Image: Hot exoplanet
C. Carreau / ESA
An artist's conception shows a hot exoplanet in a tight orbit around an alien star. The Kepler probe's practice target is thought to be in a similar setting.
updated 8/6/2009 4:41:20 PM ET 2009-08-06T20:41:20

NASA's planet-hunting Kepler spacecraft is in good shape to begin discovering Earthlike planets, according to its first science results, released on Thursday.

The space telescope, launched in March 2009, detected the giant extrasolar planet HAT-P-7b within its first 10 days of taking data. Although this planet was previously discovered by ground-based telescopes, the fact that Kepler found it and measured it in such great detail bodes well.

Kepler is on a quest to root out distant worlds that resemble our own Earth, and which might be hospitable to life.

So far, the various Earth-based and space-based planet-hunting telescopes are only able to find worlds that are significantly larger and hotter than our own, because they are easier to see. But Kepler was designed to be capable of detecting a distant Earth-sized planet in an Earthlike, yearlong orbit, which would give it roughly the same temperature range as our home.

The preliminary results indicate the observatory is up and running as expected.

"This tells us that Kepler has the photometric precision necessary to see Earthlike planets," said Jon Jenkins, a Kepler co-investigator at the SETI Institute in California. "Kepler's prospects for detecting Earth-sized planets transiting sunlike stars are good: The instrument is performing much as expected."

Practice target
Planet HAT-P-7b is nothing at all like Earth. This gas giant world circles its star every 2.2 days in a tight orbit that heats up the planet's surface to a boiling 4,310 degrees Fahrenheit (2,376 degrees Celsius).

"It is as hot as a glowing-red heating element in your stove or toaster," said David Koch, Kepler deputy principal investigator at NASA Ames Research Center in California. "That means we can see light from the planet itself."

As Kepler watched HAT-P-7b's host star, the telescope observed a dip in the star's brightness when the planet passed in front of it in what's called a transit, blocking some of the star's light. Kepler also noted a smaller dip in light when the planet passed behind the star, because the planet's small contribution to the total brightness was briefly shielded.

Previous observations of HAT-P-7b were not able to measure this second, smaller effect, called an occultation.

"The depth of the signal from that occultation, that tiny blocking of the light from the planet itself, is about the same as a transit would be for a planet as small as Earth," Koch told "So we measured the same kind of signal as if we were detecting an Earth by seeing this occultation."

Image: Habitable zone
This graphic shows the distribution of detected planets, including worlds in our own solar system and the exoplanet HAT-7-b. The green band indicates the conditions (based on planetary mass and temperature) where life as we know it could survive.

Finding Earth’s twin would take longer
That's good news for the Kepler team, although the Earthlike discovery that scientists dream about will have to wait a while. Since Kepler looks for transiting planets that produce very limited signals, at least a few orbits must be completed before a signal is noticeable.

In the case of HAT-P-7b, this requires only a few days, but for a truly Earthlike planet in an Earthlike 365-day orbit, about three years will be necessary to discern a pattern.

Kepler was launched in March 2009 on a 3.5-year mission, which scientists hope to extend if all goes well. The telescope is designed to scan one portion of the sky, looking at about 100,000 stars simultaneously to seek signs of planets temporarily obscuring their light. Astronomers hope the mission will yield small terrestrial planets in what's called the habitable zone, where a planet is not too far or too close to its star and its temperature range is similar to Earth's and can host liquid water.

"Our mission is designed so that if Earths in Earthlike orbits are common we expect to find as many as [roughly] 50 such planets," Jenkins said.

If the mission is successful, humans could be a step closer to finding alien life in the universe. No matter what it finds, the results will be revealing. If Kepler completes its term without finding any Earthlike planets, it could suggest that Earth, and life, are unique or at least rare.

The Kepler team, led by principal investigator William Borucki at Ames Research Center, detailed Kepler's first results in a paper published in Friday's issue of the journal Science.

More on extrasolar planets | Kepler

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