If you take a look around our solar system, it’s sometimes enough to make you feel slightly lonely. As far as we know, the only planet with any kind of life on it is ours. We certainly haven’t seen any sign of complex life anywhere else in the solar system. Some people take this as a sign that life in the universe is uncommon. However, there’s another possibility. It seems obvious that across the whole galaxy there are star systems which are more habitable than others. Perhaps ours is simply not a very habitable one.
Some astrobiologists think that might be the case. A team based at Ohio State University, for instance, believe that there may be planets around other stars which have cores up to 25% warmer than ours. Their reasoning is fairly simple too. Looking at a set of 8 sun-like stars, they detected much more thorium than is present in our solar system in 7 of them. Radioactive decay from elements like thorium can help to keep a planet warm, and a warmer planet means there’s more chance of water being retained.
Granted, a sample size of 8 isn’t exactly enough to draw any conclusions, but it’s certainly enough to forge a hypothesis. Planets with warmer interiors have a lot of advantages. For instance, it’s Earth’s molten core which gives our planet its magnetic field to protect us from the solar wind. For an example of a planet which has frozen nearly solid and no longer has such protection, just take a look at Mars. If Mars still had a warm interior, perhaps it would be a very different place today.
And then there’s the other matter of where exactly that habitable zone around stars may lie. As it happens, we’ve been slightly wrong about that too. Old estimates put the habitable zone for a star like the Sun at 0.95-1.67 AU. But a newer calculation puts it from 0.99-1.70 AU. Not wildly different, but different enough to exclude some newly discovered exoplanets from the search for extraterrestrial life, and to include a few more. This newer estimate, incidentally, was reached by accounting for feedback from clouds, which can help to warm or cool a planet. James Lovelock must be happy about that!
As much as we like to think that our little yellow star is quite special – and don’t get me wrong, I’m just as affectionate towards it as anyone else – it really isn’t. We are nowhere important, circling an average star, in an average part of an average galaxy. Think of it like a game of poker. Sometimes you get dealt an incredibly good hand, other times, a terrible hand. Most often though, the hand you get dealt is quite average. Perhaps less than average.
Upper – ESA, NASA, and L. Calcada
Lower – PlanetQuest/NASA JPL