The Red Edge

By Sean Carroll | April 22, 2010 4:33 pm

We had a great time last night at a panel discussion on extrasolar planets, right here at my very own institution of Caltech, sponsored by our very own Discover magazine, and hosted by our very own Bad Astronomer. The panelists included Gibor Basri, John Johnson, Sara Seager, and Tori Hoehler. They did a great job at getting across the most important message: this is a field that has taken a tremendous leap forward in the past ten years, and is poised to make comparable strides in the years to come. A lot of the excitement right now centers on the Kepler satellite, which is on track to find hundreds of extrasolar planets. You can get an idea of recent progress from a graph of extrasolar planets discovered over the years.

Extrasolar Planets by the Bushel

From the perspective of the person on the street, planets are pretty cool — but life on other planets is what’s really cool. (Or would be, if we found it.) And frankly, it’s not even the prospect of life that gets people going; it’s the idea of intelligent extraterrestrial life. Tori mentioned that he was slightly surprised, some years ago when there was a report (later discredited) that we had found evidence for life on meteorites from Antarctica, that people didn’t make a big deal out of it — it was exciting, but not Earth-shattering. I suspect that microbes, no matter where they’re from, aren’t going to shatter most people’s Earths; that will take some sort of greeting, friendly or otherwise.

Still, it’s amazing what has been done, and the prospects for doing more are pretty breathtaking. Here’s one idea that I find pretty clever: searching for the Red Edge. You know how plants appear to be really bright in infrared photographs? That’s because they reflect a lot of infrared light, but tend to absorb regular visible red light. In a spectrum, where we decompose the reflected light into different wavelengths, this phenomenon shows up as a sharp “edge” as you go from infrared (on the right here) to red light. The idea would be that something similar should happen even for very different kinds of life — so if you found a planet whose spectrum featured the red edge, that would be a promising place to hope for finding life.

The Red Edge

I have no way of judging how feasible this technique really is — in particular, I’m always skeptical of claims that rely on alien forms of life resembling ours in any way. (The authors do emphasize that an extraterrestrial red edge might not be at the same wavelength as ours.) But I like it because it relies on an underlying truth of which I am quite fond — the fact that life relies on the increase of entropy. The specific wavelengths at which different kinds of life might reflect light can undoubtedly be very different from biosphere to biosphere; but what won’t change is the general idea that a planet full of life will re-radiate energy with a much higher entropy than what it absorbs. That’s the deep principle underlying the red edge; plants absorb visible light, and radiate at longer wavelengths with higher entropy. If we eventually find life on other planets, I’d personally be pleased if entropy were at the bottom of it all.


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Cosmic Variance

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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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