Forecasting the Weather on Exoplanets?

Well, not yet, but that is the direction that scientists are headed. "Cloudy for the morning, turning to clear with scorching heat in the afternoon," may soon apply not just to typical late-summer days on Earth, but also to planets located outside our solar system, according to a new study.

Using observations from the Kepler space telescope, an international team of astrophysicists from the University of Toronto, York University, and Queen's University Belfast has discovered evidence of daily weather cycles on six extra-solar planets seen to exhibit different phases. These phase variations reportedly occur as different portions of these planets reflect light from their stars - much like the way our own Moon cycles though different phases.

And based on these phases, it seems that the studied exoplanets experience relatively pleasant weather - at least, four of them see cloudy mornings while two others boast hot, clear afternoons.

"We determined the weather on these alien worlds by measuring changes as the planets circle their host stars, and identifying the day-night cycle," lead author Lisa Esteves explained in a press release. "We traced each of them going through a cycle of phases in which different portions of the planet are illuminated by its star, from fully lit to completely dark."

Since the planets are situated very close to their stars, they are expected to rotate counter-clockwise - just as the majority of objects in our solar system do - with the right side moving in the direction of each planet's orbit. This causes an eastward movement of the planet's surface and therefore an eastward circulation of atmospheric winds. As a result, clouds that form on the planet's night side, where temperatures are cooler while it faces away from its host star, would be blown to the planet's morning side.

"As the winds continue to transport the clouds to the day side, they heat up and dissipate, leaving the afternoon sky cloud-free," said Esteves. "These winds also push the hot air eastward from the meridian, where it is the middle of the day, resulting in higher temperatures in the afternoon."

For four of the planets, the research team saw excess brightness in the Kepler data that corresponds to when the morning side is visible. For the other two, they saw an excess when the evening side is visible. (Scroll to read on...)