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Forecasting the Weather on Exoplanets?

May 14, 2015 12:57 PM EDT

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...)

"By comparing the planets' previously determined temperatures to the phase cycle measurements provided by Kepler, we found that the excess brightness on the morning side is most likely generated by reflected starlight," Esteves said. "These four planets are not hot enough to generate this excess light through thermal emission."

"The excess light seen on the two very hot planets can be explained by thermal emission," she added. "A likely explanation is that on these two planets, the winds are moving heat towards the evening side, resulting in the excess brightness."

These discoveries are all thanks to the Kepler telescope, whose precise measurements allowed astronomers to measure such tiny signals from these distant worlds. But just because their weather seems like your typical summer day in many places on Earth, don't get your hopes up that the team has simultaneously discovered habitable exoplanets.

Most of the planets examined in this study are very hot and large, with temperatures greater than 1,600 degrees Celsius (~2,900 degrees Fahrenheit) and sizes comparable to Jupiter. Although these conditions are far from hospitable to life, lucky for astronomers they are bt excellent for phase measurements.

Kepler data has been used in the past to measure the temperature of these planets, but this is the first instance in which phase variations were used to measure the morning- and evening-side specific brightness of a collection of planets.

"Upcoming space missions should reveal many more small planets around bright stars that will make great targets for detailed studies," concluded co-author Ray Jayawardhana of York University. "Someday soon we hope to be talking about weather reports for alien worlds not much bigger than Earth, and to be making comparisons with our home planet."

The results were published in The Astrophysical Journal.

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