Finally, planet hunters see the light

It's taken 10 years since the first Jupiter-like planet was discovered around a sun-like star, but for astronomers searching for worlds beyond our solar system, it's been worth the wait.

This week, two teams working independently announced the first unambiguous detection of light from planets orbiting other sun-like stars. The achievements, researchers say, help set humanity on the doorstep of a golden age in exploring solar systems beyond our own.

Until now astronomers have detected their quarry through fleeting shadows or the subtle quiver of underbrush. They've had to rely on the faint dimming of a star as a planet swings in front of it or, more often, tiny wobbles that planets impart to their parent stars as they orbit. Although the technique the two teams used also is indirect, it finally reveals infrared light coming directly from the planets.

That light carries a wealth of information about the molecular composition of the planets' atmospheres. Armed with that information, scientists will pierce a critical barrier to uncovering the range of planetary environments that solar systems in our galaxy have to offer. The ultimate hope: finding Earth-like planets whose atmospheres carry the chemical signatures of life.

"These results are historic," enthuses Geoffrey Marcy, an astronomer at the University of California at Berkeley who heads one of the world's most prolific planet-hunting teams.

Over the past decade, and especially within the past few years, astronomers have been uncovering extrasolar planets at a furious pace. Dr. Marcy estimates that solid detections of planets orbiting other stars now number about 150. "We've discovered nearly all of the Jupiters and Saturns that exist around stars out to about 100 light years from Earth," he says. Many of them are so-called hot Jupiters - huge gas-giants orbiting very close to their parent stars.

These represent a wealth of targets for the approach the two teams used to detect their planets' light. Both used NASA's Spitzer Space Telescope, which observes in the infrared portion of the electromagnetic spectrum.

One team, led by David Charbonneau at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., turned Spitzer to a planet his team discovered last year orbiting a sun-like star some 489 light-years away in the constellation Lyra. The planet has roughly 75 percent of Jupiter's mass and orbits its star at a distance of only about 3.7 million miles. By comparison Mercury, our solar system's innermost planet, orbits some 36 million miles from the sun.

Dr. Charbonneau's planet, dubbed along with its parent star as TrES-1, swings between Earth and its star with each orbit. Viewed from Earth, such "transiting" planets add their light to the star's when the planet appears between the star and Earth. The star shines solo when the planet passes behind it. Charbonneau's group measured the combined infrared light, then subtracted the star's infrared signature after TrES-1 swung behind it.

The benefit of working in the infrared, Dr. Charbonneau explains, is that stars radiate far less of their energy in the infrared than in visible light. But planets this close to a star give off enormous amounts of heat shining brightly in the infrared. Thus, while the measurements are still challenging to make, it's far easier to distinguish the planet's radiation from the star's in infrared than in visible light.

Charbonneau's team is publishing its results in an upcoming edition of the Astrophysical Journal.

A second team, led by NASA's Drake Deming, applied the same approach to a planet known as HD 209458d, which orbits a star 153 light-years away in the constellation Pegasus. The planet, with only 70 percent of Jupiter's mass, is orbiting its star at a distance of 4.7 million miles. It has the odd distinction of having a diameter 35 percent larger than Jupiter's, despite its lower mass.

In both cases, the infrared data indicate that these planets reach temperatures in excess of 1,400 degrees Fahrenheit - hot enough to melt iron.

"They're getting blow-torched by their stars," Marcy says.

Sara Seager, an astronomer at the Carnegie Institution of Washington's Department of Terrestrial Magnetism and a member of Dr. Deming's team, notes that Spitzer's three instruments will allow astronomers to follow up and compile crude spectra of planetary atmospheres. The team's work is being published in Thursday's edition of the journal Nature.

Despite the historic nature of the two teams' success, the approach has its limits. If a transiting planet is too small or too far from the star to absorb and re-emit significant amounts of heat, it will go undetected.

Still, "Spizter is a lucky break," Marcy says. The telescope "was designed 25 years ago before we even knew of extrasolar planets." The telescope, launched in 2003, will give astronomers plenty to chew on while they await three orbiting telescopes NASA hopes to launch over the next decade or so. Progressively, each is designed to detect, then analyze, planets at a range of distances from their stars.