This artist's impression of planet HD 209458b shows plumes of hot gas billowing from its surface as it skims just 4 million miles from its star.© ESA, Alfred Vidal-Madjar (Institut d'Astrophysique de Paris, CNRS, France) and NASA

Light from planets outside our Solar System has been genuinely detected for the first time. The breakthrough marks the end of a long race between astronomers to image an alien world directly, without recourse to theoretical detective work.

Although more than 130 extrasolar planets, or exoplanets, have been discovered, almost all were found by detecting the way they make their parent stars wobble as they orbit. Visual identification is difficult because light from the star swamps the relatively dim glow of a planet.

So astronomers have been looking for infrared light from these planets, because their emissions in this area of the spectrum tend to be much brighter than any visible shine they have. The infrared light can reveal details of the planet's temperature and chemistry, but conclusive evidence has proved elusive (see ""Exoplanet may have been pictured at last":http://www.nature.com/news/2005/050110/full/050110-14.html").

Now two research groups say they have definitely spotted infrared light from two different exoplanets using NASA's orbiting Spitzer Space Telescope. In both cases, the planet's orbit seems to take it across the face of its star before disappearing behind it. With each transit and eclipse, the total amount of infrared light coming from the region rises and falls. By subtracting the constant starlight, the astronomers are left with a measure of the light from the planet itself.

"It's an extremely important milestone," says Alan Boss, an astronomer at the Carnegie Institution of Washington. "These are the first steps along the path to identifying Earth-like planets."

Double whammy

The two planets are already known, despite the fact that no one has seen them before. One of them, HD 209458b, was the first exoplanet ever found to transit its star, although astronomers didn't realize this when they first discovered it. The recent observations were made by Drake Deming of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and his team. They publish their findings online in Nature¹.

The second group, led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, has made similar measurements of a planet called TrES-1, to be published in The Astrophysical Journal².

Both planets are classed as 'hot Jupiters'. These are massive objects that take about three days to complete an orbit a few million miles from their parent stars, far closer than Jupiter is to our own Sun.

"It's one of the most significant detections ever made," says Sara Seager, part of Deming's team. "We're opening up a new window in exoplanet detection."

Seager says that HD 209458b is between 850 °C and 1450 °C, and is likely to have water vapour in its atmosphere. Further observations and computer modelling should pin down its precise temperature, and reveal more about its atmospheric chemistry.

"It's an awesome experience to realize we are seeing the glow of distant worlds," says Charbonneau, who calculates that TrES-1 is about 800 °C. "When I first saw these data, I was ecstatic."

Planet hunters

Astronomers will now rush to test the technique on other star systems, says Seager. They will be limited to similar hot Jupiters that transit their stars, cautions Boss, "but it's a step in the right direction".

HD 209458b and TrES-1 are two of just six known transiting hot Jupiters. Cooler planets that lie further away from their stars take much longer to complete an orbit. Several circuits are needed to check and recheck light readings and with limited space telescope time available, Earth-like worlds that take months to complete an orbit will require long-term observations by telescopes dedicated to the task, says Boss.

But both NASA and the European Space Agency are planning space telescopes that will hunt specifically for Earth-like worlds. This is just the beginning of the story, says Boss. 

Carnegie Institution of Washington, DC

Harvard-Smithsonian Centre for Astrophysics, Cambridge, Massachusetts

Carnegie Institution of Washington, DC

• References

  1. Deming D., Seager S., Richardson L. J. & Harrington J. Nature, advance online publication, doi:10.1038/nature03507 (2005).
  2. Charbonneau D. et al. Astrophys. J., in press (2005).