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Looking for Life in Light
Gearoid_Murphy writes "Earth-like planets around distant stars may be too far away to be reached by spacecraft but scientists could still investigate whether they harbour life.
Telescope technologies are being developed that will probe the very faint light from these objects for tell-tale signs of biology.
These are the same "life markers" known to be present in light reflected off the Earth - so-called "earthshine"."
From the article:
...but not life!
"'This gives you some information on habitability,' said Wesley Traub, chief scientist on the US space agency's (Nasa) Navigator Program..."
How will this help? Radio telescopes can look at the absorbtion spectrum of the planet for the tell-tale lines of water, methane, oxygen (both O2 and O3), and other markers of highly reactive chemicals - especially when they will react with each other. When you have an atmosphere that is chemically violently unstable (as is the case on Earth), it must be being maintained by some process.
That's the first clue, but only the first. The second clue is that "dead" planets will be in equilibrium with their surroundings, but "living" planets will always be in opposition. (Organisms will always create a dynamic equilibrium that suits them, so must always counter any and all natural phenomena that would push the system away from that preferred state. Simple negative feedback.)
Simple radio telescopes can do all this now, no new optical technology need be developed, and no assumptions about the type of life need be made. (All the above assumes is that life can never be inert and that any specific organism cannot function equally under all potential conditions. That's broad enough, although there will probably be exceptions even then.)
The Km array proposed (and the hectare array already built) are just a huge stack of ordinary satellite TV dishes. This could be done by anyone at any time. A mile array would give you 2.5x2.5 pixels ast 100 lightyears - enough to discern if weather patterns exist, though not enough for any long-range forecasts.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
I haven't studied the specifics, but when I hear about similar ideas, irt usually goes something along the lines of starting by just looking at the star. Based on the spectrum, the star has so much oxygen, so much hydrogen, etc. Then, calculate when the planet passes in front of the star. Then, see how the starlight changes. If there is a spike in the apparent amount of hydrogen indicated by the spectrum of the starlight whenever the planet passes in front, then the planet probably has a lot of hydrogen, and so forth.
If you do an spectral anaysis of IR etc reflecting off the earth, you'll get certain signatures for trees, grasslands, sea, coulds cities etc. So if observers see the similar patterns they will assume that the distant planets will have a similar biology, cities,...
Of course these are all just assumptions. The scientists hope to make discoveries which they can publish for fame and glory. Luckily for them, they'll probably be dead long before they can be verified by eyeball technology.
Engineering is the art of compromise.
Where's the news? NASA's had the Terrestrial Planet Finder in the works for years now. Is it a slow news day at BBC?
Last time I checked, there exists a project already doing this. It's called SETI. They very thouroughly comb a large range of the EM spectrum for any data representing intelligent life. This proposal instead takes hazy samples from a very narrow band of data (the visible spectrum), to guess at the chemical composition of other planets. So we've replaced listening to white noise with looking at faint blue dots.
I am actually working on this exact problem as an undergraduate astrophysics researcher. My mentor came up with the quite excellent idea of looking at the difference in luminosity of specific frequencies over the course of time.
Technically, we use polarization-encoding to split a light beam into two right-angle polarized beams, run them through different color filters, and then recombine them back into a single beam. We then use a fast polarization analyzer to look at each beam independently at speeds close to 100 frames per second.
The idea here is that as the orbiting planet goes through various phases, and shows us different surface area profiles as a function of time (think about the various surface areas of the moon that you can see as it goes through its phases), so we'd expect that the difference in signal for certain frequencies to vary with a period equal to the orbital period.
The difference in signal comes about by the fact that the planetary atmosphere and surface have a specific curve of frequency vs reflected percentage of light. This differs from the emissions of the host star, which follows a theoretical blackbody curve.
In practice, the Square Kilometer Array is intended to have a collecting area close to the physical area of one million square meters - requiring almost no gaps to exist between dishes.
My first calculation would be for dishes with a wider gap, which would give you much greater flexibility on pointing the damn thing, as you can't see through the other dishes. Personally, I consider this to be a much superior design, even though it would cost on the collecting area. Unfortunately, they are the ones being paid, even if I am the one who is right...
By way of comparison, Jodrell Bank Radio Telescope is a paltry 76 meters across, for a total collecting area of 4560 square meters, and that's one of the largest single steerable telescopes out there.
I'm going to guess that a collecting area about nine times that of Jodrell Bank, combined with a resolving ability that is, well, astronomical, you would get a very respectable image of Earth-like planets around other stars. If we accept the SKA group's claims, then you've a collecting area 250 times that of Jodrell Bank.
I first heard the 100LY=1 pixel resolution with SKA from Jill Tarter, head of the SETI Institute at a talk she gave at NASA Langley. From crunching the numbers, I can see nothing that could seriously contradict the claim. Even if you assume my model is the more reasonable implementation, the complete MERLIN network that has been detecting jovian planets for some time has only a fraction of that collecting area - probably something like a quarter or a fifth. (Aside from Jodrell Bank, the next-largest radio telescope in the UK is a paltry 32 meters across.)
If we go with SKA's claims, then we're talking about collecting possibly hundreds of times the total radiation, which would definitely be enough to spot even the tiniest of worlds - provided it had some characteristic reflected in the radio spectrum.
(It's also worth bearing in mind that networks such as MERLIN, which are hundreds of kilometers across, are set up for VLBI - very long baseline interferometry. That's fine, when you're talking about gas clouds or stars, but is probably none-too-hot for spotting very fast pulsars or rocky inner planets. On the other hand, a kilometer would let you use regular interferometry, which means these things would show up quite nicely.)
There are three drawbacks to all of this, and I'm surprised none of the posters has commented on them (so far). First, interferometry requires very exact timing of all the delays in the system, or it won't work. Let's go with the SKA estimate and say the dishes are 1 meter apart. Your clock must count an integral number of ticks for every meter the signal travels from the dishes, even after allowing for the natural variation in the data lines varying the speed of the signal. This is some astonishingly serious timekeeping.
The second problem is to keep the signal noise-free. Easy, for a giant single steerable dish - you plunk it in the middle of nowhere and surround it with a huge Faraday cage that only obscures the horizon. When you've a few tens of thousands - or millions - of very small dishes, the problem isn't so easy. The terrestrial radio sources will be far harder to screen out - not just
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
I am guessing that they are talking about optical observations, since it appears to be an extra-atmospheric telescope they are designing. However, at those distances, how can they discern the difference between the shine from a planet and the light given off by the star(s) near the planet? I would think that we observe the earthshine from small enough distances that we can see it in spite of the Sun. I am curious how this would work for distant bodies.
The project is called Terrestrial Planet Finder (TPF). I don't know a ton about the details, but I know some guys who were working on it. One of the technologies being investigated (I'm not sure how well this relates to TFA, but it addresses your question directly) is an optical trick called a coronograph. The basic game is to design fancy Fourier optics that put more emphasis on small variations in off-center light. Like I said, not sure of the details, but it actually kind of works.
m0nstr42.blogspot.com
To separate the light from a planet and it's sun you need a telescope with sufficent resolution. It's just like the problem of 'splitting' multiple star systems into their separate stars, you need a large enough telescope. In this case though, we need a REALLY BIG telescope. We can't make one large enough, but we can combine the light from several telescopes separated by a long base line to get the same result. In fact such scopes are already being built and the first ones have already seen first light.
We might need to put such a multi-telescope system in space to get a long enough baseline, but it could be done.
Now imagine being able to actually see an exo-solar planet orbiting some distant star. We see it's night side and see some lights on the surface of the night side of the planet. The spectrum from that light is rich in Tungsten, Mercury, and Sodium.
I'd say THAT would be a sign of intelligent life (at least they had created electric
outdoor lighting).