Habitable exoplanets may be more like Dune than Earth

We're been looking for planets around other stars that are as like Earth as possible, but new simulations show that habitable exoplanets may be much more like Arrakis (or Mars) than Earth.

The holy grail for any exoplanet hunter is of course finding a rocky planet about the same size as Earth orbiting about the same distance as Earth from another star that's similar to the sun. The most important criteria is usually how much energy the planet gets from its parent star: it can't be too hot or too cold, which is why planets that fall into that just right range of temperatures are said to exist in the "Goldilocks Zone," which implies that there's liquid water on the surface.

Goldilocks was a picky little brat, though, and for a planet to be warm and wet, it has to exist in a fairly small range of distances from its star. New simulations run by planetologists at NASA's Ames Research Center suggest that it might be more likely that habitable exoplanets won't be warm and wet at all: just warm, like deserts. Essentially, the simulations showed that planets without large amounts of surface and atmospheric water had a much wider Goldilocks Zone than planets with oceans and lots of rain. These desert planets would still have to have water, of course, it would just be mostly underground or in small oases instead.

So, here's why lots of water might be a bad thing: if you look at the "too hot" and "too cold" edges of the Goldilocks Zone, you start running into problems where having a lot of water makes temperature extremes much, much worse. Let's say you've got a planet that's right on the "cold" edge of habitable. If you've got a lot of water in the atmosphere, you probably have a lot of snow and ice, which is going to reflect solar energy back into space, cooling the planet further until it completely ices over. On the flip side, a wet planet that's on the "hot" edge of habitable will probably end up with a runaway greenhouse effect caused by all the extra water vapor in the atmosphere, eventually causing the oceans to boil away completely. Not pleasant.

If you take the water out of the equation, though, planetary climates become much more stable. Land planets absorb and emit heat much more efficiently, helping them regulate their temperatures. The upshot of all this is that a desert planet has a Goldilocks Zone that's a full three times larger than an ocean planet like Earth, implying that we're three times more likely to find habitable desert planets than habitable ocean planets. Things are looking good for the spice trade.

Astrobio, via Physorg

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