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Weird wobbly world discovered

Kepler 413b

The super-Neptune planet orbits its stars once every 66 days in what astronomers call a circumbinary orbit (Source: NASA/JPL-Caltech)

All shook up A strange planet with erratic, rapidly changing seasons has been discovered orbiting two stars, by scientists using NASA's planet hunting Kepler space telescope.

The planet's strange seasons are a likely outcome of its misaligned and rapidly precessing orbit around its stars, according to the study's lead author Veselin Kostov of Johns Hopkins University and the Space Telescope Science Institute in Baltimore.

"We don't know if the misalignment is primordial, meaning it was there when the planet formed, or if it's a subsequent effect, caused by another planet in the system," says Kostov.

"We have also noticed a nearby star. If it's gravitationally part of the system, it could be the origin of the misalignment!"

In a paper to appear in The Astrophysical Journal, Kostov and colleagues describe the planet, named Kepler 413b, as a super-Neptune planet made up mostly gas and about 65 times the mass of Earth.

It was discovered 2300 light years away orbiting its stars once every 66 days in what astronomers call a circumbinary orbit.

The two stars in the system include an orange dwarf about 80 per cent the mass of the Sun, and a small red dwarf, half the Sun's mass.

Crazy planet dance

The planet's orbit wobbles and precesses over an 11-year period as it goes around the stars.

"There's a slight misalignment between the orbit of the two stars and the orbit of the planet around them, they're not all on the same plane," says Kostov.

"The difference is only about 2.5°, which isn't unusual. In fact the planets in our solar system don't line up exactly with the rotational plane of the Sun either."

According to Kostov, this misalignment means the gravitational pull that the two stars have on the planet, continually changes depending on the distance between each star and the planet, causing extreme orbital precession.

"Earth's orbit also precesses, but over time scales of 100,000 years," says Kostov.

Kostov believes there's a link between a planet's orbital precession and its rotational precession which would have dramatic effects on weather.

"If we make some reasonable assumptions of the planet's properties, such as mass, size and rotational speed, then we can estimate the rotational procession of the planet, what its axis is doing," says Kostov.

For Kepler 413b, Kostov and colleagues hypothesise its spin axis precession is 11 years, the same as its orbital precession.

"If both orbital and rotational precessions are similar, they could resonate, and the planet's rotational axis could vary by many degrees over very short time scales," says Kostov.

"This far-off planet is precessing on human timescales which is simply amazing. It would mean some crazy weather."

Up and down

Kostov and colleagues detected the wobble thanks to the unusual pattern it caused in the transits as the planet passed in front of its stars, as seen from Kepler.

Kepler finds planets by looking for regular dips in light from a star caused by a passing, or transiting, planet.

Because of the orbital wobble, Kepler 413b's orbit continuously moves up or down. The difference can be enough to miss passing in front of the stars, as seen from Kepler.

"Over 1500 days, we saw three transits in the first 180 days, roughly one every 66 days, then nothing at all," says Kostov.

"I was disappointed, but didn't give up, which was good because eventually the transits came back after 800 days and we had five in a row."

Tags: planets-and-asteroids