June 29, 2013
Newly Found Habitable Planets?
The Holy Grail of planetary astronomy is the elusive Earth-like planet orbiting a distant star. Such a world would be of similar size to our own, while maintaining just the right distance from its host star such that liquid water could be maintained on its surface. Of course, the planet should be rocky and possess an atmosphere too.
Why do these things matter? Because there is currently only one planet in the entire universe known to contain life on its surface; Earth. So, the search for life beyond our own existence is focused on the only type of planet that we know for sure can sustain it.
Could life exist on a planet similar, but different than ours? Possibly. But scientists are convinced that liquid water is necessary for the proliferation of life. As a result, a rocky world is needed, and the planet must receive a very specific amount of energy from its parent star in order for H2O to readily exist in liquid form.
Also, it is expected that the life forms would be carbon based — for a lot of reasons dealing with organic and bio-chemistry — but essentially, the need to bond to other atoms in certain ways is vital, as well as the need for a metabolic mechanism. Both of which are uniquely possible in carbonaceous life. (There are one or two other possibilities, but these are less likely, and the subject of a separate blog post.)
Unsurprisingly, news earlier this week that three new habitable planets had been discovered caught the attention of the media. The problem? I think that we may be jumping the gun a bit.
The three planets in question are super-Earths – planets of similar composition to earth, but several times larger. Consequently, they would have significantly higher gravity than our own planet. And while they orbit their parent star at a distance sufficient for liquid water to exist, we don’t know much about the actual surface composition or, more crucially, the atmosphere.
Which is why I was surprised when Sara Seager, professor of planetary science at Massachusetts Institute of Technology, was quoted as saying, “With three terrestrial-mass planets in the habitable zone, the likelihood of one of them actually being habitable is tremendous.” We simply don’t know how life arises; we only know that in the case of our planet it did. While on Venus and Mars, planets at the edge of our solar system’s habitable zone, we find no evidence of life. (At least not yet.) So, simply being rocky and at the correct distance from the star, does not mean that life is automatic.
It could turn out that life is rather common, but it could also be exceedingly rare. Until we find evidence of life, such as oxygen rich, we can’t really begin to guess at the commonality of life. Could this new system contain life? Of course. Is it “tremendously likely”? I’d say not.
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