Thomas R. O'Donnell

Exoplanets from a somewhat different angle

In University research on October 27, 2013 at 11:15 pm
TILT TILT pinball machine

Photo credit: via photopin cc

It’s often how the best scientific discoveries start: When a scientist says, “Well, that’s weird.”

That was pretty much Steve Kawaler’s reaction when he saw data from Kepler-56, one of dozens of stars NASA’s Kepler space telescope flagged as likely to have orbiting planets, known as exoplanets.

Kawaler, an Iowa State University professor of physics and astronomy, is part of a committee overseeing Kepler’s asteroseismic (no, that’s not a typo) research. Asteroseismology is a lot like Earth-bound seismology, Kawaler says. “It’s the same mathematics, the same physics” describing wave propagation and response to conditions like pressure.

But with stars 3,000 light-years away, as Kepler-56 is, scientists can’t plant monitors on the surface to detect and measure waves, like they do here. Instead, astronomers watch for subtle oscillations in light from the star. The frequency and magnitude of those oscillations, combined with readings from instruments like spectrometers, can tell astronomers a lot, including the radius, mass and age of a star and how fast it rotates.

Asteroseismology and other data seemed to tell Kawaler and others in an international team something about Kepler-56 and its planets that they had never seen before.

Kepler, designed by NASA specifically to search for exoplanets, concentrates its observations on a patch of sky near the constellation Cygnus and the bright star Deneb, found almost straight overhead in the night sky. Kepler-56 is in that area, perhaps visible with a small telescope, Kawaler says.

Kepler monitors stars, watching for periodic decreases in their light intensity. Those dips in brightness may be times when a planet orbiting the star is transiting (passing in front of the star). Kepler only can detect planets directly in its line of sight, meaning the planets’ orbits must be in the same plane as the observatory; we must see them edge-on.

But “the orbit could be pointed anywhere,” Kawaler told me. Astronomers have calculated that only about one in a hundred stars will have planets with orbits aligned with our perspective. “That’s why Kepler is looking at so many stars: not all will have transiting planets.”

Kawaler said Kepler-56 is part of a larger study mission asteroseismologists were conducting into the stars’ inclinations – the angle at which their equators are tilted. In our solar system, Earth and all the planets orbit pretty much along the same plane as the sun’s equator.

That was proving true of stars the Kepler team evaluated, too. “In almost all of the cases we saw nice alignment,” Kawaler recalled. When Kepler-56 came up, though, “we said, ‘Well, that’s weird.’”

What was weird: The data indicated that Kepler-56’s equator was tilted, but the two planets circling it were not. Their orbits are about 45 degrees off from the plane of the star’s equator.

Illustration of the tilted axis of Kepler-56 and its orbiting planets.

Kepler-56, with two planets orbiting at a tilt to their host star. Image: Daniel Huber/NASA Ames Research Center.

Researcher Daniel Huber of NASA’s Ames Research Center in California “is the smart guy who pointed out early on” that oscillation data indicated Kepler-56 had to be tilted, Kawaler said. Huber “did the lion’s share on the asteroseismology side” and is first author of a paper published earlier this month in the prestigious journal Science (paywalled).

The data, combined with computer modeling, told the team that Kepler-56 is fairly old – about 4 billion years – and large, with a radius a bit over four times that of the sun, but only about a third more massive. Its temperature and surface gravity indicate the atomic fusion process generating the star’s light and energy has consumed the hydrogen in its core. It’s now burning hydrogen in a shell surrounding a helium core.

The planets the observatory spotted, meanwhile, are gas giants, about the size of Neptune or Saturn in our solar system. Planet b has a radius 6.5 times and a mass 22 times that of Earth. Planet c has a radius almost 10 times and a mass 181 times that of Earth. Both are orbiting fairly close to their star, Kepler-56. (Under astronomical convention a refers to the star itself.)

It wasn’t like scientists hadn’t seen planets orbiting at an angle to their star’s equatorial plane before. But usually they’re single “hot Jupiters”: gas giants at least as big as Jupiter in our solar system that orbit close to their stars. “Hot Jupiters can be all over the place,” Kawaler says, but generally are the only planet in their system; in order to orbit closely, they apparently scatter any other planets.

(By the way, “Hot Jupiters” would be a great band name. Wait, it already is. But they’re Canadian, so maybe it doesn’t count.)

(Just kidding, Canada! Please don’t send angry emails.)

What makes planets b and c unusual: It’s the first time researchers have seen a system with more than one planet orbiting at a tilt.

That meant there had to be a third body, Kawaler says. “The only way to have this stable (system) is if there’s another planet farther out continually torqueing the other planets” out of the standard equatorial orbit. (Don’t get confused: It’s torqueing, not twerking.) Theoretical modeling predicted just such a body, orbiting much farther out from the star.

Armed with that prediction, researchers from the University of California, Berkeley, booked time on the W.M. Keck Observatory, a giant telescope installation in Hawaii. With an instrument called the High-Resolution Echelle Spectrometer, they measured Kepler-56’s velocity through space and found a slow drift, verifying that there’s a third, massive object circling the star in a wide orbit.

The discovery is a great example of how theoretical calculations mesh with observations to advance science. “It was a prediction from theory that suggested there was a planet out there. That theory then guided some members of the team to go out and look for it,” Kawaler said.

Although Kawaler calls it a planet, the team isn’t exactly sure what this third companion is. The paper says it could be a gas giant within a few astronomical units of the star or a brown dwarf or star within several dozen astronomical units.

Regardless, the Science paper says, if this giant body is itself orbiting at an angle to the planets’ orbits, it could be torqueing them out of their equatorial orbits. The inner planets themselves would stay aligned with each other because their orbits are strongly coupled.

How did this happen? Physicists and astronomers believe most planets form out of a platter of dust swirling around new stars – a protoplanetary disc. In Kepler-56’s case, “something interesting – I don’t want to say catastrophic – must have happened relatively early in the formation of the system” to push the third companion so far out of whack, Kawaler says. Whatever that was, Kepler-56 “does tell us there’s lots of ways to make planetary systems.”

Meanwhile, a tilted orbit like the ones planets b and c experience could make life there more interesting, he adds. Rather than following a relatively dull path solely around their star’s equator, as our planet does, the Kepler satellites travel around the upper and lower latitudes.

The sun and other stars have strong magnetic fields. Solar flares, which throw off huge quantities of charged particles, typically follow those field lines. When those particles hit gas particles around Earth, the gases glow, producing the famous auroras.

“If we were orbiting the sun around its entire latitude, we would have a very different set of auroras and a very different experience,” Kawaler says – perhaps some wild light shows, all around the world.

  1. […] Steve Kawaler, an ISU physics and astronomy professor, is part of a Kepler project committee focused on asteroseismology: stars’ inner rumblings, similar to the quakes and tremors seismologists study to learn about Earth’s innards. In 2013 I wrote about another Kepler paper involving Kawaler that described a star with an unusual tilt. […]

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