This year has been a spectacular one for exoplanets. New discoveries and new insights have truly pushed the gateway to other worlds even further open.

Should We Expect Other Earth-like Planets At All?

By Caleb A. Scharf | December 26, 2012

In the past 12 months we’ve gained increasingly good statistics on the incredible abundance of planets around other stars and their multiplicity. We also finally seem to have evidence that our neighboring star Alpha Centauri B does indeed harbor at least one world. It is by any set of standards, a great haul.

But I continue to be a bugged by the claims of ‘habitable’ worlds and ‘Earth-like planets’ that seem to beset many scientific announcements (including I’m ashamed to say my own). In the spirit of closing out the passage of our 4,500,000,000 th or so orbit around the Sun I thought I’d try to set the record straight, because I think we have so much more to look forward to than simply finding ‘another Earth’.

First, when press releases state that a ‘habitable’ world may have been found, the truth is far more complex. Astronomers and astrobiologists tend to use the term habitable as a shorthand for the presence of liquid water on a planetary surface, implying a range of temperatures between the freezing and boiling point of water. But this also requires a surface atmospheric pressure high enough for water to exist as a liquid without boiling off to vapor, and an atmosphere will alter the transfer of radiation to and from the surface – often by way of a greenhouse effect.

And this is just the beginning. By this simple criterion even the Earth is only partially habitable – about 85% of its area remains amenable to liquid water over a year (a fact that my colleagues Dave Spiegel, Kristen Menou and I reiterated a few years ago). So strictly speaking ‘habitable’ includes a range of environments that we would find appallingly hostile, including high-pressure, high-temperature climates and those in a sub-arctic category with thin atmospheres.

Read more: Should We Expect Other Earth-like Planets At All? | Life, Unbounded, Scientific American Blog Network.

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