Astronomers have found three new planets that are the most similar to earth so far. The scientists used NASA's Kepler spacecraft to observe variations in star brightness to find the planets. No question, this is a great discovery. But reading the popular press, you'd think they'd found little green men peering back at us with big, sad eyes.
The New York Times article is typical, with the headline "2 Good Places to Live, 1,200 Light-Years Away." That's a colossal overstatement, much like their claim that Mars is "habitable."
Mars is in the habitable zone, to be sure, but it's not habitable in any real sense. It is far too cold and the atmosphere too thin and lacking oxygen for a person to survive without wearing a space suit. Crops will not grow except in hermetically sealed green houses. So far there's no trace of any form of life on the surface. Bacteria could likely be persuaded to live there in the soil fairly easily as they do in Antarctica. And, yes, humans could colonize Mars and perhaps thrive there. But it would be little different from living on the moon
The two parent stars, Kepler 62 and Kepler 69, are more than a thousand light years away. Kepler 62-e is on the inner edge of the habitable zone, and is 60 percent larger than earth. Kepler 62-f is only 40 percent larger than earth, and is at the outside edge of the habitable zone. A third planet, 1.7 times the size of earth, was found in the habitable zone of Kepler 69, a star almost identical to the sun. But those are the only things we know about the planets: we don't know if they're made of rock or gas. And we don't know what the atmospheres consist of.
Why? The atmospheres: Mars was too small to hold on to its atmosphere for very long; what oxygen remained combined with carbon or other elements. It's only got a wispy envelope of carbon dioxide.
Venus, a bit smaller than earth, has a very thick atmosphere of carbon dioxide and clouds of sulfuric acid. The temperature on the surface of Venus is almost 900 degrees Fahrenheit. The atmospheric pressure is 92 times greater than earth's at sea level. It's the greenhouse effect gone mad: the same thing would happen to earth if we pumped enough CO2 into the air.
Finally, the only reason that earth is habitable in the sense that people can live here is that it has an oxygen-nitrogen atmosphere. But such an atmosphere is impossible without bacteria and plant life that constantly produce the free oxygen required for animal life. Without those simpler forms of life, the reactive oxygen would oxidize everything around it. The entire earth would rust and perhaps wind up looking like Mars. So these planets we're finding out in the galaxy wouldn't be truly habitable for humans unless something is constantly replenishing atmospheric oxygen -- something we'd probably call life.
There's no question that this is a very cool discovery: it shows that earth-sized planets are common around stars like the sun at the distance necessary for the right temperature for human habitation. But it's way too early to claim they're "habitable" without any knowledge of the planets' compositions and the constituents of their atmospheres.
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