Cosmic Irony - Planets Orbiting Cooler Stars More Likely To Remain Ice-Free
Than Planets Around Hotter Stars

20 July, 2013

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Follow us: - Planets orbiting cool stars actually may be much warmer and less icy than their counterparts orbiting much hotter stars, even though they receive the same amount of light, shows a new study led by Aomawa Shields, a doctoral student in the University of Washington astronomy department.

This is due to the interaction of a star's light with ice and snow on the planet's surface.

Stars emit different types of light. Hotter stars emit high-energy visible and ultraviolet light, and cooler stars give off infrared and near-infrared light, which has a much lower energy.

Illustration of a planet with two moons orbiting a red dwarf in the habitable zone. Credit: David A Aguilar(CfA)

It seems logical that the warmth of terrestrial or rocky planets should depend on the amount of light they get from their stars, all other things being equal.

That's because the ice absorbs much of the longer wavelength, near-infrared light predominantly emitted by these cooler stars.

This is counter to what we experience on Earth, where ice and snow strongly reflect the visible light emitted by the Sun. Around a cooler (M-dwarf) star, the more light the ice absorbs, the warmer the planet gets.

The planet's atmospheric greenhouse gases also absorb this near-infrared light, compounding the warming effect.

The researchers found that planets orbiting cooler stars, given similar amounts of light as those orbiting hotter stars, are therefore less likely to experience so-called "snowball states," icing over from pole to equator.

However, around a hotter star such as an F-dwarf, the star's visible and ultraviolet light is reflected by planetary ice and snow in a process called ice-albedo feedback. The more light the ice reflects, the cooler the planet gets.

"The last snowball episode on Earth has been linked to the explosion of multicellular life on our planet," Shields said.

"If someone observed our Earth then, they might not have thought there was life here - but there certainly was. "So though we'd look for the non-snowball planets first, we shouldn't entirely write off planets that may be ice-covered, or headed for total ice cover."

"There could be life there too, though it may be much harder to detect."

The research paper is published in the August issue of the journal Astrobiology, and published online ahead of print July 15.

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