A New Recipe for Hunting Alien Life

I think this is a step in the right direction, but it still has the central issue that looking for oxygen rich planets (or any atmosphere with a disproportionately high quantity of some suitable agent for reduction (respiration). We didn't even get around to the whole respiration thing on this planet for quite a long time, and there's no requisite that ATP/ respiration is the direction life is going to take, and equally, in the event of some kind of reductive respiration, there's not a prerequisite that oxygen be that agent, although its a pretty good candidate.

> there's no requisite that ATP/ respiration is the direction life is going to take

Based on the article I linked in my other comment I would say that on oxygen-rich planets, respiration is likely to happen simply because life needs to find a way to deal with it's toxicity, or perish. Although that presupposes that photosynthesis is inevitable, which also doesn't have to be true.

I kind of wonder how much we truly know about the strength of lock-in effects of evolution - of how much life is "stuck" with old hacks that we can no longer get rid of or that lead to dead ends. I doubt we'd ever evolve avian lungs from where we are now, for example. I know there are examples like that all over the place, but just how strong is the effect?

That would be the drake equation. The fermi paradox is just a philosophical question.

Study: Disequilibrium biosignatures over Earth history and implications for detecting exoplanet life

Citation: Joshua Krissansen-Totton; Stephanie Olson; David C. Catling; Science Advances 24 Jan 2018.

Link: https://dx.doi.org/10.1126/sciadv.aao5747

DOI: 10.1126/sciadv.aao5747

Abstract: Chemical disequilibrium in planetary atmospheres has been proposed as a generalized method for detecting life on exoplanets through remote spectroscopy. Among solar system planets with substantial atmospheres, the modern Earth has the largest thermodynamic chemical disequilibrium due to the presence of life. However, how this disequilibrium changed over time and, in particular, the biogenic disequilibria maintained in the anoxic Archean or less oxic Proterozoic eons are unknown. We calculate the atmosphere-ocean disequilibrium in the Precambrian using conservative proxy- and model-based estimates of early atmospheric and oceanic compositions. We omit crustal solids because subsurface composition is not detectable on exoplanets, unlike above-surface volatiles. We find that (i) disequilibrium increased through time in step with the rise of oxygen; (ii) both the Proterozoic and Phanerozoic may have had remotely detectable biogenic disequilibria due to the coexistence of O2, N2, and liquid water; and (iii) the Archean had a biogenic disequilibrium caused by the coexistence of N2, CH4, CO2, and liquid water, which, for an exoplanet twin, may be remotely detectable. On the basis of this disequilibrium, we argue that the simultaneous detection of abundant CH4 and CO2 in a habitable exoplanet’s atmosphere is a potential biosignature. Specifically, we show that methane mixing ratios greater than 10−3 are potentially biogenic, whereas those exceeding 10−2 are likely biogenic due to the difficulty in maintaining large abiotic methane fluxes to support high methane levels in anoxic atmospheres. Biogenicity would be strengthened by the absence of abundant CO, which should not coexist in a biological scenario.

One softball answer to that is that any civilization that doesn't get off of its home planet is likely to go extinct, so there's less chance of it actively modifying the environment in ways we can detect. It's also less likely to need to develop long range, high powered communications that we can detect.

At the moment I think the idea that we're only looking for life that fits some definition of success is specious, we're looking for any life we can find! It happens that a big, spread out civilization that looks like us is more likely to emit something we can see, be it atmospheric, electromagnetic, etc, just by nature of being in more places in space and having a larger footprint.

There are astrobiologists who spend a lot of time thinking about what life may look like, but when it comes to where to put limited resources, we start with what we know to look for (us), and go from there.

You might be interested in the book 'weird life' by David Toomey, which tries to theorize about what some more exotic living things might look like in the galaxy, and expounds on some of these definitional questions.