Theories of how planets form may have to be re-examined following the detection of a distant giant gas planet "too big for its star," Australian astronomers say.

The unexpected size of the observed exoplanet in relation to its star has scientists scrambling for an explanation.

"We have found a small star with a giant planet the size of Jupiter orbiting very closely," says George Zhou of the Research School of Astrophysics and Astronomy at The Australian National University.

"It must have formed further out and migrated in, but our theories can't explain how this happened," he says.

The planet is so close to its star that it completes a full orbit every three days.

Because it is so close, it is impossible to see it directly. Its existence was detected by a decrease in the light coming from the star as the planet moved in front of it, in what is known as a transit.

To confirm initial observations captured by robotic telescopes at the university's Sliding Spring Observatory, the astronomers sought help from one of the world's largest telescopes, the Magellan Telescope in Chile, and from an amateur astronomer, Thiam-Guan Tan, using his small telescope in the back yard of his home in Perth.

It's not the first time Tan has helped discover or confirm an exoplanet, with at least 11 to his credit.

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

Astronomers have determined the exoplanet's mass is similar to that of Saturn, although in size it is much more like Jupiter, making it "quite a puffed-up planet," Zhou says.

The planet's host star, classed as an M-dwarf and sitting some 500 light years from Earth, emits only a twentieth amount of the light or our sun, which makes such stars difficult to study.

It also makes the exoplanet unlike any discovered before, Zhou says.

"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 says.

After two decades of searching, the number of exoplanets found orbiting distant stars now stands at more than 1,800. An exoplanet is the term used to refer to a planet that orbits a star outside the solar system.

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