Life has a great impact on the composition of the atmosphere of Earth. It is also true that the atmosphere of Earth has a great impact on life on the planet. It could be said that the two, the atmosphere and life, are an interacting system. If someone in the astronomy business is going to go out and make estimates of what planets among those newly discovered ones might have life, they should certainly incorporate this system into the conceptualization.
The simplest phenomenon is the greenhouse effect, controlled by the well-known greenhouse gas, CO2. If plants start doing photosynthesis, and in so doing produce some CO2, which gets into the atmosphere, some heating may occur. A planet with no greenhouse gases whatsoever has its temperature set by two things, the input solar flux from the star it orbits, and its own internal heat, seeping up to the surface. The internal heat comes from the formation of the planet, when the kinetic energy of the gas, caused by gravitational attraction, is converted to heat when the gas condenses onto the proto-planet. It takes a long time for this heat to dissipate.
A planet with greenhouse gases simply has an atmospheric layer blocking much of the heat from radiating away from the planet. In its place, the cold upper parts of the layer radiate much less energy. The right gases do not interfere with solar energy, which has a higher temperature than the absorption bands of the gases. Another way to say this is that the LWZ, liquid water zone, which some astronomers like to dub the habitable zone or the Goldilocks zone, moves further away from the home star.
Another interesting part of this system is the oceanic circulation. Depending on where the continents are, there could be more or less warm water from the tropics moving toward the poles to warm them. On a planet with a lot of this circulation, polar ice would be smaller than otherwise. If something happens over the eons to reduce the circulation, which might be atmospheric greenhouse gases, there would be more ice. Ice is bright and shiny, and reflects solar photons back into space more than does rock or ocean. So, as ice cover increases, a feedback effect happens and things get colder, meaning more ice. Ice age!
Volcanic activity is another player, as it dumps large amounts of greenhouse gas into the atmosphere. An ice age might be terminated if sufficient volcanic activity occurs, but this might mean a basalt flood to get the quantities high enough.
The system might function like this. Chemotrophs come into existence in the deep ocean by rifts, where there is good chemical energy flowing into the ocean. They could care less about ice cover. But in some shallower areas, given enough time, the various steps along the pathway to photosynthesis can be taken, and then CO2 is made. Photosynthetic organisms break their connection to the shallows, and spread through the seas. Now CO2 gets significant enough to have an effect, and less ice for a while. Then ocean circulation takes a hit, and the ice cover albedo feedback brings back the ice age.
Volcanoes step in, and the ice retreats, and life spreads to the land, where more CO2 is made, even more efficiently than in the ocean. Now the ice age cycle is somewhat reduced, but still goes on.
Regrettably for Earth science, figuring out geological evidence for ice ages is difficult and their extents and even durations are still controversial. Since we have not yet figured out the evolutionary pathways for life from some self-reproducing chemical up to ourselves, we have difficulty in coming up with an integrated picture of this system. And this does not even count the difficulties in figuring out the locations of the continents at any given epoch of geological time, much less the oceanic circulation.
Is there anything that can be usefully extracted from what we do know, or can estimate, about the life-atmosphere-ocean-volcano system that will provide a narrower window for finding planets that might have evolved life? Note we are not looking for intelligent life here, so radio waves and lighting are non-starters. Note that greenhouse gases have effects when they are very low in concentration, so observing the constituents of the atmosphere during a transit, as we are now learning to do, would not tell us about their presence or absence. If we did find, for example, a large fraction of an atmosphere was CO2, this would probably be an indicator that life was not present, rather than the other way around.
Perhaps the most visible possibilities are the polar caps. At this point, seeing an exo-planet as a single pixel is an amazing accomplishment. Seeing it as a 10x10 image will require bigger instruments, which may not even have been thought of yet, to say nothing of being checked for feasibility or, gasp, cost. However, if we did see a polar cap, perhaps by looking at multiple transits on planets with significant obliquity, this would be a red flag for a solo planet. Maybe we could name it Peary or Shakleton.
The other side of this nudge upwards in understanding would only come from a better balance of funding for other critical areas of research. Astronomy gets a large share of public research monies because their launches and instruments take a lot of money, and frankly, because they excite and interest a large number of taxpayers. But learning better about where our continents were two billion years ago, or how to derive some overview of ancient oceanic circulation, or for that matter, what are the essential steps in the evolution of life, would have a large payoff for the big picture of where life could be in the Milky Way, as well as better understanding of our own little Earth.
I would be the last person to suggest that money be taken away from our astronomical probes and instruments and diverted to geology or ancient biology, as those probes and instruments are necessary for our steps forward and, honestly, exciting and interesting, but instead some other source should be found so we move forward with a balanced portfolio of science projects, all pertaining to the question of where aliens are hanging out.
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