For the first time, astronomers have seen the glow of alien planets circling sun-like stars. 鈥淭his is a new era,鈥� says the leader of one of the teams, Drake Deming from NASA鈥檚 Goddard Space Flight Center in Greenbelt, Maryland, US. 鈥淭his is the first time we have actually seen light.鈥�
Although planet hunters have bagged almost 150 extrasolar planets since the first one was spotted 10 years ago, until now they have only inferred the planets鈥� presence by measuring the wobble in the host star鈥檚 orbit or the dimming of the starlight as the planet passes in front of it. No one had yet seen the light from a planet directly.
Earlier this year, Glenn Schneider of the University of Arizona in Tucson, US, and his colleagues directly saw an object that was hailed by many as an extrasolar planet. But Schneider鈥檚 object, visible in infrared light, is five times as massive as Jupiter and is in orbit around a small, failed star known as a brown dwarf.
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Schneider himself refuses to use the 鈥淧-word鈥�, preferring instead to call the thing a 鈥減lanetary-mass object鈥�. 鈥淓veryone was calling this a planet in the press,鈥� says Schneider. 鈥淏ut this wouldn鈥檛 form as planets do.鈥�
鈥淗ot Jupiters鈥�
By contrast, Deming鈥檚 team and another led by David Charbonneau at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, have each studied a planet of the class called 鈥渉ot Jupiters鈥�, which orbit Sun-like stars. These gas giants probably evolved in a manner similar to those in our solar system.
The orbits of both planets, as observed from Earth, take them behind their host stars. The teams took advantage of this to tease out the planets鈥� radiation from infrared images captured by NASA鈥檚 Spitzer space telescope.
First they measured the radiation from each star-planet system when the planet was to one side of its star as viewed from Earth. Then they measured the radiation when the planet was behind the star. The difference between the two readings showed the radiation coming from the planet.
鈥淔rom the first moment we saw the data, we were ecstatic,鈥� says Charbonneau. 鈥淲e could see the signal of the planet passing behind the sun and coming back again.鈥�
Surface temperatures
Based on these measurements, the teams estimated the temperature of the planets. Both gas giants have surface temperatures of at least 700掳C.
鈥淚t is clearly a direct measurement of the flux from the planet,鈥� says Didier Queloz from the Geneva Observatory in Sauverny, Switzerland, who was behind the discovery of the first extrasolar planet 10 years ago. 鈥淚t is very impressive.鈥�
Deming鈥檚 planet, HD 209458b, has a mass two-thirds that of Jupiter and orbits a sun-like star in the constellation of Pegasus about 140 light years from Earth (Nature, DOI: 10.1038/nature03507).
Charbonneau studied a more distant planet called TrES-1, which has a similar mass and zips around its star in just three days. He reports these calculations in a forthcoming issue of The Astrophysical Journal.
