A ‘decoder’ to gauge exoplanet climate

Astronomers from Cornell University in the US have developed what they call an environmental colour “decoder” to tease out climate clues for potentially habitable exoplanets in galaxies far away.

“We looked at how different planetary surfaces in the habitable zones of distant solar systems could affect the climate on exoplanets,” says Jack Madden, co-author, with Lisa Kaltenegger, of a paper in the Monthly Notices of the Royal Astronomical Society.

“Reflected light on the surface of planets plays a significant role not only on the overall climate but also on the detectable spectra of Earth-like planets.”

Madden and Kaltenegger combined details of a planet’s surface colour and the light from its host star to calculate a climate. 

For instance, they say, a rocky, black basalt planet absorbs light well and would be very hot, but add sand or clouds and the planet cools. However, a planet with vegetation and circling a reddish K-star would likely have cool temperatures because of how those surfaces reflect their sun’s light.

“Think about wearing a dark shirt on a hot summer day. You’re going to heat up more, because the dark shirt is not reflecting light. It has a low albedo (it absorbs light) and it retains heat,” says Madden. “If you wear a light colour, such as white, its high albedo reflects the light – and your shirt keeps you cool.

It’s the same with stars and planets, Kaltenegger adds.

“Depending on the kind of star and the exoplanet’s primary colour – or the reflecting albedo – the planet’s colour can mitigate some of the energy given off by the star. What makes up the surface of an exoplanet, how many clouds surround the planet, and the colour of the sun can change an exoplanet’s climate significantly.”

Madden says forthcoming instruments like the Earth-bound Extremely Large Telescope will allow scientists to gather data in order to test a catalogue of climate predictions.

“There’s an important interaction between the colour of a surface and the light hitting it,” he says. “The effects we found based on a planet’s surface properties can help in the search for life.”

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