Warmer Planets Found Around Cooler Stars, Vice Versa

Planets orbiting cooler stars may be less likely to contain ice than those around hotter stars, researchers at the University of Washington have found.

This is due, their study shows, to the fact that ice absorbs much of the longer wavelength, near-infrared light predominantly emitted by these cooler stars. This is counter to what takes place on Earth where ice and snow strongly reflect the visible light emitted by the Sun.

For this reason, planets around a cooler star absorb more light and the planet gets warmer. Meanwhile, the planet's atmospheric greenhouse gases also join in, absorbing this near-infrared light, which compounds the warming effect.

As a result, the researchers found that planets orbiting cooler stars, even when given similar amounts of light as those orbiting hotter stars, were 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, meaning that the more light the ice reflects, the cooler the planet gets.

This feedback can be so effective at cooling, the scientists found, that terrestrial planets around hotter stars appear to be more susceptible to the snowball state. Though, they point out, this is not necessarily a bad thing seeing as Earth itself is believed to have experienced several snowball states during the course of its 4.6 billion year history.

However, the researchers also discovered that both of these interactions are less pronounced near the outer edge of the habitable zone, or swath of space around a star where liquid water could potentially form. There carbon dioxide appeared to be king, building up as temperatures decreased.

This is the case because planets at that zone's outer edge are more likely to have a thick atmosphere of carbon dioxide or other greenhouse gases, which blocks the absorption of radiation at the surface, causing the planet to lose any additional warming advantage due to the ice.

Knowing this, the scientists argue, means that astronomers hunting for possible life can prioritize planets less vulnerable to that snowball state -- that is, planets other than those orbiting hotter stars.

Though, that doesn't mean they will rule out the cooler planets.

"The last snowball episode on Earth has been linked to the explosion of multicellular life on our planet," Aomawa Shields, a doctoral student in the University of Washington astronomy department, said in a press release. "If someone observed our Earth then, they might not have thought there was life here -- but there certainly was.

The researchers' findings are documented in a paper published in the journal Astrobiology.