Most of the markers exobiologists use to identify a planet as potentially habitable are possible red herrings. We’ve already talked about how methane, often seen as a key biosignature, can be abundant on planets that almost certainly don’t harbor life. And while geological activity may indeed indicate the possibility of a habitable underground ocean on Eris or Pluto, Jupiter’s moon Io—discovered by Galileo in 1610—is an intensely geologically active planet that is much less likely than most of its less geologically-active neighbors to harbor extraterrestrial life.
Asking whether there’s life on Io is rather like asking whether there’s life in Hell. Geological activity has given Io a warm underground ocean, certainly, but it happens to be a warm underground ocean made of boiling molten lava—much less habitable than the kind we’ve found on Enceladus and Europa. Could there be life there? Yes, and there could be life on the Sun, too—but the odds are against it.
Io is a relatively easy call because it’s in our solar system and we know a great deal about it, but as we study more exoplanets for possible habitability, we’re going to find lots of evidence of methane and, occasionally, evidence of geological activity. It’s going to make us very excited and it will be important, in these cases, to remember that methane and geological activity are still common on (ostensibly) lifeless worlds. Making the case for life on another world is less about identifying a single physical trait, and more about building a forensic case for life based on multiple criteria—acknowledging, all the while, that it’s very possible we won’t find life where we expect to, and that we will find life where we don’t.