HD 209458b and TrES-1 were previously discovered using indirect methods: HD 209458b by measuring the "wobble" it causes in the star it orbits around and TrES-1 by so the so-called transit method -- where a planet causes its host star to "blink" as it passes in front it. Both planets are known as "hot Jupiters" because they are about the same size as the gas giant but orbit their respective stars much more closely than Jupiter orbits our Sun. This means they receive large amounts of radiation from their host stars and subsequently emit strongly in the infrared part of the spectrum.

The two teams used the Spitzer Space Telescope to make their observations. First they measured the intensity of light coming from the star and planet combined. Next, they measured the intensity as the planet passed behind the star in a secondary eclipse. The difference between the two values was due to the light coming from the planet.

Based on its spectral signature, Deming's team found that HD 209458b has a temperature of about 857°C (Nature advance online publication). Meanwhile, Charbonneau and colleagues found that TrES-1 is cooler with a temperature of around 787°C (The Astrophysical Journal at press).

"These are very exciting and highly significant new results," says Andrew Collier Cameron of St Andrews University. "Not only do they represent the first direct detection of radiation emanated by extrasolar planets, but they also yield the brightness temperature in the thermal infrared."

Since the size of the planets is well known, the observations will allow astronomers to probe the temperature and other properties of the atmospheres of the planets. The results should also help explain why some hot Jupiters like HD 209458b are oversized for their mass and age, while others such as TrES-1 are closer to the expected size for Jupiter-like planets.