Distant gas giant hottest planet known, scientists say

A Saturn-sized planet outside our solar system may the hottest yet known, with temperatures over 2,000 degrees Celsius, scientists said Wednesday.

The authors of a paper published in the journal Nature used infrared data taken from NASA's Spitzer Space Telescope to record the temperature on HD 149026b, a planet orbiting a star in the constellation Hercules.

The planet's dense atmosphere is said to contribute to its recorded temperature of 2,050 C which is about equivalent to the temperatures on the surface of red giant stars and less than half the surface temperature of our sun.

The planet is also dark, meaning it absorbs most of the starlight that reaches it, and then re-emits the light in the infrared range, the authors said.

"This planet is off the temperature scale that we expect for planets," said Drake Deming, a co-author of the paper, from NASA's Goddard Space Flight Center, Greenbelt, Md.

The authors also released the first ever temperature map of another planet outside our solar system using the Spitzer telescope.

The planet, HD 189733b, a hot Jupiter-sized gas giant, orbits a star in the constellation Vulpecula much closer than Mercury does to our sun. It is also tidally locked to its star the way our moon is to Earth, meaning one side of the planet is always facing the star.

The warm spot of the planet facing the star had a temperature of 930 C, while the planet's dark side had a temperature of 650 C.

Both planets are transiting planets, meaning each was first observed after it passed in front of its star. There are about 200 planets outside our solar system that have been discovered.

The super-hot HD 149026b is located 279 light years away from Earth, while the planet tidally locked HD 189733b is about 60 light years away from Earth. A light year is the distance light travels in a year, or about 9.5 trillion kilometres.

The findings of characteristics very different from planets in our own solar system will help our understanding of planets, the scientists said.

"These hot Jupiter exoplanets are blasted by 20,000 times more energy per second than Jupiter," said co-author David Charbonneau of the Harvard-Smithsonian Center for Astrophysics.

"Now we can see how these planets deal with all that energy."