Kepler Spacecraft Sets Sights on Earth-like Planets

If Friday?s launch goes according to plan and successfully lobs NASA?s new Kepler space telescope into orbit, the mission stands to potentially change the way we look at the universe.

Kepler is designed to turn its eye on thousands of stars in our own Milky Way galaxy and look for signs of Earth-sized planets orbiting in a region conducive to supporting life.

"Kepler will push back the boundaries of the unknown in our patch of the Milky Way galaxy, and its discoveries may fundamentally alter humanity's view of itself," said Jon Morse, director of NASA's Astrophysics Division at the agency?s headquarters in Washington, D.C.

The probe is slated to launch on Friday from Cape Canaveral Air Force Station in Florida aboard a Delta 2 booster. NASA delayed the mission by one day for extra rocket checks after the loss of another probe just after its launch last week.

The $600 million Kepler mission is named for Johannes Kepler, the 17th century German scientist who pioneered the fields of optics and planetary motion.

"Now, 400 years later, we're using his discoveries in order to answer a profound and fundamental question about our place in the universe: Are there other Earth-like planets out there?" Morse said.

Kepler will use an unprecedented combination of light detectors (adding up to about 95 million pixels) to capture the subtle shifts in light that characterize an extrasolar planet orbiting its star. For comparison, a high-end digital camera on Earth might have 10 megapixels, but Kepler?s detectors add up to 95-megapixel array, mission researchers said.

Looking for Goldilocks

Over the last two decades, scientists have spotted more than 300 extrasolar planets circling other stars in our Milky Way galaxy. Most of these planets have been about the size of Jupiter or larger, making it unlikely they would harbor life. But those aren't what Kepler is aiming to find.

"We're interested in planets like that of Earth, rocky planets in an orbit where life might be possible," said principal investigator for Kepler science William Borucki of NASA's Ames Research Center in Moffett Field, Calif.

Kepler will be pointing its 0.95-meter diameter telescope and array of 42 charge-coupled devices (light-sensitive microchips also found in standard digital cameras) at a pre-selected target group of 100,000 stars.

Kepler will be looking for fluctuations in the light of each star that result from a planet transiting ? or moving in front of ? its parent star as seen from the perspective of Earth. These fluctuations can tell scientists how big the planet is, as well as how far away its orbit is from its parent star.

"When a planet goes across a star, it blocks some light," Borucki explained. "The bigger the planet, the more light it blocks, so we get the size of the planet from the size of the dimming."

By looking for multiple transits, the Kepler team can determine the planet's orbital period, or how long it takes the planet to circle its star. Each planet will need to be observed through at least three transits, Borucki said, to determine the period and to make sure the dimming isn't due to some other astronomical phenomenon, such as a spot on the star.

"We don't want to have false discoveries; we want to be very, very sure that when we say it's an Earth, it really is an Earth," Borucki said.

If the potential planet has a short orbital period (a few days or weeks), that means it orbits very closely to its sun. A long period (of several years) would mean it sits closer to the edge of the star's gravitational grip. In general, such extreme orbits would make the planet too hot or too cold, respectively, for life to arise there.

Borucki and the rest of the Kepler team are interested in finding a planet with a more middle-of-the-road orbit, one that is "not too hot, not too cold, but just right," he said. Such orbits, which fall in what is called the sun's "habitable zone," would mean that the temperature of the planet was mild enough for liquid water to exist on the surface.

Stellar continuum

Of course, this Goldilocks orbit won't be in the same place for every star, which can have large variations in the amount of light they put out.

Kepler will be looking at three main types of stars: A-type stars, G-type stars (the group our sun belongs to), and M-dwarfs.

A-type stars are the hottest of the bunch, which shifts their habitable zone further away from the star.

They "are very luminous, they put out a lot of energy, and their habitable zone is actually much further away from the star than for our Sun," said astronomer Debra Fischer of the San Francisco State University, who is not directly involved with the Kepler mission.

The habitable zone for G-type stars would lie in the range of Earth's orbit, so any likely exoplanet candidates would have an orbital period of about one year.

M-dwarfs, meanwhile, lie on the other extreme.

"These very low luminosity stars are not giving out as much energy," Fischer explained. "To be in the habitable zone around one of these low-mass stars, you have to move in a little bit so you can warm up, basked in the starlight."

So planets in the habitable zone of an M-dwarf will have much faster orbital periods than those in the habitable zones of G-type or A-type stars.

"And so what Kepler will be doing is watching as the planet goes across, blinks the starlight out, and with the M-dwarfs, the frequency of the blinks of planets in the habitable zone is going to be much higher," Fischer explained.

Hot Jupiters and twin Earths

Kepler won't be sending back news of a twin Earth right away though. Once it is up in orbit and properly calibrated, it will likely churn out detections of bigger planets first.

The first planets to "roll out the Kepler assembly line" will likely be the so-called 'hot Jupiters,' Fischer said. The planets are intriguing to astronomers because they are the size of Jupiter but sit at the orbit of Mercury.

"The shear number of these objects that Kepler will find is going to help us learn a lot about these systems," Fischer said.

Next to roll out will likely be the similarly-named 'hot Neptunes," and then finally, "the hardest detection, and by far the most exciting, is going to be the detection of bonafide Earths," Fischer said.

While Kepler won't be able to tell astronomers what the planets look like, Fischer thinks there will be great variety in the other potential Earths out there, for example, 'water worlds' covered entirely by oceans.

"I think that the science fiction writers are going to be challenged to imagine the diversity that we could expect to find even in these types of planets," she said.

Just how many other Earth's Kepler will find is anybody's guess though. "because we don't really know what's out there," Borucki said.

"Kepler's designed to find hundreds of Earth-size planets if such planets are common around stars; dozens of these planets if they're in the habitable zone," Borucki said. "If we find that many, it certainly will mean that life may well be common throughout our galaxy, because it's an opportunity for life to have a place to evolve."

"If on the other hand, we don't find any, that will be another profound discovery. It will mean that Earths must very rare, we may be the only extant life in our universe," Borucki added. "It'll mean no 'Star Trek.'"

Borucki emphasized that no matter what Kepler finds, it won't find little green men.

"Although Kepler will not find E.T., it's helping to find E.T.'s home," he said.

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