George Dvorsky takes on the ‘rare earth’ hypothesis in his Sentient Developments blog, calling it a ‘delusion’ and noting all the reasons why life in the galaxy is unlikely to be unusual. The post reminds me why the book that spawned all this was so significant. Rare Earth: Why Complex Life is Uncommon in the Universe (Copernicus, 2000) is Peter Ward and Donald Brownlee’s take on our place in the cosmos, concluding that complex life is rare because an incredibly fortuitous chain of circumstances must arise for it to occur. Indeed, the authors argue that large parts of our galaxy are composed of what they call ‘dead zones.’

The argument is complex and looks at factors ranging from a planet’s place in the galactic habitable zone (itself a controversial subject), its orbit around its own star, its size, its satellites, its magnetosphere, its plate tectonics, and more. I’m surprised to realize, looking through our archives here, that I haven’t managed to do a complete review of Rare Earth in the past five years (although we’ve certainly kicked its ideas around during that time). I’ll remedy that soon, because I think it’s a hugely significant work. Agree with it or not, the authors have laid out a strong case that has inspired spirited rebuttal, and the study of astrobiology has benefited from the insights they’ve brought.

Back to Dvorsky, who notes Charles Lineweaver’s work suggesting that planets began forming in our galaxy long before our own Sun ever ignited, and goes on to look at progress in exoplanetary science, which is uncovering so many new worlds. The point George is making is that there have been vast amounts of time for life to arise, and a huge range of galaxies within which it could evolve. He’s absolutely right, but I’ll part with him on the use of Gliese 581 c to make the case on life’s odds:

…shockingly, the first Earthlike planet was discovered in 2007 orbiting the red star Gliese 581. It’s only 20 light-years away, 1.5 times the diameter of Earth, is suspected to have water and an atmosphere, and its temperature fluctuates between 0 and 40 degrees Celsius.

If we are one in a billion, then, and considering that there are only 0.004 stars per cubic light-year, what are the odds that another Earthlike planet is a mere 20 light-years away?

But the odds are still where they were, Gliese 581 c now being generally considered too close to its star to be habitable. The next planet out, Gl 581 d, is possibly within the extreme outer edge of the habitable zone, and perhaps a better bet for life than what now seems to be a hellishly hot Gl 581 c, but in any case neither of these massive ‘super-Earths’ are truly Earth-like, and the question of whether they support life or not awaits further data. Recent work by Brian Jackson, Richard Greenberg and Rory Barnes suggests tidal heating for Gl 581 c that may be three times greater than what we see on Io, not a promising sign.

We still, in other words, don’t have a good read on how close the nearest habitable planet is, or how rare our Earth may be. Could Ward and Brownlee still be right? Absolutely. At this point we lack the data to know.

Alan Boss has much to say about all this, and George goes on to quote him to good effect. Author of the recent The Crowded Universe: The Search for Living Planets, Boss (Carnegie Institution) sees Earths as ‘incredibly common,’ going on to make this astounding statement: “…every solar-type star probably has a few Earth-like planets, or something very close to it.” Take that, rare Earthers!

I just finished The Crowded Universe the other day and will have more to say about it soon — we looked at its author’s ideas on habitable planets in a previous post. I tend to go along with Boss, as least in so far that I suspect life is quite common elsewhere in the universe, and probably complex life at that. But the question of sentient, technology-building life is quite another matter. It wouldn’t surprise me at all to find many living worlds in our galaxy, but of necessity we await the results of our explorations, one of which was so recently launched. One day Kepler’s successors will doubtless flag the spectroscopic signature of a living world, but barring a SETI breakthrough, the question will remain. Are there other civilizations? Do they too dream of traveling to the stars?