RACE TO FIND ALIEN EARTHS

NASA is gearing up for a space race that's expected to point to the first truly Earthlike worlds beyond our solar system - and, like the race to put the first human on the moon, this marathon will take several years to run.

The roots of the race go back more than a decade, as astrophysicist Alan Boss explains in his new book, "The Crowded Universe: The Search for Living Planets." That's when pioneers in the planet-hunting field started detecting worlds around suns beyond our own.

The techniques used back then couldn't find other Earths in planetary systems like our own. The first method, pioneered by Polish astronomer Alexander Wolszczan at the Arecibo Observatory in 1991, could detect Earth-scale planets (and perhaps even the first known extrasolar dwarf planet) around radio pulsars - but those planets were thought to be burned-out cinders and not Earthlike at all.

In 1995, astronomers began reporting the detection of Jupiter-scale planets around normal stars, by precisely measuring the gravitational wobble those planets induce in the stars themselves. (Our interactive tutorial explains how it's done.) As the years have gone by, planet-hunters have gotten smarter about using that "Doppler wobble" technique, and they've also trained sensitive telescopes on faraway stars to measure the slight dimming in their light as alien planets make their transits over the stars' disks.

This transit method takes center stage in the next phase of the planet-hunting space race: The European Space Agency's Corot satellite, which was launched a little more than two years ago, has a head start. Just this month, members of the Corot science team announced the discovery of a "hot super-Earth" that is less than twice Earth's size.

Next month, NASA picks up the pace with the launch of its Kepler satellite, equipped with a planet-seeking telescope that has some advantages over Corot. Astronomers expect Kepler to turn up some true Earthlike planets, in Earthlike orbits, around sunlike stars.

"If Kepler comes up empty-handed - boy, it'll turn out to be virtual harakiri," Boss, a member of the Kepler science team, told me earlier this month. "But there's little chance of that."

The first fruits of the $550 million Kepler mission won't be the coolest alien Earths, Boss cautioned. "Often the oddballs are the earliest ones to find, for some reason," he said. Boss expects the Kepler team to announce the mission's first discoveries of hot Jupiters and hot super-Earths within a month after science operations begin.

The biggest factor behind that schedule has to do with the time scale of a planet's orbit. It takes at least three orbits for astronomers to confirm that the dimming of the star is really caused by a planet rather than, say, the brightness cycles of a variable star or a binary-star system. If the planet is extremely close to its star - which would be an oddball orbit by solar system standards - that won't take long. For example, the hot super-Earth identified by Corot completes an orbit in just 20 hours.

Farther-out planets will require more time to orbit, and therefore more time to detect.

"The earth, by definition, will take at least three years to get," Boss said. "Roughly four years from now, we will be beginning to make our claims for Earthlike planets around solar-type stars."

Boss' book traces the buildup to the Kepler mission through a series of time-stamped entries, reading almost like a diary. It's often been said that politics can get as messy as sausage-making - and based on Boss' accounts of Kepler's budgetary travails, the same can be said for pre-launch mission planning.

Along the way, Boss also delves into the deep scientific issues of the planet search:
Are new planets built from the core up, like dirty snowballs, or do they whirl into shape like stars are thought to do? (Boss says both processes come into play.)

How do you define stars, brown dwarfs, sub-brown dwarfs, planets and dwarf planets? (Boss was involved in many of those discussions, including the IAU's efforts to define planethood.)

What will it mean if (or when) Kepler finds those alien Earths? (Boss says finding out how many such planets exist among the more than 100,000 star systems that Kepler is expected to survey will reveal "the most basic parameter in any estimate of the prevalence of life in the universe.")

Kepler's primary mission is due to last three and a half years, but Boss hopes that the spacecraft will be up for some extra laps around the racetrack. Which mission will be the first to reveal just how common alien Earths are? Corot or Kepler? Considering that Boss is on Kepler's team, he's not the best person to handicap this race objectively. But in the end, it doesn't really matter who reaches the finish line first.

"Either way," he writes, "after centuries - if not millennia - of speculation and wondering, we will finally know just how crowded the universe really is."