Comments on: The Red Edge

By: Scientia Pro Publica #27 « Melliferax

Scientia Pro Publica #27 « Melliferax — Mon, 26 Apr 2010 12:44:57 +0000

[...] was about life on earth. What of elsewhere in the universe? Sean at Cosmic Variance tells us of The Red Edge, a new way to possibly detect life at other planets. “You know how plants appear to be really [...]

By: Brian Too

Brian Too — Mon, 26 Apr 2010 08:23:44 +0000

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.

By: Mark B.

Mark B. — Sun, 25 Apr 2010 16:51:56 +0000

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?

By: Lab Lemming

Lab Lemming — Sun, 25 Apr 2010 10:37:56 +0000

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.

By: lemuel pitkin

lemuel pitkin — Sat, 24 Apr 2010 18:44:41 +0000

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?

By: onymous

onymous — Sat, 24 Apr 2010 13:42:11 +0000

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.

By: olderwithmoreinsurance

olderwithmoreinsurance — Fri, 23 Apr 2010 19:08:02 +0000

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.

By: Low Math, Meekly Interacting

Low Math, Meekly Interacting — Fri, 23 Apr 2010 16:32:19 +0000

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.

By: Sean

Sean — Fri, 23 Apr 2010 15:52:25 +0000

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.

By: Charon

Charon — Fri, 23 Apr 2010 10:25:25 +0000

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.