Kepler to locate new Earths?

The Kepler probe, launched 11 months ago to hunt for Earthlike worlds around other stars, has already discovered planets, an astrophysicist said Monday night. Although none yet announced is Earthy, five were detected in the probe’s first 43 days of taking data — and eight months have passed since then.

Jason Steffen of the Fermilab Center for Particle Astrophysics told a capacity crowd at Clark Planetarium about Kepler’s first results, which were announced in January. A graduate of Davis High School and Weber State University, Ogden, Steffen earned his master’s degree and doctorate at the University of Washington, Seattle.

[Jason Steffen of Fermilab discusses the Kepler probe’s search for Earthlike exoplanets Monday at Clark Planetarium. Photo by Cory Bauman]

The probe downloads photographs of 150,000 stars every 30 minutes, searching for a relatively brief dimming of starlight that might indicate a planet is crossing in front. As it keeps watch on the same stars month after month, patterns develop that show how often planets transit.

Kepler’s 37-inch-diameter telescope is armed with a 96-million-pixel CCD camera. Its target is a patch of stars in the Cygnus-Lyra region, a field about 15 degrees across.

According to NASA, the $600 million Kepler project could help answer a major scientific question: among the hundreds of billions of stars in our galaxy, what proportion could host life as we know it? If Kepler were to find many Earthlike planets within the stars’ Goldilocks zone — just the right distance away, where oceans neither boil nor freeze solid — the answer would be much different than if it were to spy none.

The crowd’s size was a pleasant surprise to planetarium director Seth Jarvis. “We are excited to see so many people interested in learning about extrasolar planets,” he said. Half an hour before the lecture began, cashiers reported that only about 40 tickets were sold. Then the auditorium filled.

“Right now this is the only planet that we know of that is habitable,” Steffen said. But that could change within a few years as Kepler’s results come in.

Planets are not uncommon; while only eight are known in our solar system, within the past 15 years astronomers have found about 400 around other stars, and the number increases monthly. Most are found by observing a planet’s gravity tugging at a star, which limits the discoveries to planets so big, and often so close to the star, that life on them is highly unlikely.

Kepler uses a different technique, studying the intensity of a star’s light.

If a Jupiter-size planet were to transit its star, the starlight would drop by a factor of one part in 1,000, a minuscule dimming of light from a star already so faint that it can’t be seen by the naked eye. But to detect an Earth-size planet, the instrument needs to be able to tell the difference between full-strength starlight and that light dimmed by one part in 100,000. Kepler is sensitive enough to do that, Steffen said.

Unfortunately, nine times out of 10 a dimming is a false alarm. It could be that the star is part of a binary pair, with one star eclipsing the other and changing the light intensity. Or perhaps background stars throw off the measurements. Determining whether the dimming is a true transit or an artifact is the job of several ground-based observatories, which make detailed readings of promising candidates found by Kepler.

When a transiting planet is confirmed, however, scientists can deduce a great deal of information about it, based on the time of transit, amount of dimming, and effect on the star itself. Using spectroscopic analysis and other observations, they can calculate its size and mass, determine whether it orbits around the star’s equator or at an angle, and even know which direction it is moving relative to the star’s own rotation.

Three or more transits must be recorded to confirm a planet’s presence, besides the other studies needed to weed out the many false positives. That means if an Earth-size planet were orbiting at Earth’s distance from a sun-like star, taking a year to go the circuit, the team would need three years to confirm that they had found a sister planet to ours.

In January Kepler scientists released data on the first five planets discovered, the smallest of which was Neptune-size, Steffen said. None was in a star’s habitable zone. In fact, mysteriously, one object is three times hotter than the star it eclipses, and nobody knows why.

Since then a huge amount of data has been collected but the team isn’t ready to announce further findings.

Steffen said celestial mechanics vastly reduces the number of planets that may be detected. Most would orbit at an angle that does not bring them directly between Earth and the star, and Kepler would miss them.

“Out of that 150,000 stars we expect to find 200 planets,” he said. How many of these would be Earth-size and in the habitable zone is anybody’s guess — maybe five or ten.

After the lecture, Nightly News asked when an Earthlike planet will be confirmed.

“That’s a very good question. I don’t think it’ll be within the next year.”

Steffen said additional Neptune-size planets may be announced first, as well as “super Earths” up to 10 times as massive as this one.

Three years may not be needed to prove the discovery of a habitable planet, however. Kepler is looking at various sized stars, including those smaller than the sun. A small, cooler star’s habitable zone would be closer in, allowing a quicker orbit, so the required three transits might happen much sooner than three years.

Meanwhile, plenty of possible planets are turning up. “We’ve had hundreds of candidates,” Steffen said.

“But which ones are planets? That’s bulk of the work.”

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