The Economist had two great piecees last week about why moons may be the next big thing in the search for life. The articles are wonderful, and I highly recommend that you read both of them. And also highly recommend The Economist as a place for science news. They are one of the few general newspapers/news magazines where I think nearly all their science and technology articles are well written. My only complaint is they don’t always have articles I find interesting.
The one thing I’d like to expand on a bit is our poor (or at least, I think so) definition of the “habitable zone”, or if you watched Battleship this summer, you might also be familiar with the other name for of “Goldilocks planet“. Currently, if you hear someone talking about a habitable zone, they probably mean one thing: the region where a planet can orbit a star and maintain liquid water. But that actually is a really vague definition. A lot of this also depends on the planet you’re looking at. How much light a planet reflects is a big factor in how much heat it can keep. (In fact, Ice Ages are feedback loops because of this – the ice caps are really shiny compared to dirty and water and can reflect off a lot of heat and prevent their melting, leading to more ice and less heat) And we need to consider the composition of the planet. Without the greenhouse effect of carbon and the salt content of our oceans, Earth’s water would freeze over a lot more often. Of course, there are still ways to control for this. People typically define the properties of a hypothetical planet they look at when calculating habitable zones. And generally, there is a limit to where you can put something a certain star and expect liquid water (Mercury’s position would be a no-go, unless you were dealing with some particularly crazy atmospheres I think).
But that’s not what bothers me. It’s that while some form of habitable zone/Goldilocks planet has gone on to permeate the broader culture, we’ve also kind of forgot to explain how this is an oversimplification. Like the first article mentions, we expect water on lots of moons on planets outside the habitable zone because of tidal or magnetic heating. And we could probably use better explanations of why NASA’s focus for astrobiology is to “follow the water”. Even though I do worry about “carbon chauvinism“, there are good reasons people expect life to use water and carbon instead of other biochemistries.