including telescopes at the ANU Siding Spring Observatory.

To confirm the signal was a planet and not a blip in the system, Bayliss called in help from one of the world's largest telescopes, the Magellan Telescope in Chile, and an amateur astronomer, T G Tan, who operates from his backyard in Perth.

T G Tan has been really helpful on our projects. He was able to catch the transit of the planet from Perth, after it had set over the horizon, Zhou said.

Zhou noted that the planet has a similar mass to Saturn, but its radius is similar to Jupiter, so it's quite a puffed up planet and because its host star is so cool it's not heating the planet up so much, it's very different from the planets we have observed so far.

He added that the atmosphere of this planet will be an interesting target for future study.

The study is published in Astronomical Journal.