A new study suggests that planets that orbit cooler stars were more likely to remain ice-free compared to those around hotter stars due to the interaction of a star's light with ice and snow on the planet.
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.
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.
But new climate model research led by Aomawa Shields, a doctoral student in the University of Washington astronomy department, has added a surprising new twist to the story: 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.
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 research is published online in the journal Astrobiology.