The largest known planet in the universe, TrES-4, described for the first time last August, is a real head scratcher. This giant has about five times the volume of Jupiter but only four-fifths the mass. Given a 151,000-mile-wide bathtub, the planet could float on water. TrES-4 is so large and light that it challenges existing ideas about planet formation, says Georgi Mandushev, an astronomer at the Lowell Observatory who led the team that found the planet. “This planet is impossibly big,” he says.
But for the modern-day planet hunter, breaking records is all in a day’s work. The discovery of TrES-4 comes only months after the news of Gliese 581c, the smallest planet discovered outside our solar system. To top it off, nearly 40 discoveries this year have swollen the ranks of exoplanets to about 250, up by 20 percent.
The rapid rate of discovery of exoplanets can be attributed to the maturity of Doppler spectroscopy, by which astronomers measure a planet’s gravitational tug on its host star, and by a technique involving “transiting” planets—looking for planets that move between their host stars and Earth, the method used by Mandushev to find TrES-4.
Planet hunting is still in its infancy, and the search for the holy grail, an “alien Earth,” continues. The first foreign planet orbiting a star was confirmed a mere 11 years ago, and promising swaths of space like the Goldilocks zone, where the conditions are just right for liquid water, have yet to reveal habitable planets.
Today “the discovery of planets around other stars has become blazing hot for administrators of big telescopes,” says Geoff Marcy of the University of California at Berkeley. “We are being allocated enormous amounts of time [on those instruments],” he says. With increased funding for planet hunters, NASA’s plans to launch the $550 million planet-seeking Kepler mission in 2009, and the French national space agency’s launch of the alien-Earth-hunting COROT late last year, the exosolar ranks should continue to grow.