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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