The Search for Life On Habitable Exoplanets

New submitter Benzainload895 writes "The Verge has an article about why life on other worlds would be far stranger than we might expect. They also interview some astronomers who are trying to narrow down the most likely locations for life. Quoting: 'As it turns out, the small planets with long orbits that Kepler was finding were the ones it was least disposed to find. [After estimating how often red dwarf stars have planets and taking into account their expanded habitable zones, they] came up with an estimate Cowan says is "starting to get really close to a hundred percent, where for every [red dwarf] out there you should expect there to be a habitable rocky planet." Furthermore, research exploring these planets suggests weirdness — and lots of it — in what life they might harbor. For instance, the dim light coming from a red dwarf may not be enough for plant photosynthesis like on Earth. This may lead plants to be black instead of green in order to absorb more available light. Even weirder, these planets likely don't spin as they orbit. Since red dwarfs are smaller and cooler than the sun, planets circle them at close range, creating greater tidal forces than on our planet. While the tidal force on Earth moves the ocean up and down a few meters, that force on a red dwarf planet would be so strong it'd gradually slow down the rotation of planet completely. The result? One side of the planet would face its star in a permanently sunny day, while the other side would face the stars in an endless night."

Comments

Cats...

[JoeMerchant]

Higher life form evolved from cats.... as any BBC viewer of Red Dwarf knows.

Re:Cats...

[l0ungeb0y]

Actually everyone that has read Venus on the Halfshell knows that humansand cats alike evolved from cockroach shit

Re:Cats...

[Jason Levine]

And, as any Doctor Who fan will attest to, they make excellent nurses. Though exactly WHY is a dark secret.

We need to be more open to "life"

[metrix007]

Always looking for water looking for life, or where the 6 building blocks for life as we know it could form.

That's what we know and it makes sense, but there are surely other types of life that have different building blocks.

Re:We need to be more open to "life"

[gstoddart]

Always looking for water looking for life, or where the 6 building blocks for life as we know it could form.

That's what we know and it makes sense, but there are surely other types of life that have different building blocks.

You know, someone asks more or less this exact question every time this topic comes up -- "why not look for other forms of life unlike our own?".

And the answer is always the same -- how do you look for something you don't know the first thing about? What do you look for? How do you look for a lifeform which has completely different biology from us? How will you know if you've found it?

If you're physically there, you can look and see. If you're doing inference from spectroscopy and the like, what, exactly, do you look for to find a bit of life which is so alien from our own that we don't have any idea of what to look for or what processes would be involved?

You can't just take random chemicals and decide that they do, or do not, suggest a lifeform we can't even imagine.

It's simply not possible to look for signs of something when you have no basis on which to even speculate what those signs would be -- because you could look at anything and say "gee, maybe that's alien life".

But you'll never know, and can never make any hypotheses or predictions. At which point, you're well outside of what can be called Science, and straight back to speculative fiction. There's really no point in trying to look for life built around other building blocks, because we don't know anything about what that hypothetical lifeform would look like or how to spot it.

I'm not saying it couldn't exist. I'm saying that until we know about it and how it works, there's no basis to look for it.

Re:We need to be more open to "life"

[the gnat]

Always looking for water looking for life, or where the 6 building blocks for life as we know it could form.

I'll summarize the standard reply to this: we're looking for these signs because that's all that we know enough to look for. Every life form we've encountered is carbon-based and requires liquid water and a certain temperature range. We also know that O2 in the atmosphere indicates photosynthetic activity. Now, it's theoretically possible that somewhere, there exist silicon-based, chlorine-breathing lifeforms. But since we've never encountered these, we have absolutely no idea what constitutes habitable conditions, or what chemical signatures to search for. So, rather than guess wildly and look for something we can't identify, we focus on the environments that are most conducive to known forms of life, because we have some confidence that we could detect something interesting. Is this really that difficult to grasp?

Re:We need to be more open to "life"

[JoeMerchant]

If you think about Conway's game of Life - there are many equation forms that make some kind of "Life" but only a few that make anything interesting.

I suspect that Earth provided a robust environment for many forms of life and Carbon-Water came out dominant, as it probably will in similar environments.

You'd have to have a robust (temperatures supporting a liquid solvent, capable of forming complex molecules without destroying them quickly, etc.) non-Earthlike environment for "other" life to evolve, and many of these "other" forms might not be as fast to evolve and adapt as Carbon-Water has been.

Re:We need to be more open to "life"

[cusco]

Carl Sagan took a look at what an alien spacecraft could sense as it approached our Solar System, to see at what distance life could be deduced. His conclusion was that as soon as the spacecraft could detect molecular oxygen and methane in the same atmosphere (an inherently unstable and unmaintainable combination) it would know that something unusual was happening on the third planet out from the Sun. Detecting light on what should be the dark side of the planet would be the confirmation.

Re:We need to be more open to "life"

[gstoddart]

Right, but it's entirely based on understanding the biology as it exists on planet Earth. And is how we would try to identify life similar to ours on other planets.

Now, tell me, what do we look for with, say, silicon based life forms or anything which has a completely different chemistry than ours?

The answer ... we haven't got the faintest idea, and therefore we can't look for it, because it would be pure speculation, and therefore not 'science'.

Unless you can tell us the chemical signatures/indicators of life based on different elements than our own, there is no way to search for them. And, until we've encountered it and understood it, we have no basis on which to look for it based on its properties, because we don't know those properties.

But if they're based on silicon, breath cyanide gas, and fart out helium, and make xenon gas when they have sex ... we'll never know it unless we've already met them. (And, yes, that's a completely arbitrary list. ;-)

Re:We need to be more open to "life"

[flaming error]

Plus a place like that might support us.

Re:We need to be more open to "life"

[cusco]

No, Sagan wasn't looking at oxygen as a product of life, he was looking at molecular atmospheric oxygen as something that was inherently unstable, especially in the presence of methane, and very especially in the concentrations that we breathe. That meant something unusual was happening. An atmosphere rich in helium and xenon would attract attention as well, and prompt investigation. The default hypothesis would be complex radioactive processes, just like the default hypothesis for atmospheric O2 would be complex chemical processes. The discovery of artificially concentrated radioactive elements (for example) would be analogous to artificial light sources on Earth. We don't have to know what kind of things life produces, we just need to have a handle on what typical planetary evolution produces and look for variances from this norm. We'll probably end up with situations where investigators ask, "Is that life, or just an obscenely complex self-maintaining chemical process?"

Re:We need to be more open to "life"

[khallow]

If you're doing inference from spectroscopy and the like, what, exactly, do you look for to find a bit of life which is so alien from our own that we don't have any idea of what to look for or what processes would be involved?

It's worth noting that it would be glaringly obvious from spectroscopy that Earth had life. The atmosphere is inherently unstable. You wouldn't have an oxygen rich atmosphere in the absence of life.

Any similarly prolific ecosystem will have similar, observable chemical instabilities. It's far harder when life is marginal (say on a very cold, dark, or mostly inhospitable world). Then any chemical imbalances it creates could be swamped by nonliving processes.

So I think we could with sufficiently sensitive spectroscopy find worlds where the life present has heavily modified the surface or atmosphere.

Re:We need to be more open to "life"

[Kjella]

I think that once you have life, the incentive to fill every niche in the available biosphere is huge, which is why you find life in the most inhospitable places from ice bears to desert scorpions despite a tropical rain forest being much nicer. Maybe it's deep underwater or in deep caves or other places shielded from the normal environment where advanced life can begin. But even there it will be competition and resource pressure, so being able to survive exposure is an advantage meaning they'll progressively evolve shells and self-repair systems to deal with it. For all we know we're the slow ones, there's no basis for saying.

Re:We need to be more open to "life"

[JoeMerchant]

Based on where we stand today, if we were relatively slow, I'd expect to have been visited by "others".

I am almost certain there are "others," and lots of them - but if any of them are even twice as fast developing as we have been, they'd be spreading something to other stars by now.

Re:We need to be more open to "life"

[Kjella]

Plus a place like that might support us.

This. If we can't find inhabited planets, inhabitable planets are the second most interesting. Perhaps the most interesting for those that look at Earth as a single point of failure. What are the essentials?

1. Temperature, we only need the warmest spot to be arctic range or the coldest spot to be desert range.
2. Gravity, not sure exactly how far from 1G we could sustain but almost certainly a good range.
3. Magnetosphere, both to shield from radiation and sustain an atmosphere, including water vapor.
4. Water, no human colony could live long without water. If we have water and sunlight, we can grow food and produce oxygen.

If we could hit four of four, I think that would be habitable. Doesn't matter so much what it is like today, just put up pressurized greenhouses with plants and let them gobble up as much CO2 from the outside as they can. Dead plants become more soil - hopefully we can find the other bits we need - and we'll be able to grow bigger and bigger fields sustaining more and more people. That is, if we can get there but I think if you found a planet where we could truly say "If only we could get there, we could live there" all effort would go in that direction.

Re:We need to be more open to "life"

[Anonymous Coward]

Is that life, or just an obscenely complex self-maintaining chemical process?

po-TAY-to...po-TAH-to...

- T

Re:We need to be more open to "life"

[Nivag064]

"Is that life, or just an obscenely complex self-maintaining chemical process?"

Referring to "Life" on Earth??? :-)

Re:We need to be more open to "life"

[RubberDogBone]

You know, someone asks more or less this exact question every time this topic comes up -- "why not look for other forms of life unlike our own?".... There's really no point in trying to look for life built around other building blocks, because we don't know anything about what that hypothetical lifeform would look like or how to spot it.

I'm not saying it couldn't exist. I'm saying that until we know about it and how it works, there's no basis to look for it.

Look around, human. There are currently around 8.7 million different kinds of life on Earth. Only a handful of them look like YOU, and yet they are as alive as you are. Some of them are in your gut right now ensuring you can digest the dinner you will have tonight.

Out there in the vast universe, the odds are whatever life is there almost certainly looks even less like you than the 8.7 million kinds that are actually from the same planet. And yet there is this persistence to look only for life we know. We should be looking for what we do not know, which is oddly enough what most of science is supposed to be about. Except this one niche, where they only want to find what they know. Makes no sense to me.

It may be all we are capable of looking for, but then we should say that and understand our lack of ability to recognize life IS probably going to screw up any results we might get.

Re:We need to be more open to "life"

[mcgrew]

We'll probably end up with situations where investigators ask, "Is that life, or just an obscenely complex self-maintaining chemical process?"

Life IS just an obscenely complex self-maintaining chemical process. It isn't like bacteria are sentient.

Re:We need to be more open to "life"

[rubycodez]

all the life on this planet looks the same, carbon based and needing liquid water, using RNA and/or DNA.

we look for favorable places for this kind of life. it makes sense.

Re:We need to be more open to "life"

[TrollstonButterbeans]

They would notice whatever gas your Star Trek rock monsters exhaled is unusual in an atmosphere.

A lifeless world is going to have an undisturbed atmospheric composition.

Like: hydrogen, helium, carbon dioxide, nitrogen oxide, methane, water, ammonia, sulfur dioxide. The type of gases common in the universe for raw gas giants and rocky planets.

Detecting anything else in unusual quantities means something worth examining is altering the atmosphere.

On Earth, the oxygen and nitrogen would arouse suspicion -- pure oxygen molecules and pure nitrogen aren't found elsewhere in any significant quantities (for reasons that don't require too much explanation ... oxygen is highly reactive, hence that dangerous "Oxider" warning sign you see from time to time.).

Timely... sat a lecture on exoplanets last night

[ackthpt]

Part of the questions we were posing to the grad student from UC Santa Cruz: many of the planets they're identifying are about White Dwarfs - which means planets which have survived the exit of main sequence and a expelling of gas, at high velocity it could strip atmosphere and perhaps scour water from the surface - these are likely to be dead worlds, if not mostly frozen.

Those in the Goldilocks are still very hard to detect, which is why so few have been. There's a couple hundred thousand candidates which Kepler identified, and these are still being evaluated and processed - no small task. Exciting times.

Obligatory XKCD

Cold

[kanweg]

With tidal force locking, I'd expect the edges to be rather uninhabitable too, and the center that's facing the sun comparatively hot. There air will rise up, move towards the other side of the planet, cool down and drop. It would cool even further down, get even colder and move towards the lit side. When it enters that region, there is little light there, and the air is very cold. So, even though there is some light, it is uninhabitable. Then the air starts picking up heat when it moves back to the center. It would in particular in that region you could expect life to be.

However, what if that life requires CO2. It might condense on the other side of the planet.
Perhaps if there's a lot of it, you'd get a greenhouse effect, and sufficient CO2 for plants anyway.

And you thought life on earth was tough.

Bert

Re:Cold

[NEDHead]

At least we know on which side to find the astronomers .

Re:Cold

[Jane Q. Public]

"With tidal force locking, I'd expect the edges to be rather uninhabitable too, and the center that's facing the sun comparatively hot."

This is a point OP does not make sufficiently. While it says tidal forces would be stronger, that's only when the planet is young. Eventually, when it stops spinning ("sidereal rotation"), there would no longer be any oceanic "tide" at all, presuming an ocean in the first place.

Re:Cold

[dryeo]

Assuming no large satellite or the inhabitable world being a satellite or twin planet system.

Re:Cold

[Jane Q. Public]

"Assuming no large satellite or the inhabitable world being a satellite or twin planet system."

Yes, good point.

Re:Cold

[Jane Q. Public]

"Tidal forces don't need an ocean. Reply to This Share"

Correct. But a tide does. I did make the distinction, even if I didn't put it as well as I could have.

Science

[Skiron]

That is the trouble, science. We have billions of galaxies which contain billions of stars which most have planets.

The issue starts with thinking as we are the only life form, then every thing else has to be like us.

Re:Science

[firewrought]

The issue starts with thinking as we are the only life form, then every thing else has to be like us.

When exhaustive exploration is not an option, you use what you know to prioritize your search, even though that knowledge is incomplete and imperfect. For SETI, it makes sense to use the profile of life as we know it to focus our efforts, even though there may exist lifeforms well outside that descriptive envelope. Doing so gives us (1) a much smaller area to examine, (2) concrete strategies for investigating such areas, and (3) better odds of successfully recognizing extraterrestrial life (should it fall under our surveillance).

Re:Science

[Jason Levine]

Suppose you grew up only being able to speak English and not even knowing that other languages existed. One day, you find a stack of papers with weird writing on them. Some have familiar symbols, some don't. How do you differentiate between writing that is just random symbols and writing that is actually language? The low hanging fruit would be to look for English letters and words in a familiar sequence. This is what we are essentially doing: Looking for life using the "alphabet" that we know. Once we spot this low hanging fruit, we will know more about life and will be able to expand our search to find more and more life.

Re:Science

[khallow]

It's the drunk under the street light problem - looking for car keys there, because the light is better. Except that he's looking for any keys, not just a particular set half a block away in a gutter.

Re:Science

[Tablizer]

No, they have ridges on their foreheads.

Re:Redundancy

[NEDHead]

Actually, it is more like spinning with a periodicity of one year

The search for spam targets

[gmuslera]

Getting humans (and maybe even robots) to the next solar system might will be impossible in practical terms, and even harder for more distant solar systems. Finding habitable planets in our local group of stars will mean hear attently all of them to check if they were transmitting something in our direction N years ago, and to spam the close enough ones (10-20 light years away?) them with information in the hope that they might answer in some decades. And the answer will come back to a very different world, considerating our rate of change.

Re:The search for spam targets

[mmell]

Hey, I'll go. You keep breathin' the air here 'til that rock/supervolcano/solar extinction event hits. We'll go bring Truth, Justice and the American Way to some nice stable planet circling a red dwarf somewhere. It's only a temporary solution to that whole "the sun will burn out eventually" thing, but I'll take it. Send us a postcard when that yellow star you're orbiting is burning out - we'll be happy to sell you some rocket fuel and help y'all get outta Dodge!

Unless you'd rather round up all the Dems and send them hurtling into deep space?

Re:Timely... sat a lecture on exoplanets last nigh

[compro01]

XKCD #1298 is considerably more interesting for this discussion.

surely?

[geoffrobinson]

Our imagination does not give us "surely". The only thing they are sure about is how life works with water on Earth, which is why they are looking for water.

magnetic field

[confused one]

If the planet is tide-locked, there might also be limitations on core circulation. No circulation, no magnet field. No magnet field, atmosphere gets stripped by solar wind and the planet surface gets blasted with protons. Therefore, No life.

Re:magnetic field

[T.E.D.]

Who says you need an atmosphere for life? We've found life at the bottom of our oceans where there's no oxygen or light. Pretty much the only thing we know for sure to be a requirement is liquid water. Any other supposed "requirement" has either been proven wrong, or is conjecture.

Re:magnetic field

[T.E.D.]

Actually, I take that back. We don't even know for sure liquid water is a requirement. But what we do know about liquid water is that everywhere we've looked at it so far, we've found life.

Re:magnetic field

[khallow]

But what we do know about liquid water is that everywhere we've looked at it so far, we've found life.

Mars, Europa, and Enceladus all have water somewhere, but we haven't seen life on any of them yet.

Re:magnetic field

[confused one]

Actually my comment had more to do with the planet surface being blasted by protons and UV. As in high radiation flux. Cooked. Toasted. Fried. Nuked. Call it what you will, it equals death if you're on or close to the surface.

Re:magnetic field

[Tablizer]

Venus has no real magnetic field, but has a thick cloud cover.

Re:magnetic field

[Gavagai80]

We haven't examined the sub-surface liquid water of any of those places yet to test it for life.

Re:magnetic field

[khallow]

Hence, the use of the term, "yet".

Re:Redundancy

[Overzeetop]

And yet most moons are tidally locked. If you believe wikipedia (as I'm too lazy to confirm it), mercury is in a 3:2 lock due to its eccentricity, not some inherent stability of 3:2 ratio.

Dynamo

[jmn2519]

If these planets are tidally locked wouldn't that kill the dynamo process that produces a planets magnetic field? If that magnetic field is gone wouldn't they suffer the same fate as Mars and lose their atmosphere to the solar wind?

Re:And the atmosphere will freeze out

[Gavagai80]

Not necessarily, a thick atmosphere with winds evens out temperatures to a significant degree.

No you are posturing that assertion

[TrollstonButterbeans]

"I'm saying that until we know about it and how it works, there's no basis to look for it."

Why?

Astronomers, biologists, people at NASA have been considering many possibilities for life for 60+ years.