Earlier this week it was announced that a solar system containing small planets around the red dwarf Gliese 581 has been identified. Of course there was immediate speculation (starting with the discoverers themselves) that one of them, 581c, might be Earth-like, since scientists think it is rocky and has a temperature that might allow for liquid water.
This then is a planet which just might be favorable to life as we know it, and even better, it's right next door, only 20 light years away. So let's look at what other factors we will need to maintain life on a planet and look at what observations we might make at this distance which could provide evidence for the presence of life. Let's start with the Earth itself.
More below the fold -- it's a bit long, but then this is a big subject (I hope you don't mind my borrowing the Science Friday tag) ...
To see just how unusual our planet is, it helps to look at our neighbors, Mars and Venus. Their atmospheres are nearly all carbon dioxide, and neither has a magnetic field to speak of. If we go further out, we find that Jupiter and the other gas giants have atmospheres mostly of hydrogen and helium, and what carbon there is is in the form of methane. It turns out that there are two thermodynamically stable forms of carbon: CO2 in an oxidizing environment, and methane (CH4) in a reducing environment (see refresher on redox chemistry at the end of the article if you need it). Mars and Venus have oxidizing atmospheres; the gas giants have reducing atmospheres. Our neighbors in the solar system are thermodynamically stable: because they are already at the bottom of the thermodynamic energy well, they are also chemically and therefore biologically dead. From the absence of a magnetic field, we note that Mars and Venus are also tectonically dead -- we'll revisit this point later.
Earth, on the other hand, has relatively little carbon in the atmosphere, and an immense amount of molecular oxygen. There are also significant pools of reduced carbon on the surface of the Earth, mostly in the form of plants, but also animals and microbes. At this point I hope you're thinking: how very strange. Oxygen is very reactive -- metals rust, wood and rocks weather -- surely it should be rapidly consumed, especially on a geological timescale. Certainly there is no free oxygen to speak of in our neighbors' atmospheres. But oxygen is the second most common component of our atmosphere, 21 times more abundant than third place argon (nitrogen is the most abundant, at 78%). Our atmosphere is obviously a strongly oxidizing one -- yet there is a lot of reduced material growing, eating, walking around. What this means is that our Earth is very far from thermodynamic stability, and that's a good thing: life uses the chemical potential arising from that instability to power growth, maintenance, and reproduction. Even better: life goes out of its way to maintain that condition of thermodynamic instability.
Life processes
We have no idea where life originated but almost certainly it started out being heterotrophic: it used whatever chemical materials were available to power life processes (we are heterotrophic). Such life forms are limited to regions where such materials were regularly replenished, perhaps such as hydrothermal vent zones. Possibly existing reduced carbon, likely in the form of carbohydrates (a bunch of CH2O strung together: we'll call this CH2O -- do not confuse this with formaldehyde!) was used as the energy source:
2 CH2O -> CO2 + CH4
This process is known as fermentation or methanogenesis and is still common today anywhere reduced carbon is available and oxygen is not: beer bottles (where we prevent the reaction from going to methane by stopping it at ethanol), swamps and other wetlands ("swamp gas" is methane), landfills, ocean sediments, the silt that deposits behind hydroelectric dams.... But if the source of CH2O gets used up, so too are the fermentation bacteria. Fermentation is good for about 70 kJ per mole of carbon.
Some 3.5 Ga (billion years ago) life invented autotrophy: instead of scavenging whatever chemical energy sources were available, it would make those sources. Of course, you still need an energy source: for the earliest forms it is thought that chemical sources were used. Perhaps hydrogen was used to reduce CO2 to form carbohydrate. This is a form of chemoautotrophy (BTW, if you stick H2 and CO2 together nothing is going to happen. The reaction may be thermodynamically favorable under reducing conditions but it is not kinetically favorable. We do part of this industrially in the reverse water gas shift reaction, requiring high temperatures and exotic metal catalysts. Life does this at room temperature using vastly more sophisticated catalysts made from commonly available C, H, O, N -- aka enzymes. This form of chemoautotrophy may be "primitive" but do NOT underestimate the complexity involved in making it work! 3.5 billion years later we still can't do it as elegantly as those ancient bacteria.) Ultimately, though, the limited availability of free hydrogen limits how far you can take this.
Early photosynthesis
About 3 Ga, life chose to use solar power in addition to chemical autotrophy. Unlike hydrogen, solar energy is plentiful, at least near the sea surface. This new form of photoautotrophy -- photosynthesis -- was limited to sunlit regions, but the ability to carry your lunch around and float around the sunlit ocean opened enormous new ecological regions to explore. Probably the first forms of photosynthesis used hydrogen sulfide to reduce CO2 instead of water: this is still used today by purple sulfur bacteria:
H2S + 2 CO2 + 2 H2O + hv --> SO4= + 2 CH2O + 2 H+
Here "hv" refers to light energy. Ultimately the supply of free H2S limits purple sulfur populations. The carbohydrate stores may be consumed by fermentation or by a new form of respiration known as sulfate reduction (the reverse reaction of the one shown above). Sulfate reduction is good for about 126 kJ/mol C, better than fermentation. Sulfate reducing bacteria remain very common today in anoxic (oxygen free) sediments, particularly in seafloor sediments but also in those wetlands where sulfur is available (generally only the first 2-3 mm of marine or wetland sediment is well oxygenated). That rotten egg smell from H2S is a dead giveaway that you're getting anaerobic sulfur respiration. In the presence of iron, which is common in runoff and ocean sediments, the sulfide generally precipitates out as iron sulfide, FeS2 (the chemistry is a little different from that above, but not significantly for our purposes), aka iron pyrite, or Fool's Gold, for its metallic yellow appearance.
A digression on sulfur
Pyrite is commonly found in organic rich sediments: shales, coal, oil, for reasons which should be obvious. Those deposits laid down in fresh waters tend to be low in sulfur, or "sweet" if you're talking about fossil fuels; those laid down in seawater, which has a high concentration of dissolved sulfate, tend to be sulfur rich, or "sour". FeS2 is reduced sulfur; mine tailings usually contain pyrite in greater or lesser quantities. When brought to the surface, it will readily oxidize to form sulfate in the form of H2SO4, or sulfuric acid. Mine tailings are usually very acidic as a result. Burning the sulfur, such as that in coal, converts it into SO2, a partially oxidized gas phase sulfur species. SO2 rapidly oxidizes completely to H2SO4, which washes out of the atmosphere as acid rain. So digging up sulfur-rich deposits has negative consequences. But we'll see that buried sulfur is important to maintaining atmospheric oxygen.
Green plant photosynthesis
The history of life and the history of the Earth changed dramatically about 2.5 Ga when life invented a form of autotrophy we know as green plant photosynthesis. Its waste product would change the structure and composition of the atmosphere, the radiation balance of the planet, climate, the chemistry of the oceans and crust, drive earlier forms of life to niche environments, and generate a huge chemical potential that would make much more complex lifeforms possible.
CO2 + H2O + hv -> CH2O + O2
Conveniently for green plants, the waste product is highly reactive, and therefore 1. toxic to its competitors -- oxygen is the original biological toxic waste, and 2. very useful as a "terminal electron acceptor" (oxidant) for metabolizing CH2O. This reverse reaction is of course aerobic respiration, and is good for about 478 kJ/mol C, vastly more efficient than other forms of respiration (actually, denitrification is fairly close at 398 kJ/mol C, but for more complex lifeforms the fixed N is too valuable for protein manufacture).
On the amazingness of green plants
The free energy change associated with aerobic respiration is immense. We can run the reaction backwards to find what sort of solar energy we need to drive photosynthesis, and find that if the reaction consumed one photon per CO2, we'd need photons of less than 250 nm or so. That's deep UV, and the solar flux at those wavelengths is close enough to zero. Cyanobacteria 2.5 Ga said "Fine, I'll use several photons per CO2". This is so nontrivial it is impossible to overstate. But green plants do it every day, with a staggeringly complex array of pigments, proteins, and organometal complexes, which 2.5 billion years later we still can't hope to emulate. Pretty amazing for stuff we dismiss as pond scum.
So take a look outside as the leaves are sprouting and things are growing, making stuff out of air, water, and light; using a nuclear fireball 150 million km away to maintain a massive planet-wide thermodynamic potential; and if you aren't struck dumb in total wonder there has got to be something seriously wrong with you.
As photosynthesis became widespread, the O2 produced gradually swept the oceans of reduced material, and, with no place else to go, started outgassing. Eventually, the atmosphere was swept of any reduced gases, and oxygen built up. O2 is not stable with respect to deep UV, and at high altitudes it breaks up, converting UV to heat. As it does so it forms a huge thermal bulge which we call the stratosphere (the bottom half of the bulge) and the mesosphere (top half). In the lower stratosphere is a layer of enhanced ozone, which serves to block deep UV from reaching the surface and shields biomolecules like DNA from UV damage. So by what is arguably the most important chemical reaction on the planet, plants (which are also aerobic respirators) generate and maintain a huge chemical potential to drive life processes and by screening deep UV also make it possible for life to exist on land, opening up yet a whole new frontier. Finally, plants cooled the planet, by drawing down CO2, a greenhouse gas, and replacing it with oxygen, which is not a greenhouse gas (but is radiatively active in the UV). If it is impossible to overstate just how amazing photosynthesis is, it is equally impossible to overstate its effect on the Earth system.
On the benefits of a molten core
Actually, it's not quite so simple. A model with just plants and atmosphere ends up with very little free O2, as O2 is used up in respiration. To get high levels of something as reactive as O2, you need to keep stuff that would react with O2 away from it. That is, you need to bury it. As it is, recycling efficiency of organic matter is very high, but there is a small (~0.1%) leakage term which is lost to the buried sediment. That burial term is key. But if you only bury the material, you will also lock up and ultimately deplete the trace nutrient supply, so you also need to regenerate it. Ideally you would like a very large, slow moving reservoir that sinks reduced material and releases it very slowly.
Enter a molten core. Nearly all chemical processes at the surface are driven by the sun, but the planetary heat source plays a crucial role. The molten core provides a source of energy to drive mantle convection, which forces the crustal plates floating atop the mantle to move around and bang into each other, occasionally diving under each other. This latter process is called subduction and buries great quantities of organic sediment (primarily marine). Eventually it gets brought back up by uplift and volcanism, but the turnover time is on order 108 years. This allows atmospheric oxygen to build up and provides a buffer that damps rapid changes to oxygen levels. Estimates vary, but the general consensus is that buried reduced carbon accounts for about 4/5 of atmospheric oxygen and buried reduced sulfur accounts for the other 20%. (Iron pyrite may be called Fool's Gold but it is in fact of biologically vital value, provided we don't dig it up. Unlike true gold, which is basically useless and just sits there looking shiny.) Uplift also regenerates biologically necessary minerals and makes it possible to maintain life.
What else does a molten core do? The core is metal, some fraction of which is charged. Moving charges = magnetic field, so our planet has a magnetic field (which also keeps extremely high energy particles out of the lower atmosphere). Venus and Mars have no magnetic field, their cores are no longer dynamic, and they are tectonically dead. From their atmospheric compositions, we know they are also chemically dead, and if either currently has life, it is in trace amounts.
On Earth-like planets
To get back on topic ... so what would we look for in searching out Earth-like planets. At this point it should be clear that we'll need to look for the presence in large quantities of thermodynamically unstable compounds, such as oxygen is in our atmosphere. Significant quantities of oxidized and reduced species coexisting would also be highly intriguing (though that may only indicate redox ~neutral conditions). For life as we know it, we'd need water as well.
We should also look for a tectonically dynamic planet. Spectroscopically this is detectable by the presence of auroras and at least in principle direct detection of the magnetic field. Realistically we'd need a space probe.
There is only so much we can deduce at these distances, yet there is also a lot we can do. It will be interesting to see what new turns up with Gliese 581c. But let's also spare a thought for what current observations of our own planet are telling us about the third planet from the sun and our only home.
Chemistry Refresher
Just in case you need a refresher on redox chemistry, here's a quickie. When atoms bond together to make molecules, they tend to share electrons. Sometimes the sharing is equitable, such as in molecular nitrogen (N2) or oxygen (O2), sometimes one atom will completely give up an electron to others, such as in sodium chloride (NaCl). (In the latter case, the atoms are said to be ionized -- they have electrical charge -- and the bond is electrostatic in nature and called an ionic bond.) More frequently, though, the electron sharing is partial, with one or more atoms taking the lion's share of the shared electrons (they have greater electron density). Such is the case with CO2, H2O, CH4, etc. An atom which gives up electron density is said to be oxidized, one which gains electrons is reduced. Oxygen is great at sucking up electrons, so it is a good oxidizer or oxidant (no, the terminology is not coincidental). Hydrogen tends to give up its electron, so it is a mild reductant. Atoms with oxygens attached are (almost always) oxidized; those with hydrogens attached are usually reduced (the exceptions are metal hydrides, where hydrogen is usually the oxidant). CO2 is oxidized carbon, methane and petroleum (a bunch of CH2 strung together) are reduced carbon; nitrate (NO3-) is oxidized nitrogen, ammonia (NH3) is reduced N; sulfate (SO4=) is oxidized sulfur, H2S and FeS2 are reduced S; etc.
Important: reduced species are not stable in an oxidizing environment -- they'll want to become oxidized. Similarly, oxidized species are not stable in a reducing environment -- they'll be reduced.