Astronomers hunting alien Earths

Alien worlds beckon to us Earthlings today, promising visions of a sister planet to our own orbiting a nearby star.

NASA's Kepler space telescope team this month unveiled the first indisputable detection of a rocky planet spotted in another star's solar system. The find lifted hopes that planets with rocky cores like Earth fill many stars' solar systems. Mission scientists led by William Borucki of NASA's NASA Ames Research Center in Moffett Field, Calif., promise to report on hundreds of more planetary candidates next month.

Dubbed "Kepler-10b," the recently-announced rocky planet roasts in an orbit far too hot for life, circling less than two million miles above its sun-like star. "You have to see this as a warm up for Kepler, literally," says mission team scientist Sara Seager of MIT, however, pointing to expected future finds.

But will we be looking in the right places when the time comes to hunt for another Earth?

"Although no such planets have yet been discovered, the first will be found within the next couple of years," write astrophysicists Jonti Horner of Australia's University of New South Wales and Barrie Jones of the United Kingdom's Open University in the current Astronomy & Geophysics journal. "And so the time is right to discuss exactly what factors might come together to make such planets more, or less, suitable for life to develop and thrive."

Since the 1995 discovery of a planet orbiting the star 51 Pegasi, which resides about 50 light years away from us, by Swiss astronomers, more than 500 planets have been detected orbiting nearby stars, according to the list kept by Jean Schneider of the Paris Observatory. Most of them look jumbo-sized — "gas giant" planets that resemble Jupiter in our solar system. But with the recent discovery of Kepler-10b, a planet 1.4 times wider than Earth but 4.6 times more hefty, an era of more Earth-like planet discoveries appears at hand.

"Once one is found, many more will quickly follow, and the search for life beyond our solar system will begin," Horner and Jones write. A key concept is the "habitable zone" for planets, a region that is too warmed by their star for water to freeze and too cooled for water to boil off — a region stretching from Venus to Mars in our solar system. But pointing to the (apparent) lifelessness of those two planets despite them inhabiting this zone, they ask: "How do we differentiate between one exo-Earth and the next? What factors help determine which planets are most suited to the development of life?"

Nobody knows for sure. Undaunted, the two astronomical experts propose a number of things to look for when starting the hunt for another Earth:

•The right star: Stars much bigger than our sun burn out too quickly, within a billion years, too soon for life to arise. Scratch big stars. Some stars, such as our nearest neighbor, Proxima Centauri at some 25.4 trillion miles away, are "variable" and release massive flares that make them 100 times brighter, in a few minutes time, which might bake growing things on any planets they possess.

•The right solar system: Based on our solar system, some astronomers initially suggested that a gas giant such as Jupiter might serve as a shield for Earth-like planets, soaking up comets that would otherwise smack into the worlds that harbor life and sterilize them. But, say Horner and Jones, "this idea is, simply, wrong. In fact, giant planets in a planetary system are more of a double-edged sword." Gas giants might send more comets than they deflect towards their Earth-sized brethren, they found in an earlier analysis.

•The right orbit: One surprise of the exoplanet era has been the elongated "eccentric" orbits of many of the planets. More than half of the discoveries have orbits more elongated that Earth's nearly circular one. Such orbits can take a planet into and out of a star's habitable zone. And they might expose the Earth-sized planet to the gravitational pull of other planets that pull them into periods of freezing or frying climates that wipe out chances for life, says the study. "Just because a planet's orbit makes it currently appear habitable, it does not necessarily follow that that planet's orbit will have been the same for a protracted period of time."

•The right continents: Water on Earth appears to lubricate plate tectonics, the movement of light continental crust across the planet's constantly reworked surface. Continents feed minerals necessary for life, such as calcium, into the environment. Ultimately, plate tectonics is driven by the heat of the Earth's core, which also provides a strong magnetic field that shields our atmosphere from being stripped away, like the Martian sky.

On the good news side, these conditions leave a lot of potential Earth-like planets still out there. The "M" class stars, smaller and longer-lived than our sun, are the most plentiful kind in the universe, and a Nature journal study last month found there may be three times more of the stars filling galaxies throughout the universe than expected. The dimness of those stars might mean that Earth-sized worlds in their habitable zones orbit so close to the star that they become gravitationally locked so they always have one face perpetually looking onto the star and potentially baking. But a 1999 study led by Martin Heath of the Biospheres Project in London, found that even then, worlds with atmospheres would distribute that heat evenly, leaving habitable conditions intact.

Some caution about planet discoveries might be in order, though. The September announcement of the planet Gliese 581g by a team led by Steven Vogt of the University of California-Santa Cruz and Paul Butler of the Carnegie Institute of Washington (D.C.), has come under fire. Circling just such an M-class dwarf star, the planet was announced as orbiting in a potentially habitable zone. Other astronomers, such as the Geneva Observatory's Francesco Pepe, have since said they couldn't confirm the planet's detection, raising questions about its existence. (Noting that three outside groups have now reported they can't find Gliese 581g, Pepe says in an email, "It turns out today that Vogt's conclusions were wrong.")

Life, of course, might not be confined to watery worlds such as our own. Other commentaries in the current Astronomy & Geophysics ponder whether glacier-lined volcanoes on Mars might shield life, or if the frigid plains of Saturn's cloud-draped moon, Titan, might harbor microbes.

Not an easy business, looking for life elsewhere. Nevertheless, given the pace of verified planet finds, roughly 10 a month now, and Kepler's coming cornucopia of candidates, we should turn to thinking about which stars to aim future telescopes at today, so we can begin planning for even more exciting discoveries, Horner and Jones conclude.

"We have just a few short years to prepare ourselves for that search, which promises to yield the most exciting result science has ever witnessed — the detection of life beyond the Earth."a

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