13 June 2012

Some comments on the prevalence of habitable planets in the universe

The SETI institute (seti.org) has a series of fascinating talks, some of which focus on planet formation, and the likelihood and likely form of habitable planets in our Galaxy. I highly recommend them. (Many are on YouTube). This is a serious and very rapidly developing area of scientific investigation, and if you’re one of those people whose eyes glaze over when it comes to anything involving the wider universe and life beyond Earth, well, too bad for you, and I suggest you read something else. Bye. For the rest of you: it's really very interesting.

A quick summary of what I’ve gleaned lately, mostly from this source: planetary disks last about 1 million years on average; not much more, after the formation of a star, which generally takes place in starforming regions, which is a whole topic unto itself that I won't go into here. The disks of gas and dust from which planets form are ubiquitous (i.e., planets are very common), BUT, the region where the so-called habitable zone will later be located (i.e., for a sun-like Star the region of the disc centered around 150 million km from the star; or for any star the concentric region where at reasonable atmospheric pressures water can remain liquid) — is, during this planet forming phase, VERY HOT (~500K) throughout the planetary formation process, which effectively precludes the solidification of either rock or water. Since both of these are obviously necessary for the formation of terrestrial planets, what that means is that rocky planets with water on their surfaces do not form in situ. Instead, terrestrial planets (like Mercury, Venus, Earth and Mars) invariably form from accretion through collisions of rocky and/or icy bodies that have migrated from further out in the protostar system. It also means that planetary systems just like the Sun’s, with only rocky planets in the inner region and only gas planets in the outer region, contrary to what was once believed, are probably relatively rare. It's just luck of the draw, since many of the processes involved are essentially random. What sometimes happens, as we now know, is that large gas giants migrate inwards, but when they don't, something somewhat like what happened here, i.e., the migration of rocky and/or watery bodies further inward, is likely. In fact, the mathematics of turbulence and other physical effects mean that proto-planets always move around in the disk during the formation stage; and, on average, on the order of one planetary-mass object per star is actually ejected from the system entirely, (which, tangentially, means that there are approximately as many so-called "rogue planets" in any given galaxy as there are stars).

Probably equally likely as the suite of terrestrial planets in the inner system that we have here in the Solar System, is a Neptune-sized planet in that area. Such a Neptune-sized planet could easily have a large moon, which, all things being favorable, could be habitable. Another possibility in the inner systems of typical stars would be the presence of a so-called Super-Earth. This is a class of planet which does not exist in the Solar System, but which is believed to be quite common in the universe at large. The definition usually given is a planet between approximately 1 and 10 Earth masses. (Not large enough to retain hydrogen, which would make them Neptunes, or, if you prefer, Neptunoids). These planets, especially in the upper range of this class are likely to be uninhabitable, often with very thick CO2 atmospheres that would result in serious greenhouse heating. However, planets like this could exist in the habitable zone, and could have companion planets (or large moons) somewhat after the fashion of the Earth and the Moon, where the smaller planet in a binary planet pair could be habitable (with or without other, smaller, moons en suite). One reason that these binary planets, or large planets with slightly less-large moons, are especially interesting, is that they could conceivably exist in orbit around red dwarfs, which, as we know, are by far the most common type of star, consisting of roughly 90% of all stars in the universe. Although a single planet in close orbit around red dwarf, i.e. close enough to have liquid water, would probably be rotationally stopped with respect to the star, which would cause all kinds of problems, a planet sized moon of a super Earth or Neptunoid planet in close orbit around red dwarf, could have a day night cycle, and could conceivably sustain complex life. Given the prevalence of red dwarf stars, it may turn out that life-bearing worlds are actually typically this type of object, and single planets in orbit around larger stars like the sun, i.e., like Earth, may be less common.

This is not to suggest that actual gas giants are not also common in the habitable zones of protostar systems. Such gas giants (that is, comparable to Jupiter or Saturn, although some of them are actually much larger than Jupiter), in the habitable zone, are perhaps even more likely to have habitable moons. Thus, this kind of planetary body (i.e. an approximately Earth-sized moon orbiting a gas giant), could be even more common, especially in low-mass star systems (Class M and K; red and orange dwarfs), where the habitable zone is rather narrow and close to the star. (Just as red dwarfs (Class M) are the most common stars, the next larger class, K dwarfs, are more common than larger stars, and so on). Imagine a Saturn in orbit around Epsilon Indi or other orange dwarf, with a moon just enough bigger than Titan to retain an earthlike atmosphere and oceans. Such a world could easily resemble Earth. And even in orbit around a M dwarf, such a large gas planet's moon could have liquid water and a day/night cycle making the whole planet habitable, if rather different from our world, since red light is, after all, red light, and will give rise to a significantly different biosphere than ours, almost by definition.

So we may someday learn that more living worlds are actually moons of larger planets than are major planets of their stars in their own right (like Earth), across the board.   

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