Is a moon really necessary to stabilize the the earth’s rotation and the obliquity of the ecliptic? Is hat stability really necessary, or can life adapt to the lack of it?

Can a magnetic field radiation protection be substituted by anything else?

If we are tidally locked to the star will our weather be screwed up, or will the atmosphere freeze/boil away?

Do M dwarfs have too many flares for life to evolve on their planets?

Is plate tectonics a requirement?

Even, is where and when in the galaxy’s evolution we’re formed make a difference in the chemical composition of the solar nebula?

We don’t have any examples to say for sure if any of these things is a game-channger. And we do know life is incrediubly adaptable. Not to mention, look at how much the earth has changed just since life arose here. We’ve been severely hammered dozens of times, and pulled through OK.

Besides, gas giants can have dozens of satellites, all differnt and each a potential earth.

No, I think the biggest spoiler is our own imagination. We always imagine aliens as being too much like us, terrestial creatures of roughly humanoid configuration, living on a world roughly like our own. I think all you really must have is long term dynamic temperature stability and an orbit that stays in the Habitable Zone most of the time.

Any stable late main sequence subdwarf that can last a few billion years is a suitable prospect. And that is the most common star there is. nd although our world may seem awfully stable for us, but for a subterranean or aquatic creature from a planet orbiting Barnard’s star, Earth could be a real hell-hole. Come to think of it, about 90% of our own planet is uninhabitable for hominids without technology and social organization.

And for the search for an “Earthlike” planet, there was always something new to add, such as:

The axis of the Moon’s orbit is located within the Earth’s mantel, churning it and keeping our magnetosphere charged. A nessessity for the development pf higher lifeforms.

The proximity of our Moon also acts as an impactor shield. Just look at Apollo’s map of the lunar farside.

Jupiter acts as an impactor deflector and attractor. Recall Shoemacher’s multiple impacts in the Jovian atmosphere, with many splashes larger than the Earth’s diameter.

Now, Rob’s posted article touting the natural oddity of an oxygen-rich atmosphere seems to be yet another nail in the coffin for the quest for an Earthlike planet.

Granted, my ignorance on what is in the universe is truly infinite, but if I had to bet on it now, I would say that microbial life is very common in the Galaxy, perhaps as many as a billion worlds.

Metazoans, multi-cellular creatures comparable to vertebrates, are probably several orders of magnitude rarer.

Intelligent life, probably no more than several species at a time. Intelligent life capable of making its presence known to us by technical means: maybe just one.

I haven’t always felt this way. I published this essay about a decade ago:

REALITY CHECK

The last few years have been kind to SETI enthusiasts. As hard scientific facts continue to accumulate, the factors in the Drake equation seem a little less unknowable and the evidence, although circumstantial, is starting to look very compelling. We know now that the molecular clouds where solar systems form are seeded with the chemical precursors to life and with the comets and meteoroids which can transfer this chemistry to planetary surfaces. The detection of organic compounds in meteorites and in the spectra of celestial bodies, as well as the serious proposal of fossils of Martian microbes, all argue that science is comfortable with the idea that life arises spontaneously wherever conditions are favorable. Our planet’s own geological history corroborates this; it appears that life began here almost as soon as the primitive Earth cooled. The non-photosynthetic food chains found in submarine vents and the recent discovery of deep crustal microbial communities have made scientists comfortable with speculation of life on Europa and other solar system sites. It now seems likely that life can gain a foothold in any suitable environment and will arise spontaneously in many places throughout the universe.

We may add to this evidence the results of one of the more mature branches of astronomy: stellar structure and evolution. Stable and long-lived stars suitable for nurturing life seem to be the rule, not the exception, and planets are probably a normal by-product of the formation of these systems. The recent detection of extrasolar planets, albeit non-earthlike, suggests our own solar system is not unique. All the evidence is not in, but it is not unreasonable to assume that life is quite common in the Galaxy. Our own planet’s past supports the conclusion that living worlds are self-regulating and quite capable of surviving all but the most devastating of cosmic catastrophes. The combined speculations of astronomy, geology, and biology are in agreement that SETI is on the right track. In my opinion, there are tens of millions of life-bearing worlds at this moment in our galaxy, give or take an order of magnitude.

And now, the bad news… The evidence for extraterrestrial intelligence, that is, extrasolar species capable of constructing communications devices, is totally non-existent. We have very little to go on other than our own example. The Earth has been the abode of life for at least 3 billion years, but for most of that time it was represented by only the most primitive micro-organisms. For a substantial portion of Earth’s history life consisted solely of anaerobic microbes. The development of photosynthesis appears to be a bottleneck in evolution, but not as severe as the invention of multi-cellular organisms which did not appear until about half a billion years ago. If this pattern is not peculiar to Earth and is typical of life-bearing worlds, the consequences for SETI are troubling.

SETI requires reasonably complex and active life forms as participants–on both sides. We must expect that candidate species share an oxygen metabolism and a multicellular structure comparable to vertebrates in sophistication. We know that intelligent primates, and possibly intelligent cetaceans, appeared on Earth only a half-billion years after the first appearance of multicellular creatures, a relatively short time by geological standards. But we should not conclude that intelligence was inevitable, simply awaiting a mammalian nervous system of sufficient capacity to support it. Mammals have been around for as long as the reptiles and have spent most of this time without developing any technical abilities. The dinosaurs and birds never chose intelligence as a strategy, and the cetaceans prove that even intelligence does not necessarily lead to tool-making. Primate cooperative technical intelligence, aided by language, arose only a heartbeat ago in the cosmic time scale. It’s late arrival and explosive growth suggests it was not inevitable, and probably accidental. We cannot even conclude from this history that intelligence is a survival trait; it is possible that its very advantage to a species is destabilizing to the biosphere as a whole. I shall omit at this point the usual cautionary remarks about nuclear war and environmental pollution… Neither should we forget that highly advanced civilizations need not necessarily be technological, even if they start out that way.

If we determine that technical societies capable of interstellar communication are common and not prone to self-destruction; it is still not obvious that they will be eager to communicate with us. Many will be cautious about talking to strangers and choose only to listen, others may tire of SETI after the first success–just as I suspect we will, as much as I hate to admit it! In fact, the more likely a culture is to engage in SETI, the more likely that it already has all the correspondents it wants. Once a SETI-prone species makes contact, it may prefer to cancel expensive searches using energy-inefficient methods and concentrate on upgrading its existing communications and refining its technologies. In other words, if SETI is common and widespread in the Galaxy, newcomers will probably not be sought out very aggresively.

The limiting factor in interstellar communication is not distance, but time. Even neighboring civilizations might be highly separated in time, time also affects the conduct of individual contacts. Centuries have to pass before each transmission’s success can even be evaluated. If we assume that during the course of its existence the Galaxy has hosted a million technical societies, and each of them survived an average of a million years, at any one time we could expect only a hundred active cultures in the entire Galaxy–about one for every billion stars. These numbers are not very encouraging, the average age of a civilization is one of the big teasers in the Drake Equation. For ETIs to be common enough to find easily this value has to be inordinately high. It seems inescapable that even if civilizations are very common, at any one time they are few and far between.

It is risky to make predictions about ETIs, but let’s try a few very liberal assumptions in order to further explore some issues in SETI targeting. Assume that there a million species in the Galaxy right now capable of and interested in communicating with their neighbors using microwave or laser technology (the only ways we know how!). With about a hundred billion stars in the Galaxy, this works out to about one civilization for every hundred thousand stars. The stellar density in the solar neighborhood is such that there are approximately that many stars within two hundred light-years of Earth. This line of reasoning suggests that civilizations are at least several hundred light-years apart and that for them to have the slightest chance of contacting their neighbors they must be prepared to transmit (and listen) to hundreds of thousands of stars for milennia just to have a fighting chance of making a one-way contact. Even if our hypothetical ETI is capable of using advanced astronomical techniques to eliminate unsuitable stars, the magnitude of this task is enormous. Any society contemplating a passive search for the first time (like us) must hope that its neighbors have been carefully tracking thousands of targets and transmitting continuously at them for thousands of years in order to have even the remotest statistical chance of intercepting a signal. Passive SETI is easy, but it depends primarily on what the other guy is doing. If the passive party further expects the active member to be transmitting at him in a narrow, energy-efficient beam, and has designed its listening strategy accordingly, in my opinion, they are wasting their time and squandering their budget.

Most SETI passive strategies are based on high signal-to-noise ratio directed searches of nearby stars, selected for their high probability of harboring intelligent life (old and stable Pop I stars on the main sequence). But as we have demonstrated, even a perfectly suitable world is highly unlikely to be transmitting at us when we look at it. To have a good chance of intercepting a signal, we must simultaneously observe large numbers of stars, i.e., large volumes of space. The active partner will also realize this, and will know that transmitting directly at nearby sunlike stars on narrow beams will be energy-efficient but highly ineffective. To listen (or transmit) using narrow “spotlight” methods allows weak signals to be heard or sent immense distances, but does little to maximize the number of stars reached. A “floodlight” or broadcast approach will saturate small volumes of space with signal, but is very wasteful and reaches few stars for the energy expended. I suspect advanced SETI species will adopt an intermediate “searchlight” methodology where a wide beam, on the order of a radian (approximately 60 degrees) in diameter, will be aimed at regions where large numbers of stars are found. I envision active transmitters to be aimed at the galactic plane, the signal footprint wide enough to encompass the thickness of the entire Milky Way and nearby disk stars. The best way to listen for these signals would be to concentrate on wide areas near the galactic equator, and to worry about the location where the signal originates only after it has been detected. Design receivers for sensitivity, not resolution. In other words, sweep the galactic plane first, particularly in areas of high galactic longitude. ETI will most likely be transmitting towards the nucleus, where most of the old stars are. Those civilizations at low galactic longitudes might even be transmitting across our line of sight, towards the galactic center, and we would not be able to hear them at all.

We have just recently learned, in cosmic terms, how to conduct SETI. At present, we cannot develop the antenna power to transmit a searchlight signal that would stick out above the noise for more than a few light years. We can only hope that others have been at this a lot longer than we have and will be able to do so.

In conclusion, indirect evidence suggests that SETI-capable civilizations will be highly separated in space and even more so in time. There may be a much smaller probability than previously thought to make contact with these cultures. In order to maximize this probability, we may have to rely on search strategies which are counter-intuitive. We can only hope that these civilizations will also anticipate these conditions and modify their procedures accordingly. It may be a long time before we make contact, perhaps never. We are almost certainly not alone, but we may never be able to know for sure.

It does appear that photosynthesis appeared very late, about half-way through life’s history on this planet, so that suggests it is not an easy or inevitable chemical solution to the problem. It may be a binary gate, a bottleneck, in the evolutionary process leading to high-energy life forms.

We know anaerobic life evolved very early in Earth’s history (almost as soon as it cooled off!). This, and the presence of simple organic compounds in the interstellar medium, and the presence of extremophiles on our own world, argues some forms of primitive life are probably widespread throughout the universe. But the transitions to an oxygen metabolism and the emergence of multicellular organisms (both essential to complex life-forms capable of achieving sentience and rapid evolution) may be a fluke unique to our world.

I certainly hope not, but that is what the meager evidence suggests so far. There may be other possible chlorophylls, just as there may be other possible DNAs; the fact they aren’t around today may simply be because more efficient modern versions just crowded them out by out-competing them. But until we find alternatives here, or in space, we must be prepared to accept that these two compounds are both necessary preconditions for complex life forms.

As much as I hate to admit it, we may very well be alone.

Until we actually get there, whose to say? Physics is physics, but chemestry gets weird.

Are there any alternatives to chlorophyll currently existing on Earth?

Are any other compounds, natural or artificial, capable of replacing it as a reducing agent for alien life.?

This report is troubling. If there are non-biological sources for O2 then identifying extrasolar life becomes much more problematic.