Prime Real Estate

It's been awhile since I posted anything form our from our good friends at Astrobiology, but I ran across a outstanding lecture by Vicki Meadows of CalTech on what astronomers are looking for when they seek out new life on other worlds.

The first item is planetary mass - obviously if the planet is too big, then it develops into a gas giant and the atmospheric turmoil and pressure on such planets is not conducive to life developing. At least life as we know it. Interestingly enough, a rocky planet up to ten times the size of Earth might be a potential life harboring planet.

The other thing a planet would need is an atmosphere that would protect the planetary surface and allow watery oceans to develop. Oceans are important as they provide the lubricant for plate tectonics to occur, which is critical in sequestering carbon dioxide and preventing a runaway greenhouse effect. However, a world has to be large enough for plate tectonics to develop and anything less than one third the size of the Earth doesn’t cut it. So sorry, Mars. Other important attributes to the atmosphere are its reflectivity and how well it absorbs radiation to warm the planet.

“I’m sure you’ve all heard of global warming. If you don’t have plate tectonics on your planet, over very long periods of time the carbon dioxide builds up, and that leads to global warming. So it’s always nice to have enough mass to have plate tectonics. Mars, for example, doesn’t have plate tectonics –- it’s too small. You need to be about a third of the size of the Earth to have plate tectonics that function over a reasonable amount of time.

In our search for habitable planets, we also look at atmospheric composition, what the atmosphere is made of. We’ll look at how well the atmosphere reflects light, how well it absorbs radiation and warms the surface of the planet.”

In addition, the orbit of the planet is critical, as the planet needs to be at a relatively constant distance from its parent star to keep it warm. Some variability is alright if the atmosphere is thick enough, but you don’t want the temperature too warm or too cold. The other major factor is the distance from the star. A planet has to be in the star’s habitable zone where liquid water can develop, rather than freeze or boil away. The star also has to be stable over the long term to allow life to develop. Large stars run through their stellar fuel far too quickly, usually under a billion years, which isn’t likely to let one celled organisms time to evolve into higher life forms. However, the star can’t be too small either, because a small star’s habitable zone would be so close to it that a planet might become tidal locked to it and only present one side of the planet to the star.

So you need a stable, middle aged middle sized star (more than half the mass of our sun). Another item astronomers have identified is that planets tend to form more readily around stars with a high metal content. Stellar types likely to have stars are F, G, K, and M (the sun is type G). Another issue is that the habitable zone will “migrate” outward as the star ages and grows brighter over time. The Earth will fall inside the inner edge of the zone around the Sun in another 500 to 900 million years – so we’d better get on the stick and get some O’Neill colonies going pronto!