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"."

Comments

Earthshine? Pah.

[ScentCone]

For some real signs of life, try a little moonshine.

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[account_deleted]

Comment removed based on user account deletion

By the sound of it, they will be using optics

[El Cubano]

Telescope technologies are being developed that will probe the very faint light from these objects for tell-tale signs of biology.

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.

Re:By the sound of it, they will be using optics

[Poltras]

Probably will look at the shadow of the planet. What I am wondering though, is that if a planet is at, say, 700 light-years from here, and we're seeing a "civilization", or just plain life at least, then that life will be from 700 years before. Now with what we've been through in the last 100 years, put that a 10000 lightyears away and you have a heck of an evolution...

PS: funny part is, if they see our earthshine from the same distance, we humans wouldn't even exist. Talk about being stealthy :)

Re:By the sound of it, they will be using optics

[forkazoo]

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.

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.

Re:By the sound of it, they will be using optics

[MindHack]

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.

Re:By the sound of it, they will be using optics

[m0nstr42]

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.

Pardon me...

[GillBates0]

Pardon me while I illuminate my GIANT blinking "Welcome to Earth" sign.

If nothing else, it'll show up on the earth shine and indicate we're inhabited. On the other hand, they probably already know that...

Earthshine

[Mr_Tulip]

We may call it 'earthshine', but advanced extraterrestrials probably call it 'signs of parasitic infestation', and warn tourists to stay away in case they catch something.

Seems primitive.

[jd]

If/when the kilometer array is built (it's an array of small radio telescopes, where the array has a diameter of a kilometer and a density of one dish every couple off metters or so), they will be able to resolve Earth-sized planets at a distance of 100 light years.

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.

By making some big assumptions

[EmbeddedJanitor]

Of course I have not RTFA because that is cheating.

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.

Why telescopes?

[talkingpaperclip]

I can't understand why these guys are searching so hard for life in light. There have to be at least a dozen half-dead bugs in the ceiling light about 7 feet away from me.

Re:What's the point?

[Daniel Dvorkin]

You're right, of course. We shouldn't be wasting our time with this head-in-the-clouds nonsense. Say, what's the latest news about Britney Spears' baby?

Re:Seems primitive. (Resolution v. Lightgathering)

[ookabooka]

Are you sure about the capabilities of such an array? There are two main properties to a radio telescope (or any telescope for that matter) and that is Resolution and Lightgathering. By increasing the diameter of the collecting dish you increase both the resolution and lightgathering capacity. By creating an array using interferometry you can increase resolution to create a "virtual" dish with a diameter equal to the distance from one end of the array to the other. This doesn't, however, increase lightgathering capacity the same way. Let me propose a crude analogy: think of dishes as buckets, you can put two buckets 50ft apart and infer how much rain fell between them by adding them and dividing by two, but if it was just a drizzle, your data wont be so hot. If however, you have a 50ft bucket, you're gonna collect a lot of water.

So an array of a bunch of teeny TV sattelite dishes wont have as much surface area as a dish a kilometer wide. So yes we could resolve a planet, but it would have to be bright enough to be seen.

Re:Seems primitive. (Resolution v. Lightgathering)

[jd]

Yes, I'm certain.... but it is why you need the density of dishes. If you had one dish on each corner of a square, one kilometer on a side, then you would have the collecting area of those four dishes. Which, if they are TV dishes, is very little. If, however, you have that same square but one dish every five meters, you would have 200 x 200 dishes, for a total of 40,000. If each dish has a collecting area of one square meter, you then have a total collecting area of 40,000 square meters.

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

Re:Isn't this called SETI?

[Urkki]

SETI has that pesky 'I' there, meaning intelligence. It's looking for signs of radio communications, based on assumption that only intelligent beings might communicate with radio. Though if a non-intelligent life communicating with radio was found, I don't think anybody would be majorly disappointed ;-).

TFA is talking about finding planets that have *any* life that can significantly change the atmosphere of a planet. Earth could have been discovered like this probably at least since we've had O2 (regular oxygen gas) and O3 (ozone) in our atmosphere, starting from about 2 billion years (*) ago. Contrast this time with the time we've used radio communications, less than 100 years.

(*) reference:
http://www.globalchange.umich.edu/globalchange1/cu rrent/lectures/first_billion_years/first_billion_y ears.html