July 19, 2013
Planets May Keep Warmer In A Cool Star System
Lee Rannals for redOrbit.com - Your Universe Online
According to a new study published in the journal Astrobiology, planets orbiting cooler stars may be more likely to remain ice-free than planets around hotter stars.
Stars emit different types of light, with hotter stars emitting high-energy visible and ultraviolet light, and cooler stars giving off infrared and near-infrared light. A new climate model found that planets orbiting cool stars actually may be warmer and less icy than their counterparts orbiting much hotter stars. The scientists say this is because the ice on cooler-star planets absorbs much of the near-infrared light predominately emitted by cooler stars.
On Earth, ice and snow strongly reflect the visible light emitted by the Sun. However, ice on a planet orbiting one of these cooler stars would absorb the light and make the planet warmer. The exoplanet's atmospheric greenhouse gases also absorb this near-infrared light, compounding the warming effect.
The researchers found that planets orbiting cooler stars are less likely to experience "snowball states," which is icing over from pole to equator. Terrestrial planets around hotter stars appear to be more susceptible to the snowball effect. Scientists believe that Earth has even experienced several snowball states during the course of its 4.6-billion-year history.
The team found that the interaction of starlight with a planet's surface ice is less pronounced near the outer edge of the habitable zone, where carbon dioxide is expected to build up as temperatures decrease. Planets at the zone's outer edge would most likely have a thick atmosphere of carbon dioxide or other greenhouse gases, which blocks the absorption of radiation at the surface.
Aomawa Shields, a doctoral student in the University of Washington astronomy department who led the research, said astronomers looking for life shouldn't rule out those circling cooler stars. Scientists prioritize planets less vulnerable to that of the snowball state, but Shields pointed out that even our own planet has been through this stage.
"The last snowball episode on Earth has been linked to the explosion of multicellular life on our planet. If someone observed our Earth then, they might not have thought there was life here -- but there certainly was," she said. "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."
NASA recently published the outline astronomers need to sift through in order to determine whether or not a planet is habitable. The space agency said exoplanets must meet a handful of different criteria before they can even be determined as potentially habitable.