New York (PTI): Astronomers have discovered a second exoplanet orbiting its star backwards, a day after the first "retrograde" exoplanet was spotted.

Using the Japanese Subaru telescope to observe planet HAT-P-7b, two teams, one led by Joshua Winn of MIT and another led by Norio Narita at the National Astronomical Observatory of Japan, have discovered the second exoplanet.

"It is funny that the two good cases for really misaligned orbits, even retrograde orbits, have come at around the same time.

"We don't know if it's a slowly rotating star that we're seeing edge-on, or a really rapidly rotating star that we're seeing pole-on. It could be like the solar system -- but reversed, or it could be going pole over pole. Either way it's cool," Winn said.

However, both the teams disagree on the tilt of the exoplanet's orbit.

While the US team's measurements indicate that the planet either runs backwards along the star's equator, at a tilt of about 180degrees, or it orbits the star's poles, at a 90degrees tilt, the Japanese team claimed that the orbit is inclined by 227degrees with respect to the star's equator.

"Statistically, they're highly discrepant. It could be due to differences in how the two teams used the telescope, or in the models they used to interpret their findings. They will not know for certain until they can exchange their data," Mr. Winn was quoted by the 'New Scientist' as saying.

HAT-P-7b is a previously known planet about 1000 light years from Earth that's recently observed by NASA's new planet -hunting satellite Kepler.

Both HAT-P-7b and the first planet WASP-17b, are "hot Jupiters" -- gas giant planets extremely close to their stars.

HAT-P-7b, which is 1.4 times as wide as Jupiter and 1.8 times as massive, is smaller and heavier than WASP-17b, which may be the biggest and least-dense exoplanet found to date.

Both planets may have been thrown into their bizarre orbits by a close encounter with another planet, or by the gradual gravitational pull of an unseen companion star (see Planet found orbiting its star backwards for first time).

"The theories are all on the same theme: You need at least one other body in the system that does something to mess up the orbit," Mr. Winn said.