03 July 2023

Lithic worlds, Earths and Super Earths

If the whole subject of speculation about the prevalence of life and the nature of exoplanets makes your eyes glaze over, you may want to skip this musing. 

I have come to what I consider an inescapable conclusion, which I believe will eventually just be one of those "background wisdom" things everyone knows.

Our Solar System (our star system, or SS) has four inner "terrestrial" (terrestroid would be a better word, or perhaps even better, lithic) planets. Some star systems do not have inner lithic planets; some have all or mostly lithic planets, not necessarily close in; and a good fraction of such planets are of the SE type (Super Earth), mass between 1.1 and 2.0 Earth masses, a common enough type that it seems the majority of SSs have at least one. Another type, still lithic but clearly hopeless for life, would be the 2.0 to 3.5 or so range, which are also common and usually lumped in with SE, but actually are clearly different. Beyond 3.5 or so they hold hydrogen and are Neptune-type. No example of a Super Earth exists in our SS. 

But here's my conclusion: 

Our SS has examples of 3 extremely common types, which exemplify, respectively, the large majority of lithic worlds in the universe. Runaway greenhouse, which can happen easily inward of the socalled habitable zone (Venus), but can also happen within and even beyond the habitable zone (colder, farther from star). The second is cold, dry (Mars) type, which can range from Moon size up to a good deal larger than Earth, which is where liquid water did not take hold sufficiently to create an atmosphere that retains water vapor and ozone (a neat, tightly balanced trick). Cold dry worlds (Mars) are probably the most common type of lithic planet, with SEs being second. The third, Mercury, is the hot dry type, which are just too hot, inward of habitable zone, to retain any liquid water or much of an atmosphere at all. Mercury would be Mars type if it were further out. Hot dry worlds are usually tidally locked; a whole other subject which limits habitability even further, but I won't go into that further. If Venus, even though almost Earth size, were where Mercury is, it would look more like Mercury than it does. So this is a separate type, but small lithic bodies quite near stars are also extremely common. 

The gas planets in the SS, apart from being all bunched up and pretty far away, which is not typical but not rare, are nothing special, apart from Satur's especially beautiful rings. 

Which, of course, leaves Earth. Earth is the anomaly, the rarity. And not just because of life. It formed from a major collision, ending up with a large moon and relatively lower density (only slight, but definitely abnormal). It acquired a lot of probably cometary water (not necessarily typical). It is inward of a large gas giant that caused the heavy bombardment period to peter out early. It developed, of course, the unique chemistry of life, quite early on, which came to regulate surface temperature despite the universal tendency of main sequence stars to increase continually in temperature throughout their main sequence lifetimes. And, probably because of life and the advent of free oxygen and thus ozone in the upper atmosphere, it is 1) protected from lethal radiation at the surface and 2) retains water, which otherwise would eventually dissociate at the edge of space into hydrogen and oxygen, and the hydrogen would all escape, converting the Earth into Mars type. This is the usual fate of worlds situated in Earth's approximate position. The presence of oceans for billions of years, therefore, led to plate tectonics, which didn't kick in until about 1 billion years after the origin of the planet, and which is almost certainly necessary to retain habitable conditions on the surface, but which is probably highly unusual in the universe. Quite a number of just barely stable conditions, all of which are necessary for the habitable conditions remaining stable on this planet for more than 3 billion years. 

In other words, Rare Earth. It really is true: a lot of things that can easily stray into the "easy" part of the graph, and cause a planet like Earth to resemble either Venus or  Mars. Only a tiny fraction end up with conditions where life is possible for long periods of time, and of course the regulatory (Gaia-like) nature of biospheres themselves play a big role in that. 

This explains the Fermi Paradox completely, and means that naturally habitable planets are rare as hen's teeth. It is the evolution of intelligent life, however common or rare that turns out to be, that will make life in the wider range common in most of space possible. The natural origin of life really is a near miraculous concatenation of not particularly likely-in-combination factors. 

One countervailing factor: SEs may have all of these characteristics, although probably only at the relatively lower end of mass range. Planets between about 0.9 and 1.5 Earth masses are probably candidates for liquid oceans and plate tectonics. I've seen in print the "educated guess" that the majority of living worlds are probably closer to 1.5, meaning, as living worlds go, Earth may be near the lower end of the mass range. Planets like Mars, or even up to 0.7 or 0.8 Earth masses, probably cannot sustain plate tectonics or retain liquid water over eons, no matter what else may be going on. But it seems likely that most planets that have liquid oceans and plate tectonics are, in fact, SEs. 

The fact that 70%+ of all stars are probably too dim to have a habitable world at all (Class M or red dwarf stars) is another limiting factor, of course, but that's another topic. 

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