UCL in the News: Planets can survive extreme roasting by their stars

It has never been clear just how close planets could get without heating up so much that their atmospheres would start escaping or "evaporating" into space.

A new study suggests this threshold lies more than twice as close as Mercury's distance to the Sun (or about 0.15 astronomical units, where 1 AU is the distance between the Earth and the Sun). Tommi Koskinen [UCL Physics & Astronomy] led the team that carried out the study. …

Koskinen's team created a 3D computer simulation of the upper atmosphere of a planet with the mass of Jupiter orbiting a Sun-like star. They found that evaporation did not occur until the planet was within about 40% of Mercury's distance from the Sun - about twice as close as a previous estimate. …

The team says their results agree with observations of a planet with about 70% the mass of Jupiter called HD 209458b, which orbits its star at about 12% of Mercury's distance from the Sun, well within the evaporation zone the researchers calculated. …

But other researchers have calculated that even HD 209458b should last for many billions of years at the rate that it is losing matter, Koskinen says, adding that all the known planets around other stars appear to be capable of surviving where they are for billions of years. "They all look very stable and many of them are very close in to their star," he told New Scientist. "But there's a lot of uncertainty about these things and we shouldn't say anything too definite at this point." …

David Shiga, NewScientist.com