Sydney, May 2 (IANS): Australian astronomers have discovered a strange exoplanet orbiting a small cool star 500 light years away.

With an orbit of just one-tenth that of Mercury, the planet orbits its star every 3.3 days, the researchers found.

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

According to the researchers, the prevalent ideas about how planets form cannot explain the new finding.

"It must have formed further out and migrated in, but our theories cannot explain how this happened," Zhou said.

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.

To confirm the signal was a planet and not a blip in the system, the researchers used one of the world's largest telescopes, the Magellan Telescope in Chile.

The findings were detailed in the Astronomical Journal.