Seeking another Earth among stars

The first spacecraft dedicated to finding potentially habitable planets beyond our solar system is poised to blast off from Cape Canaveral tonight on a three-year mission to probe 150,000 stars in the most sweeping hunt for Earthlike objects ever undertaken by NASA.

By the end of the Kepler mission, scientists will probably know whether planets like ours -- where liquid water can exist on the surface to nurture life -- are common in the universe, or so rare that we are virtually alone in the cosmic sea.

“This is not just another science mission,” said NASA Associate Administrator Ed Weiler in a news briefing at the space agency’s headquarters in Washington. “If you ask me, are there other Earths out there, I’d say absolutely. But I can’t prove it.”

“We certainly won’t find E.T.,” added Bill Borucki, the lead mission scientist from UC Berkeley, “but we might find E.T.'s home.”


The $590-million Kepler mission, scheduled to lift off at 7:49 p.m. Pacific time, consists of the widest-field telescope ever flown by NASA. The nearly 15-foot-long instrument has a 55-inch-wide mirror that can simultaneously scrutinize thousands of stars in its search for extrasolar planets, or exoplanets. It will accomplish this by looking for periodic dimming -- or winking -- of a star’s light caused by planets crossing in front of it, which scientists refer to as a transit.

Many of the 340 or so known exoplanets, principally discovered by a team in Europe and another at UC Berkeley headed by well-known planet hunter Geoff Marcy, have been found using the same method.

But most of those planets are gas giants, like Jupiter, that orbit very close to their parent stars. Those planets would be far too hot to sustain life, even if they had a rocky surface.

No potentially habitable planet has been found outside our solar system.

Finding one by using ground-based telescopes has been extremely difficult because of atmospheric interference. Even the Hubble Space Telescope, which has a larger mirror, falls short because its field of view is too narrow. It’s designed to look deeply rather than widely.

The Kepler spacecraft -- named after 17th century German astronomer Johannes Kepler, who discovered the laws of planetary motion -- will be able to scan a region of the northern sky between the constellations Cygnus and Lyra that contains about 4.5 million stars. Of the total, according to mission scientist Natalie Batalha of San Jose State University, 150,000 stars have been selected for intense study.

These are stars that are most like our sun, middle-aged and relatively cool, so that the “habitable zone” where life might flourish is close enough to the star that Kepler will be able to see a transit.

To spot a transit, Kepler will measure the brightness of each of these stars every 30 minutes for 3 1/2 years.


Even so, finding another Earth will be a challenge. Scientists figure that the change in brightness caused by an Earth-type planet will be less than 84 parts per million, or 0.008%. That’s equivalent to measuring from several miles away the change in brightness caused by a flea crawling across a car’s headlight.

Adding to the difficulty of the task is that dimming can be caused by events other than a transit, such as sunspots. Also, it is estimated that fewer than one star system in 100 will have planets properly aligned so that they pass between the star and Kepler’s camera.

Kepler scientists are trying to overcome all these problems with brute scientific force. They figure that by looking at thousands of stars, at least some will be in the right configuration with the right temperature to produce another Earth. “It’s all a numbers game,” Batalha said.

Finding a planet is just the first stage of the process. After detecting a wink, Kepler would measure how often the dimming occurred. Knowing the orbital period and the star’s size would allow scientists to discover the planet’s location and size.


The dimming caused by an Earth-type planet orbiting its star at about the same distance as our home planet from the sun would occur once a year, or about three times during the mission.

Kepler will launch on a three-stage Delta II rocket and eventually drift about 45 million miles away so it won’t have to contend with the reflected light of the moon and Earth.

Most of the stars in its survey are relatively close, from tens of light-years to 3,000 light-years away.

The first planets to be discovered in the coming months are likely to be more of the same gas giants that have been found so far. The earliest possible announcement that another Earth has been found would be December 2010.


Whatever Kepler uncovers, it is likely to change our view of our place in the cosmos. “It’s possible that Earths are very, very common,” Weiler said.

It’s also possible that Kepler will discover how rare Earths are, he said. “That to me is scary, because I don’t want to live in a universe where we’re the best there is.”