The Hunt for Habitable Planets: Here and Now, a New Suite of Small Telescopes Are Poised to Look for Earthlike Planets beyond the Solar System
Cowen, Ron, Science News
For years, planet hunters have been preoccupied with hot Jupiters--giant, gaseous planets that tightly hug their sun-like parent stars. These massive, close-in planets, not yet directly seen, are the easiest to find because they induce the largest wobble in the motion of the stars they orbit. But now astronomers are following a rockier road--seeking rocky, icy planets only a few times as massive as Earth. Soon. astronomers predict, they will discover an Earth-sized planet that orbits within the habitable zone of its parent star. And if David Charbonneau has any say about it, that historic find will come from eight tiny telescopes his team has just finished assembling at the Fred Lawrence Whipple Observatory on Mount Hopkins in Arizona.
The telescopes, each only 40 centimeters in diameter, are designed to scan the 2,000 closest small, low-mass stars in the northern skies. The telescopes will look for signs that an orbiting planet periodically passes between the star and Earth, blocking a tiny but detectable amount of starlight every pass, or transit.
Nine years ago, Charbonneau, now at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his then adviser, Tim Brown of the National Center for Atmospheric Research's High Altitude Observatory in Boulder, Colo., pioneered the transit method of hunting planets. The technique offers a key advantage over the wobble method, which reveals both transiting and nontransiting planets but provides only their minimum mass and the time it takes to orbit.
By measuring the precise amount of light obscured by a passing planet, transit studies reveal the planet's size. When combined with the wobble data, transits also indicate a planet's true mass. In addition. the starlight filtering through the atmosphere of a transiting planet, like a flashlight shining through a fog, reveals the composition of gases clinging to these alien orbs--even though the planet itself would lie too close to the glare of its star to be imaged.
Before the discovery of the first transiting planet, HD 209458b, in 1999 "no one appreciated that these sorts of studies would be the ones" that would first provide information on a planet's composition, notes Charbonneau, "I don't think people understood at the time or really realized ... this would allow you to directly detect light from the planet without having to take its picture." Eventually, scientists from all over the world embraced the strategy.
For the first few years, Charbonneau and his collaborators, along with a slew of other astronomers, limited their search for transiting planets to relatively large, sunlike stars. An orbiting planet has a higher probability of making a transit, as viewed from Earth, when the body lies close to its parent star. And in order to block enough light from a big star to be detected, the transiting planet must also be relatively large. So the transiting objects researchers initially found were all giant, star-hugging planets--the hot Jupiters.
But around 2005, Charbonneau got another idea. If he could look for transits around an abundant group of dwarf stars. called M stars or M dwarfs, which are only about one-third the mass of the sun and smaller in size, he could find planets as small as twice Earth's diameter. "M stars are so small that you really could detect something as small as a superEarth--a planet five to 10 Earth masses--orbiting them if the planet passed in front," says Charbonneau.
Then he took another leap. Instead of just looking for a rocky planet not much bigger than Earth, he thought, "what about going for the big kahuna--habitability?" Because M Stars emit much less heat and light than sunlike stars, a planet closely orbiting one of these dwarfs might still lie in the habitable zone, where water would remain liquid. So not only could Charbonneau hope to find small planets, but also ones that might be capable of supporting life. …