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.

  • Charon

    “That’s the deep principle underlying the red edge”

    But reflection does not equal emission. Plants (on Earth) radiate at about 10 microns (Wien’s Law). The red edge is a reflection phenomenon.

  • Angus McPresley

    I’ve always thought it amusing when creationists claim that evolution violates the 2nd law of thermodynamics, when in fact life is a phenomenal entropy increaser.

    In fact, if I were writing a Star Trek script and needed a plausible explanation for how their “life form” detector actually works, I would say it detects local increases in entropy. How it actually does that, well, that’s where I’d need a bit of handwavium.

  • Mike

    Is self-replication the only mechanism capable of explaining such an entropy increasing signature?

  • db

    “life is a phenomenal entropy increaser”

    Sounds reasonable but it seems like plants sometimes lessen the degradation of sunlight instead of increasing it. Light striking a plant covering a dark surface is trapped in a medium entropy form in until the plant dies and decomposes. Will the outgoing radiation caused by the decomposition necessarily be of a higher wavelength than without the plant? I can’t see why.

    Or what about plants who due to competition for pollination cover a large part of the surface with highly reflective flowers?

  • PhilG

    A simple black body would reradiate the spectrum at the highest entropy so the entropy increase is not a good indication of life. Of course the overall change in entropy is always going to be positive by second law whether there is life or not.

    It is the details of the structure in the spectrum that tells you if there are signs of life, but the structure is a low entropy feature of the spectrum. E.g seeing the chlorophyl bump would be a strong indication of life, while indications of water in the spectrum just show a potential for life. Oxygen is also a good indicator of life because it is too reactive to remain in most environments and it is hard to find inorganic mechanisms that would produce it naturally.

  • matt

    Would highly reflective flowers not defeat the object of photosynthesis (assuming alien flowers used the mechanism) ?

  • Pieter Kok

    matt, flowers are not (primarily) for photosynthesis. They’re the genitals of the plants, and rely on reflection to attract insects.

  • Charon

    So… there’s a sense in which your statement about the red edge is correct, Sean (as one would hope, given that you’re a much better physicist than I am), but it still seems… well, silly. Yes, the red edge shows that plants absorb shorter wavelengths, and they do indeed radiate at longer wavelengths (though at 10 microns, not near-IR).

    It may be true, but it’s an irrelevant statement. As PhilG points out, black bodies do the same thing. Heck, Mars even has a red reflection bump and a thermal bump out in the mid IR. The existence of the red edge, and the details of the spectrum, are all about reflection. This is cool. I love spectra, personally. This is not, however, something “deep” about life and entropy.

  • Sean

    I don’t think this is worth over-analyzing. There are plenty of things that would cause energy to be re-radiated at longer wavelengths; the Moon does it quite well without any life at all. My main (quite trivial) observation was about the absorption at visible wavelengths. If a planet had a really high albedo, reflecting light essentially unprocessed, it wouldn’t give life the chance to process that energy. It’s interesting that the reflectance of plants is very high in the infrared, but it’s not a big deal — sorry if I made it seem like I was claiming otherwise.

  • Low Math, Meekly Interacting

    Maybe it’s not perfect, but seems like a hell of a good way to start. Of course anyone can whip out the “it’d be so alien we’d never even know how to look for it” argument, but I fail to see why alien biology of a more comprehensible variety isn’t just as plausible. Sure it’ll be very different, but does it all have to be SO different that we may as well be looking for sentient plasma vortexes or giant intelligent crystals? I say criteria such as these are exactly what we need to settle on and implement searches for, and right now. We have the technology, as they say, or we’re very close to it.

    If somebody said, hey LMMI, here’s ten billion, do what you want, I’d immediately plunk it all down on a space telescope or interferometer that could resolve and perform spectroscopy on terrestrial exoplanets. I’m all for unifying the fundamental forces of nature, but finding ET life runs a very, very close second on a scale of significance, in my book.

  • olderwithmoreinsurance

    It may not be worth over-analyzing but it is worth getting RIGHT, and thanks to Charon and PhilG
    for that. A planet could have an EXTREMELY high albedo (think Eris here, but for a different reason, like highly reflective clouds, perhaps) and still be a fine home for life as the total energy used by a planet’s biosphere (think earth) is a very, very tiny fraction of the energy received from it’s star.

  • onymous

    Yeah, I’m not getting it. Albedo is an order-one number. If a planet had an albedo of 0.99999999, I might imagine life would be very hard there. But the difference between 0.2 and 0.9 doesn’t look likely to be crucial, to my eye.

  • lemuel pitkin

    I always thought the key evidence for life in the spectral signature of an exoplanet would be an atmosphere far out of chemical equilibrium, e.g. the large amount of O2 here. No?

  • Lab Lemming

    The earth is about 4500 million years old. Land plants have existed for about 450 million years. So this technique would fail to detect life on Earth for 90% of Earth’s existence.

  • Mark B.

    It seems to me that the recent surge in discussions about SETI have largely ignored one implication of our optical exoplanet search. If we’re on the verge of seeing their Red Edge signal, then any modestly more advanced civilization has long been able to see ours. Our Red Edge signal (and our non-equilibrium oxygen signal) has been propagating outward for a billion years, much longer and more constant than any radio signal. There are other more recent atmospheric markers of industrial activities that might show out to a few hundred or thousand light years. And we can plausibly talk about technologies to detect this at 10′s of parsecs today. Given that many of these scale to much larger distances, it seems to me near certainty that we’ve been visible out to a kpc for a long time now. So where are They ? Or do They only like to watch?

  • Brian Too

    14. Lab Lemming,

    I’m not sure you’re on the right track there. So what if land plants have only existed for 450MY? Ocean dwelling plants (algae) have existed for roughly 4BY and, to the best of my knowledge, have been photosynthetic for essentially all that time.

    Now if all that is true (and it might not be–there’s a notion that early life metabolized sulphur compounds as I recall), life on Earth would have been remotely detectable for 88% of Earth’s existence. A rather different result.

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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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