NASA’s Kepler Telescope Finds a Wobbly World with Brutal Seasons

image: NASA

By now we know that exoplanets are extremely varied, and with some frequency a new sort exoplanet comes up that changes the way we think about these alien worlds. The latest is Kepler 413-b, another exoplanet found by NASA’s exoplanet hunting Kepler telescope and it’s a wobbly world if there ever was one.

Kepler-413b is a super-Neptune about 2,300 light-years away from the Earth in the constellation Cygnus. It orbits a close pair of orange and red dwarf stars roughly every 66 days, meaning its far too close to its stars for liquid water to exist on its surface making it a poor candidate for extraterrestrial life. But it’s still fascinating because Kepler 413-b wobbles as it orbits those stars. The planet precesses (or wobbles) on its spin axis by as much as 30 degrees every 11 years, a motion that would lead to fast and erratic seasonal changes. As a reference point, the Earth precesses on its spin axis by about 23.5 degrees every 26,000 years. Finding a planet precessing on a human timescale is baffling to astronomers.

But it’s not enough that the planet wobbles on its axis. Kepler 413-b appears to wobble in its orbit as well. The exoplanet’s orbit is tilted 2.5 degrees compared to the plane of the star pair’s orbit. That might not sound like a big difference, but seen from Earth that slight tilt means Kepler 413-b’s moves up and down relative to its star constantly. This makes it a hard planet to see. The Kepler telescope finds exoplanets by watching them transit their parent star or stars, and most of the planets we know about (including those in our solar system) do so regularly. A planet whose orbit fluctuates doesn’t transit its star regularly.

“Looking at the Kepler data over the course of 1,500 days, we saw three transits in the first 180 days—one transit every 66 days—then we had 800 days with no transits at all. After that, we saw five more transits in a row,” said Veselin Kostov, the principal investigator on the observation. And Kepler 413-b’s next transit we’ll be able to see isn’t predicted to occur until 2020. The Kepler telescope was lucky to find Kepler 413-b. Had it set its sights on the binary system a little earlier or later, the might have been wobbling right out of sight. It was looking at the right place at the right time.

Astronomers are still trying to figure out why Kepler 413-b is so far out of alignment with its stars. There’s no certain answer yet, but it’s possible there are other planets in the system or another nearby star exerting gravitational influence on the planet, causing it to trace out this oscillating pattern in its orbit.

Whether or not planets like this, that don’t conform to the regular orbital periods we see in our own solar system, are common has yet to be seen. “Presumably there are planets out there like this one that we’re not seeing because we’re in the unfavorable period,” said Peter McCullough, a team member with the Space Telescope Science Institute and Johns Hopkins University.

This latest discovery raises an interesting question: is there a whole class of planets we haven’t seen because their orbit have never been favorable for Earth-based observations? With Kepler now unable to focus on distant stars, it might be a while before we get any confirmation that there are more irregular exoplanets out there. Answering this question is likely a job for the James Webb telescope, when it finally launches.