Some planets that are now small, hot, and rocky may have started off as icy giants like Neptune. Orbiting too close to their stars, their gassy shells boiled off until all that’s left is a rocky core. That’s what scientists have thought for a long time, anyway. Now, for the first time, they’ve spotted a small exoplanet that seems to be in the process of evaporating. The boiling atmosphere of the Neptune-sized GJ 436b is creating a tail that’s nine million miles long and about two million miles wide.