NASA - National Aeronautics and Space Administration

Using images from the Spitzer Space Telescope, astronomers can now estimate the weather of a Jupiter-like exoplanet as it orbits a Sun-like star that is about 60 light-years away.

Left: exoplanet artist rendition

For the first time, scientists can map the atmosphere of a planet orbiting its parent star, by observing changes in the planet’s infrared brightness over time. The planet is tidally locked to its star like our moon is to Earth, which means that the same side of the planet always faces the star, and it is divided into permanent day and night. However, because the night side is nearly as hot as the day side, scientists say that fierce winds must be redistributing the heat from the starlit side to the dark side. In addition, the hottest part of the planet is shifted from the location that receives the most starlight. The results of this NASA-funded research will appear in the May 10 issue of the journal Nature.

“Now that we have this brightness/temperature map, we can learn a lot about this planet. We now believe that this planet is a very windy place; the speed of the west-to-east winds is probably several kilometers /second. We reached this conclusion because the brightest spot on the planet is not right in the middle of the map. The point in the middle of the map receives the most starlight, but it is not the hottest,” explained Jonathan Fortney, a planetary scientist at the Carl Sagan Center, Mountain View, Calif. who works at NASA Ames Research Center, Moffett Field, Calif. He is also a co-author of this study. ”The hottest (brightest) spot is offset by about 30 degrees, meaning that winds are blowing this hottest spot downstream.”

If there were no winds, the location on the planet that is permanently at high noon, with the blazing star directly overhead, would be hottest, as it constantly receives the most starlight. However, scientists have measured the hottest spot on this exoplanet, and it is 30 degrees to the east of the high noon spot. This means that the winds are so strong that they are blowing this hot gas about 25,000 miles away.

In the past, researchers have been able to detect the presence of planets outside our solar system, called extrasolar planets, and determine their size and location. Today, scientists use observations of infrared light (heat radiation) to determine atmospheric conditions and varying temperatures at different places on these planets.

For this study, scientists focused on a gas giant called HD 189733b, which is more than 30 times closer to its star than the Earth is to the Sun. It is a transiting planet, meaning it eclipses its parent star. Observations spanned slightly more than half of the planet’s orbit. As the day side of the planet rotated into view, scientists detected the increase in brightness and calculated its temperature range.

Right: Scientists discovered that the brightest point in the middle of the map is not the hottest point on the orbiting planet. The hottest spot is offset from the brightest spot by about 30 degrees, meaning fierce winds are blowing it downstream.

According to scientists, a minimum brightness temperature of 1,200 degrees Fahrenheit occurs during the transit (when the planet moves in front of the star), and a maximum brightness temperature of 1,700 degrees Fahrenheit occurs about two hours before the planet moves behind the star. The difference in temperatures indicates that energy is being redistributed throughout the atmosphere.

“What we first observed was the infrared brightness of the planet when we saw the full night side and the planet eclipsed its star. Then we kept watching for 33 more hours as we gradually saw more and more of the day side. We eventually saw the full day side, just as the planet passed behind its parent star, like an anti-eclipse. Because we can measure the brightness of the planet as we gradually see more day and less night, we can break up the planet into slices and estimate the infrared brightness of each slice. We can then use this brightness map to calculate the temperature that would cause a given brightness,” Fortney said.

To study gas giant planets, atmosphere models are used to understand the contrast between day and night temperatures. Models of atmospheric chemistry help scientists understand where in the planet's atmosphere light is absorbed and emitted. Some important gases are water vapor, carbon monoxide, and atomic sodium and potassium. If estimates are accurate, scientists think that supersonic wind speeds exceeding four times the speed of sound may be necessary to transport the energy to the night side.

This work was made possible through funding by NASA as part of a long-term research program.

This study will appear in the May 10 issue of the journal Nature, 2007.

For further information, please visit:

http://www.nasa.gov/mission_pages/spitzer/main/index.html

Ruth Marlaire
NASA Ames Research Center