The planet HAT-P-7b, which is about 1.4 times as wide as Jupiter and 1.8 times as massive, seems to orbit its star in the opposite direction to the star’s spin (Illustration: Leiden Observatory)

Just a day after the announcement of the first extrasolar planet found orbiting its star backwards, two other teams announced the discovery of a second one.

“It is funny that the two good cases for really misaligned orbits, even retrograde orbits, have come at around the same time,” says Joshua Winn of MIT, lead author of one of the new papers.

Both Winn’s team and another, led by Norio Narita at the National Astronomical Observatory of Japan, used the Japanese Subaru telescope to observe planet HAT-P-7b, a previously known planet about 1000 light years from Earth that was recently observed by NASA’s new planet-hunting satellite Kepler.

Both teams found that the planet’s orbit is wildly tilted with respect to its star’s equator.


Star’s spin

Winn’s measurements indicate that the planet either runs backwards along the star’s equator, at a tilt of about 180°, or it orbits the star’s poles, at a 90° tilt.

It is difficult to distinguish between the two scenarios because astronomers can’t tell the angle at which the star is pointed towards Earth – they can only detect that one side of the star has a component of its velocity that is rotating towards Earth, while the other side has a component that is rotating away.

“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,” Winn says. “It could be like the solar system – but reversed, or it could be going pole over pole. Either way it’s cool.”

Because HAT-P-7b is in Kepler’s field of view, Kepler could solve the mystery by observing the movement of sunspots across the face of the star.

Data discrepancy

Narita’s group, on the other hand, reports that the planet’s orbit is inclined by 227° with respect to the star’s equator – a disagreement of about 45° with Winn’s numbers.

“Statistically, they’re highly discrepant,” Winn told New Scientist. “I have no explanation.”

The discrepancy could be due to differences in how the two teams used the telescope, or in the models they used to interpret their findings. They won’t know for certain until they can exchange their data, Winn says.

“It’s not uncommon for these measurements to be difficult enough for there to be ambiguity,” says Adam Burrows of Princeton University, who was not involved in the research.

Big tilt

Greg Laughlin of the University of California, Santa Cruz, who was also not involved in the work, agrees. “I think it’s just a small systematic error that is leading to the difference,” he told New Scientist.

“I think the take-home message is that both of the analyses are suggesting that you’ve got quite likely a retrograde planet,” Laughlin says. “The quality of the data is still slightly noisy. I think after they get a few more measurements, the discrepancy will be cleared up.”

“At least both data sets are pointing to the same qualitative conclusion, that the thing is tilted quite a bit,” Winn adds. The maximum tilt for planets in our solar system is 7°.

Shadow companion

Both HAT-P-7b and the planet announced on Wednesday, 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,” Winn says.

Sibling planet?

In fact, Winn’s team says the wobble of HAT-P-7b’s host star suggests the star has another, more distant planet in its system.

Could that as-yet-unconfirmed planet be responsible for HAT-P-7b’s backwards orbit? “We don’t know,” Winn told New Scientist. “That’s clearly something there that needs to be followed up.”

Although both teams studying the star HAT-P-7, as well as the team studying WASP-17, have submitted their papers to peer-reviewed journals, none have been accepted for publication yet.

‘Oddball’ planets

Winn did not know that the WASP-17 group was going to release their paper yesterday, but he claims not to mind being scooped.

“In the larger scheme of things, it doesn’t really matter,” he says. “It’s great for the field that there are two of these objects that were found at around the same time. It’s just another way in which the exoplanets are different – different from expectations and different from the solar system.”

“We’re catching so many planets these days, we’re bound to see some of the oddballs,” Burrows adds. “These aren’t going to be the last ones.”