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Planets spotted in changing orbits

NASA's Kepler planet-hunting probe has spotted a system where two giant planets are locked in constantly changing orbits — with a super-Earth potentially pinned down in the crossfire.

Astronomers like to think of planets as a kind of celestial clockwork, keeping regular time. For example, the time it takes for the planets in our own solar system to complete their orbits can be calculated to within fractions of a second, and unless something huge happens, they'll stick to that timetable for billions of years.

In contrast, the two Saturn-size planets circling a sunlike star now known as Kepler-9, more than 2,000 light-years from Earth, shift their timetable with every go-round. Kepler-9b has an orbit lasting approximately 19.24 Earth days, while Kepler-9c has an orbit lasting a little more than twice as long, 38.91 days. But on average, Kepler-9b's orbit got about 4 minutes longer every time the Kepler astronomers checked, while Kepler-9c's averaged about 39 minutes shorter.

That suggests the planets are in the midst of a gravitational push-pull that keeps the orbits close to a 2-to-1 ratio, in what's known as a planetary resonance. In our own solar system, Pluto and Neptune are in a similar resonance (2-to-3), which is why little Pluto can't be kicked out of its orbit. The same thing applies to the Kepler-9 system.

"The system is stable in the sense that no planet will be ejected," said Matthew Holman, an astronomer at the Harvard Smithsonian Center for Astrophysics who is the principal author of a Kepler paper being published today on the journal Science's website.

"The orbits of the planets are changing, but these variations are oscillatory," Holman told me in an e-mail. "On average, the period ratio will be very close to 2-to-1. However, at any given instant that ratio may be bigger than 2-to-1 or smaller than 2-to-1."

Orbital variations has long been known to be theoretically possible, but Kepler-9 is the first confirmed planetary system where astronomers have been able to register this type of off-schedule behavior. It's actually quite a lucky break for the Kepler team. "The variations in what we call the transit times are large enough that we can use those transit timing variations to estimate the masses of thes bodies," Holman said in a Science podcast.

A question of timing
The $600 million Kepler mission looks for planets beyond Earth by having an orbiting telescope stare at a section of sky between the constellations of Cygnus and Lyra. That 15-foot-long, one-ton spacecraft looks for telltale dips in starlight that might be caused by planets crossing the disks of alien suns. By analyzing how long those dips last, and how frequently the dips occur, astronomers can figure out how large the planet could be. But they can't directly calculate how massive it is, and there's a chance that what they're seeing is not a planet at all.

The Kepler team is using other methods to make sure which among the hundreds of candidates they've found so far are truly planets — rather than, say, eclipsing binary stars or the glare of variable stars in the background. Usually, that requires follow-up observations by telescopes that look for the subtle shifts in starlight wavelengths caused by planet-induced gravitational wobbles. This interactive graphic explains how the various planet-hunting methods work.

The fact that Kepler-9's transit times were shifting immediately caught the Kepler team's attention, because that suggested a different method for confirming exactly what Kepler-9b and Kepler-9c were. Astronomers could plug those transit times into a computer model and run the numbers to see what types of objects could cause those weird orbits.

Objects the size of stars could be ruled out, because the transit timing variations would have been even larger in that case. Moreover, objects as massive as stars or brown dwarfs would be kicked out of the system relatively quickly. When the Kepler team ran a double-check with data from the Keck I telescope in Hawaii, the detection of a gravitational wobble confirmed that Kepler-9b and Kepler-9c were really, truly planets, Holman said.

"Now we have another tool to measure masses," Holman told me. "The combination of these methods is particularly powerful."

The researchers say this marks not only the first time that the transit timing method has been used to confirm a planetary detection, but also the first time that the transit method has been used to detect multiple planets in an alien solar system. That angle was touted in the NASA news release announcing the discovery.

"NASA's Kepler spacecraft has discovered the first confirmed planetary system with more than one planet crossing in front of, or transiting, the same star," the space agency declared in the news release. Of course, multiple-planet systems have been detected using methods other than pure transit observations. And with regard to the other "first," a different team of researchers previously reported using transit timing variations to study extrasolar planets, but they said the "final interpretation" of their results was still pending.

The Science research was held under embargo until 2 p.m. ET today, but the discovery came to light an hour early when NASA made its news release and other information about the observations publicly available.

Sub-Saturns ... and a super-Earth?
Holman and his colleagues estimate that Kepler-9b and Kepler-9c are both slightly smaller and less massive than Saturn. Theoretical models suggest that they're composed primarily of hydrogen and helium, like your typical gas giants. They appear to orbit in nearly the same plane, like the gas giants in our solar system. But the Kepler-9 planets have orbits that are significantly closer to their parent star than Mercury is to our own sun.

Astronomers assume that the planets formed farther out, in a colder region where ice and gas could collect, and then they circled inward in a complex orbital dance.

There might be another planet even closer in: When the researchers ran the numbers, they saw evidence that a world about one and a half times as wide as Earth was spinning around the Kepler-9 sun every 1.6 Earth days. If the evidence pans out, this planet could be about as massive as Earth, but hotter and more hellish than any world in our own solar system — sort of like the CoRoT-7b super-Earth that was identified a couple of years ago.

However, the Kepler team says it's too early to confirm that the Earth-scale candidate, currently known as KOI-377.03, is indeed a planet, let alone a super-hot super-Earth.

"The approach to confirming this as a planet will first be to exhaustively rule out all other possibilities for what could be causing the signal we see," Holman told me. "However, this will not establish the mass of this body. That will be left to future work."

Transit method


Astronomers had to unravel the patterns that Kepler saw in the dimming of starlight from Kepler-9, as shown in this graphic.

Today's Kepler findings have been the subject of intense speculation for the past few days, even before NASA announced that an announcement was on the way. Now that the news is out, the results may not be as world-shattering as some people expected.

To my mind, the announcement earlier this week that five to seven planets had been detected in a far-off solar system seems at least as significant as today's report from Kepler. But there's been an aura of mystery surrounding the hundreds of detections made by the Kepler team because those findings have been so closely held. The fact that today's news broke on Twitter and on the Web just adds to the Kepler aura.

Kepler's principal scientific investigator, Bill Borucki of NASA's Ames Research Center, surmised that the public is so interested in the mission because it could answer some of the biggest questions in astronomy: How common are Earthlike planets? How many possible homes for life exist? "We're not surprised at all about the attention that it's been getting," he told me during today's NASA teleconference.

As more of Kepler's discoveries come out, scientists — and science fans — will develop a better understanding of the wide and weird diversity of planetary systems. Let's hope this stuff never gets old.

More on planetary diversity:

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