Cooler Stars Mean Warmer Planets: Ice Freezes Near Hotter Suns

Astronomers continue to hunt for life on other planets, combing the far reaches of the galaxy as they search for potential host stars. Now, they've discovered a little bit more about what types of stars to search for. It turns out that planets orbiting cooler stars may be more likely to remain ice free than planets around hotter stars.

Stars emit different types of light. Hotter stars emit high-energy visible and ultraviolet light. Cooler stars, in contrast, give off infrared and near-infrared light, which has a much lower energy. In other words, not all stars are created equal.

When a planet orbits its host star, it receives different light different depending on how far away it is and what type of star it circles. This light, in turn, reacts with a planet's surface in different ways. For example, ice will absorb much of a longer wavelength--the near-infrared light emitted by cooler stars. In contrast, it will reflect the visible light emitted by hotter stars.

In order to examine exactly what would happen to planets if they were exposed to different types of stars, the researchers developed a model. They found that around a cooler, M-dwarf star, the more light that ice on a planet absorbs, the warmer the planet becomes. The planet's greenhouse gases also absorbed this near-infrared light, compounding the warming effect. This is a stark contrast to what happens around a hotter F-dwarf star. In that case, the star's visible and ultraviolet life is reflected by planetary ice and snow in a process called ice-albedo feedback. The more light the ice reflects, the cooler the planet becomes.

In fact, the researchers found that the ice-albedo feedback can be so effective that planets could potentially enter "snowball states." This occurs when the entire planet becomes frozen--but that's not necessarily a bad thing. It's thought that Earth entered several snowball states during the course of its history.

"The last snowball episode on Earth has been linked to the explosion of multicellular life on our planet," said Aomawa Shields, one of the researchers, in a news release. "If someone observed our Earth then, they might not have thought there was life here-but there certainly was. So thought 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 findings reveal a little bit more about the potential hunt for alien life. By expanding our view of what we consider to be "habitable" and by looking around the "right" stars, scientists may have a much better chance at discovering the potential for life.

The findings are published in the journal Astrobiology.