For 13 years, astronomers have inferred only the presence of planets circling other stars. Now, they have finally spotted them with their own eyes. In two papers published online today in Science, researchers report imaging four planets circling two other stars. Experts say this direct view could shed light on planet formation--and eventually even provide signs of alien life.
The challenge for direct detection of exoplanets has been picking out a planet's faint light in the overwhelming glare of its nearby star. All of the previously known 300-plus exoplanets had been found other ways, for example, by measuring how they gravitationally tugged on their parent star. Alternatively, the planet's gravity bends the passing light of a background star, or the planet blocks starlight by passing in front of its star. No one actually "saw" them.
The new observations required the latest, most sophisticated versions of two technologies. Large ground-based telescopes had to be fitted with so-called adaptive optics that compensate for the blurring effects of the atmosphere. These observatories and the orbiting Hubble Space Telescope also needed equipment to block most of the light from a central star.
With such modified scopes, astronomers led by Christian Marois of the National Research Council's Herzberg Institute of Astrophysics in Victoria, Canada, spotted three objects near a star 128 light-years away designated HR 8799. Another team, led by Paul Kalas of the University of California, Berkeley, found an additional exoplanet near the star Fomalhaut, just 25 light-years away and one of the brightest stars in the sky. All of the objects appeared to orbit their stars, fulfilling one requirement for a bona fide exoplanet detection.
Meeting the other requirement--a mass small enough to qualify as a planet rather than an undersized star--was trickier. Judging by the estimated young age of Marois's triple system and the infrared energy being radiated by the still-hot objects, they appear to be five to 10 times the mass of Jupiter, or under the 13-Jupiter-mass limit. With the Fomalhaut object, its minimal gravitational disruption of a nearby dust disk circling the star implies that it has at most three times the mass of Jupiter.
"The era of direct imaging is finally here," says astronomer Ray Jayawardhana of the University of Toronto in Canada. "We're finding an entirely new population of planets--more massive and distant from their stars" than detected using other techniques. With images from exoplanets in hand, astronomers will be able to infer their composition by analyzing the spectrum of their light, adds astrophysicist Alan Boss of the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C. That could reveal new secrets of how planets form as well as provide clues to alien life. Given a decade or two worth of technological progress, Boss says, astronomers could be making "the case that some exoplanets are not just habitable, but maybe even inhabited."