We have a curiosity about whether there is life on exoplanets, but how could we tell? Obviously, we have to know that the planet is there, then we have to know something about it. We have discovered the presence of a number of planets through the Doppler effect, in which the star wobbles a bit due to the gravitational force from the planet. The problem, of course, is that all we see is the star, and that tells us nothing other than the mass of the planet and its distance from the star. A shade more is found from observing an eclipse, because we see the size of the star, and in principle we get clues as to what is in an atmosphere, although in practice that information is extremely limited.
If you wish to find evidence of life, you have to first be able to see the planet that is in the habitable zone, and presumably has Earth-like characteristics. Thus the chances of finding evidence of life on a gas giant are negligible because if there were such life it would be totally unlike anything we know. So what are the difficulties? If we have a star with the same mass as our sun, the planet should be approximately 1 AU from the star. Now, take the Alpha Centauri system, the nearest stars, and about 1.3 parsec, or about 4.24 light years. To see something 1 AU away from the star requires an angular separation of about one arc-second, which is achievable with an 8 meter telescope. (For a star x times away, the required angular resolution becomes 1/x arc-seconds, which requires a correspondingly larger telescope. Accordingly, we need close stars.) However, no planets are known around Alpha Centauri A or B, although there are two around Proxima Centauri. Radial velocity studies show there is no habitable planet around A greater than about 53 earth-masses, or about 8.4 earth-masses around B. However, that does not mean no habitable planet because planets at these limits are almost certainly too big to hold life. Their absence, with that method of detection, actually improves the possibility of a habitable planet.
The first requirement for observing whether is life would seem to be that we actually directly observe the planet. Some planets have been directly observed but they are usually super-Jupiters on wide orbits (greater than10 AU) that, being very young, have temperatures greater than 1000 degrees C. The problem of an Earth-like planet is it is too dim in the visible. The peak emission intensity occurs in the mid-infrared for temperate planets, but there are further difficulties. One is the background is higher in the infrared, and another is that as you look at longer wavelengths there is a 2 – 5 times coarser spatial resolution due to the diffraction limit scaling. Apparently the best telescopes now have the resolution to detect planets around roughly the ten nearest stars. Having the sensitivity is another question.
Anyway, this has been attempted, and a candidate for an exoplanet around A has been claimed (Nature Communications, 2021, 12:922 ) at about 1.1 AU from the star. It is claimed to be within 7 times Earth’s size, but this is based on relative light intensity. Coupled with that is the possibility that this may not even be a planet at all. Essentially, more work is required.
Notwithstanding the uncertainty, it appears we are coming closer to being able to directly image rocky planets around the very closest stars. Other possible stars include Epsilon Eridani, Epsilon Indi, and Tau Ceti. But even then, if we see them, because it is at the limit of technology, we will still have no evidence one way or the other relating to life. However, it is a start to look where at least the right sized planet is known to exist. My personal preference is Epsilon Eridani. The reason is, it is a rather young star, and if there are planets there, they will be roughly as old as Earth and Mars were when life started on Earth and the great river flows occurred on Mars. Infrared signals from such atmospheres would tell us what comprised the atmospheres. My prediction is reduced, with a good amount of methane, and ammonia dissolved in water. The reason is these are the gases that could be formed through the original accretion, with no requirements for a bombardment by chondrites or comets, which seemingly, based on other evidence, did not happen here. Older planets will have more oxidized atmospheres that do not give clues, apart possibly if there are signals from ozone. Ozone implies oxygen, and that suggests plants.What should we aim to detect? The overall signal should indicate the temperature if we can resolve it. Water gives a good signal in the infrared, and seeing signals of water vapour in the atmosphere would show that that key material is present. For a young planet, methane and ammonia give good signals, although resolution may be difficult and ammonia will mainly be in water. The problems are obvious: getting sufficient signal intensity, subtracting out background noise from around the planet while realizing the planet will block background, actually resolving lines, and finally, correcting for other factors such as the Doppler effect so the lines can be properly interpreted. Remember phosphine on Venus? Errors are easy to make.
ianmillerblog 
Another possibility, unforeseen and new, is life evolving outside of the traditional habitable zone… in WOWs… Worldwide Ocean Worlds… Such as Europa, Enceladus (which clearly have liquid water oceans). Europa may have twice the water oceanic volume of Earth.
There could be more than refueling possibilities for humanity there. There could be native life. Viroids, maybe?… (Once we have mastered thermonuclear fusion, hydrogen, thus water, will be important because that’s what we will need the most in space, for colonization…) In any case, the concept of WOW augments considerably the inhabitable zones. In the Solar System, the presence of water may make more than half a dozen worlds colonizable… including Pluto. (Mercury, Luna, Mars, Ceres, Europa, Ganymede, Callisto, Enceladus, Pluto…)
In my not so humble opinion, I think it is more likely that WOW will have no life, because life requires lightning and cosmic & solar radiation to appear and evolve ever greater complexity in a timely manner, in the genetic material of said life… Both lightning and radiation are found only on the surface of planets… Intriguingly, this idea may be why terrestrial life became more advanced than oceanic life on Earth! Marine mammals are advanced, right… But they evolved on the continents.
Huge tides would help too, by mixing things up… Plus a mighty nuclear engine inside the planet to bury carbon with plate tectonics in an homeostatic way, and fabricate a huge magnetic shield. (I have proposed that nuclear fission helped in the creation of Luna; my theory would require that Luna and Terra have the same exact isotopic composition… As observed… If that is correct, a huge IF, it would mean that having a big moon, or being a big moon, is a requirement for evolving sophisticated, Earth style life…)
But of course whether WOWs can evolve sophisticated life is an experimental question: missions should be flown to try to detect organic activity… And the Viking experiments on Mars (which apparently found life!) should be repeated (that was refused for “Perseverance”!) it is rather baffling that a big effort was not made by, say, Europe, to find evidence of life on Europa. Are they all spiritually dead yet, out there? Just waiting for America to design vaccines for them?
My argument is there is no life under ice on Europa. Reasons include no nitrogen (there is more sodium in the extremely tenuous atmosphere than nitrogen, unless the analyses are wrong), and no significant carbon. Additional reasons include no possibility of reproduction because besides no nitrogen and carbon: phosphates would sink to the bottom, no real mechanism to make lipid equivalents (too wet and cold), and no mechanism to make phosphate esters. The only one so far discovered that is plausibly abiogenic is photophysical, so it needs light, and no phosphate esters, no reproduction. Enceladus at least has nitrogen and carbon, but I think the light is still a killer, as is the limited phosphate.
There is one further issue. Abiogenic chemistry in oceans suffers from the problem of dilution. Even if you can get condensation reactions to work (very difficult in water, without light or enzymes and you can’t start with the latter) the dilution effect means polymers are always far too short to be useful, like two mers.
Wow. Very interesting comment Ian! One sees the chemist unfolding wings of understanding. Basically you say Europa’s chemistry is not rich enough. However, there maybe 80 kilometers of ice… so what we may see may not be much. And Callisto seems also to have liquid water (it generates a magnetic field).
You seem to be saying that life started on land? One can have concentration in ponds… However, continents seem to be an emerging feature on Earth, and may have started after life (????)
But, on the face of it, except for cephalopods (no culture there), culture bearing animals on Earth evolved mostly on land (that’s my point). Instead of dilution, I look at the greater occurrence of genetic evolution through mutations… on land.
Finally there is this:
Now it turns out that bacteria in sediments below the ocean floor use radioactivity as power source: The contribution of water radiolysis to marine sedimentary life
(Justine F. Sauvage, Ashton Flinders, Arthur J. Spivack, Robert Pockalny, Ann G. Dunlea, Chloe H. Anderson, David C. Smith, Richard W. Murray & Steven D’Hondt
Nature Communications volume 12, Article number: 1297 (2021))
Abstract:”Water radiolysis continuously produces H2 and oxidized chemicals in wet sediment and rock. Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface [This hydrogen is mostly produced by alpha and gamma radiation inside the sediments; this is the point]. Radiolytic products may also be significant for sustaining life in subseafloor sediment and subsurface environments of other planets. However, the extent to which most subsurface ecosystems rely on radiolytic products has been poorly constrained, due to incomplete understanding of radiolytic chemical yields in natural environments.
… we show that all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. In electron equivalents, the global rate of radiolytic H2 production in marine sediment appears to be 1-2% of the global organic flux to the seafloor. However, most organic matter is consumed at or near the seafloor, whereas radiolytic H2 is produced at all sediment depths. Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years. Where water permeates similarly catalytic material on other worlds, life may also be sustained by water radiolysis.
Yes, Patr5ice, what I am saying is that the Jovian system is deficient in nitrogen and carbon, which is why Ganymede and Callisto have no atmosphere worth mentioning, while Titan has. It has to do with the ices that formed the planets. What I am saying is that as the ices warmed up, first the N2 and CO that helped form Neptune vaporized, then the argon and CH4 that helped form Uranus, then the methanol and ammonia that helped form the Saturnian system, leaving the Jovian system essentially with only water and solids.
I think the basics of life came from underground, with geologic processing, and life probably started around fumaroles. the reason being it was easy to get wet-dry cycles. With splashing cycles it is possible to get APM and UPM to form RNA fragments of u to 100 mers in a few hours, so the next trick is how to get AMP and UMP, and that is where sunlight comes in. It is not so much to provide energy, but rather to provide very high vibrational energy from an excited state decaying through internal conversion, which is why we use ribose – it is the only sugar that forms a furanose, and the furanose form is the only form that can relay the vibrational energy. That’s my view, anyway.
The radiolytic provision of hydrogen is interesting because life had to use hydrogen for the early anaerobes. There is obviously a lot more to life than what I outlined above, but I think that would be the way it started because this alone gets reproduction AND catalysis started.
Fascinating, Ian. Indeed, I have long been wondering why Ganymedes and Callisto, which are huge worlds, have no atmosphere to speak of. But I still do not understand: why would a N2 or CO atmosphere fly away?
We both seem to agree that hellish ponds on the surface was the way to cook the recipe of primitive life… (Thermophile bacteria are very old, indeed…)
People rarely mention that Earth was seriously more radioactive in the beginning. The half-life of uranium-238 is about 4.5 billion years, uranium-235 about 700 million years, and uranium-234 about 25 thousand years…
This means there should have been around one sixty times more U235 around, and twice more U238… Now the present U235 is .7% of Uranium.
One can make a reactor with natural uranium, and certainly with 3% U235 (natural Oklo reactor is Gabon, 1.7 billion year old)…Around 20% U235, one can make a fission bomb… Deposits of Uranium ore with more than 50% uranium are common… ergo, natural reactors, or even explosions should have happened a lot. I suggested that helped in the creation of the Moon (in a bold alternative theory to the all-too-obvious impact)….
“Why would a N2 or CO atmosphere fly away?” It wouldn’t from Ganymede or Callisto; my argument is they never got there. The ices from deep space contained N2 and CO (and other stuff) but the N2 and CO went past its melting point a little inside Neptune’s orbit and just sublimed away. The nitrogen on Titan, in my opinion, was made from ammonia, which made it as far as the Saturnian system. and was converted to N2 by geology similar to serpentinization.
Yes, our radioactive elements are passing away, but I do not think that 20% U235 can make a bomb. About 3% is required for a reactor, so I gather, but the bomb needs much stronger stuff. The 20% figure is bandied around as an excuse to criticise Iran, in my opinion. However, 20% is a good starting point for a final concentration.
Why shouldn’t Iran be allowed to enrich Uranium from its natural .7% presence of U235 up to 20%?
A sphere of 400 kilograms of 20% U235 with 80% U238 is 34 centimeters in diameter, and gets spontaneously supercritical (extremely dangerous, sometimes fatal) studies have shown. Now, adding a few tricks to this 20% enriched core enables to lower the critical mass a lot. (Beryllium reflects neutrons back to the core, quadrupling neutron density; thick, strong tamper for example in U238, bottles down the chain reaction, while adding to it with fragments of exploding U238; core with tritium, lithium deuteride adds neutrons of appropriate energy; an exterior neutron source starts the chain reaction.)
The Iran bomb problem is geopolitical: if Iran gets the bomb, a world war is much more likely. Indeed several Arab countries and Turkey, maybe Azerbaijan and its deadly enemy Armenia then of course large and mighty Kazakhstan, would get the bomb… So those who have the bomb already (China, Pakistan, India) would bomb up some more, meaning they would have to acquire much more capability to engage in nuclear war, thus lowering the trigger point… (India purchased Rafales from France in part because they can be used as hard-to-stop nuclear bombers.)
Logically Israel would have to strike first (with the approval, not just of the West, but of plenty of Arabs, who should, and do detest Aryan Iran much more than Semitic Israel…)… and strike hard. Some Iranian facilities are deeply buried. Although they tend to spontaneously explode in recent years, maybe one will need to use nuclear bombs to help them with the on-going fireworks.
A few countries agreed to stop or reverse their nuclear bomb programs (Argentina, Brazil, and South Africa which had officially seven bombs). Why? One cannot have dozens of countries with nuclear bombs… if one wants to avoid a nuclear world war. If bombs start to get used, the tendency will be to go all out and use reduced arsenals before they get taken out.
Pakistan has many of its bombs into deep caverns below its enormous mountains. hard to get to without precision bombing or nuclear bombs. The Islamist Republic of Pakistan is actively developing nuclear weapons, and long range rockets to carry them. Experts project that Pakistan may have the 5th largest nuke arsenal by 2025 with 220-250 warheads.A nuclear war between Pakistan and India would have a high probability to involve China if India “wins”.
Meanwhile, for the first time in more than a generation, the French military is preparing for an “Engagement Majeur” (major war)… within a few years and France has augmented its war preparation to 2% of GDP. Differently with what happened in 1870, 1914, and 1939, this time, at least under Trump, the preparation for war is involving the USA… which has now replaced Britain as France’s major military collaborator… Not that Britain is unwilling: it simply does not spend enough although it is close to the 2% of GDP, and is also augmenting its military spending, and so is Germany (greatly at France’s urging). The US spends 3.4% of GDP on defense… Should Iran develop nuclear weapons, European military spending will augment enormously.
More thorough version:
There is a difference between being critical and being of bomb quality. To make bomb, besides getting a very fast chain reaction, you have to be able to separate the components so they do not blow up when you don’t want them to. The general literature states that 20% is inadequate for a bomb, b ut I am not an expert on that, so I just quote the literature.