23 November 2020

Some thoughts on whether K stars are more likely to be the energy centers for living worlds than G stars, and some implications

Farflung correspondents, this is somewhat rambling and off the top of my head, reflecting my peculiar interests in universal principles of life and civilization. Posted it on Facebook. Got "TLDNR" (too long did not read") from my friend Sue, who often seems to think that's a helpful comment (it is not). Anyway, it occurred to me this might interest you. And if not, of course, you can simply not read it!


• I've found it fascinating to think about and read what is known about what makes Earth particularly suitable for life, and why there are good reasons to believe it is actually, after all, a highly unusual planet, such that while life of some sort (unicellular, bacterialike) is probably fairly common in the universe, there is pretty good reason to suppose that extremely complex life, in multilayered and globally interconnected complex ecosystems, evolved over literally billions of years of relative climatic stability... what we have here on Earth... may turn out to be really exceedingly rare. The universe is so vast that even very rare things happen many, many times, but that means they are widely scattered and probably unlikely to encounter one another. At least so far. The universe is larger in space than in time... if that doesn't make sense to you, think of it this way. There is a dimension, time, that is 13½ billion units long... back to the Big Bang or its aftermath. That is the operational degree of freedom for change and diversity in the time dimension. But, if some version the Inflation models which are apparently absolutely necessary to make cosmology work is correct, the spatial dimensions go far, far beyond 13 billion light years, because most of the universe is not the observable part, but the part that is too far away for light to ever get here from there. Think of it as having a short dimension in time (if you can get your head around 13 billion years as "short"), and the other three dimensions are almost unimaginably vast. Bottom line: the universe is yet young, yet developing, and while almost all of its "all-time" time as a dynamic and evolving reality is in the future, most of it will remain forever unknown to us because it lies in the untold hugeness of space the light from which can never reach us.
But back to abodes for life, and the conditions for it. Recently, it's been noted in several places where folks who think about this stuff talk about it that there are actually quite a few things about the Solar System and the Earth that are NOT quite ideal for life. Earth is prone to freezing over. It's happened several times, and eventually, if the internal radioactivity settles down too much for volcanism to break through the iceball, it would've been permanent. But even before that, our star is a little too active, and growing too bright (Main Sequence Stars get brighter continuously through their regular, hydrogen-burning lifetimes before entering their red giant phases). In a time less than the time that complex organisms have been present on Earth's surface (about 550-600 million years), the threshold for the beginning of loss of the oceans will be crossed; water vapor will begin dissociating and the hydrogen from water will begin to be lost at the edge of space. It's estimated that in something like 700 million to 1 billion years or so the Earth will have dried up and begun to resemble Mars. Other things, like the gradual diminution of Carbon dioxide from the failure of the carbon cycle (I recommend the book Oxygen, a Four Billion Year History [Canfield] for a description of this), will cause the Ice Ages to return and become permanent. In fact, the current brief epoch of global warming, destructive as it is (and it is), is actually only a temporary reversal of a very long term trend towards the failure of the greenhouse effect, which keeps Earth habitable. Without greenhouse gases, our position vis a vis the Sun would keep the Earth very cold. As the Sun grows warmer, paradoxically, the Earth would grow colder, but then at some point the temperature wouldn't be the main consideration, because global drying is what will make our planet ultimately inhospitable to life.
All this is quite far in the future, and if our descendants survive, they will have plenty of time to migrate elsewhere, but that's not really the point.
The question that I'm intrigued by is, given a world much like Earth, different in details perhaps but basically similar, would a different class of star be preferable? Lead to a longer period of time during which life might not only arise but flourish and the planet remain naturally habitable for longer?
(There are other interesting questions, such as would a slightly more massive planet be better suited (probably, to a point), etc., but this is the question that interests me at the moment).
And the answer seems to be, pretty clearly, yes. The Sun is a G2 star.* Estimated main sequence lifetime somewhere just under 10 billion years. It's just over halfway through that now. Stars as bright as the sun are relatively rare. It is well over the the 90th percentile in mass and brightness. Most stars are little red stars with less than a tenth of the Sun's light output, and, even more problematical, their peak output is well into the infrared and thus perhaps not too suitable for photosynthesis, and they tend to be quite prone to deadly flares (comparable to solar prominences, but to be warm enough for life a world orbiting such a star would have to be close enough to be seriously harmed by such flares). Plus, any planet in the "habitable zone" around such a star will likely be stopped or at least very slow in its turning with respect to the star, so it will have no day and night cycle like Earth, and that leads to all kinds of problems for habitability.
But what about the K type stars? Like Tau Ceti, Epsilon Indi, or 61 Cygni, for some relatively nearby real-world examples. These stars, dimmer than the sun but perfectly capable of illuminating somewhat closer-in Earthlike planets with stable and adequate luminosity. A little more orange, less white, but surely capable of supporting photosynthesis, and, here's what's most important. More stable. Slower rate of increase in brightness (which can be offset by changes in atmosphere or even deliberate advanced engineering). And, the big difference: such stars remain stable and continue shining much longer. The lifetime of a K0 (brighter end of the range) star, like 70 Ophiuchi, is about 20 billion years, while a K7 like 61 Cygni (smaller, dimmer) might live for 50 billion years or longer. And, importantly, they are something like ten times more common than G type stars. All spiral galaxies (and there are untold billions of them in the observable universe, and still more beyond that), have far more K type stars than stars like the Sun, and all those stars are just as likely to have planets. So relatively earthlike planets are substantially more likely to exist in orbit around K type stars than G type, and substantially even less likely to form around the next brighter class, F type, although that is perfectly possible also. The complication that the majority of stars are members of binary or multiple systems is another wrinkle, but at least some of those may be able to support habitable worlds as well. So planets capable of supporting life, whether introduced from elsewhere by intelligent beings like ourselves or naturally evolving on those planets, while not exactly common, almost certainly exist in incalculably large numbers in the wider universe. And even in very large numbers (billions) in our own galaxy. Even if problems causing delays in the development of life are common, we should consider the possibility that the age of "assisted development" (importation of complex life on purpose by intelligent spacefaring beings like our future selves) may be just getting started here and there in the universe. Remember that, overall, the whole universe is about the same age, and life, in all its complexity, takes time and is prone to failures from catastrophes. The past is long but somewhat limited, but the future is unimaginably vast; and some things take a really, really long time to get going.
Anyway, the periods of time that K stars can exist in the main sequence are longer than the universe is old. This leads me to speculate that by far most of the complex living worlds that will ever come into existence have yet to do so; and most of them will arise in systems with dimmer, longer lived stars than the Sun. The Age of Life is just beginning, and the Age of Civilization is mostly in the future.
Fun to think about, and these things may seem remote from our everyday lives, but they are real. We are the Ancients. The Elders. In the prime of the Dawn of Time... from the point of view of most creatures who will live in this part of the universe in all of time. If we don't blow it, we will play a major role in the future not only of this galaxy but beyond it, and will likely seed life from our world on many other worlds, where it will grow and flourish.
[And Donald Trump will be forgotten forever].
* "Main sequence stars" refers to stars in the main part of their lifetimes, where they are primarily fusing hydrogen for energy and are relatively stable. From brightest (and shortest lived and least common) to dimmest (and longest lived and most common), they are designated O, B, A, F, G, K, M. These designations refer to color, so other stars that have "evolved off the main sequence" also have these letters, but the ordinary so called dwarf stars form a normal continuum. The Sun is G2, meaning brighter end of G. (The scale subdivision runs from 0 (brightest) to 9, dimmest; a G9 might well be classified in another catalog as a K0). About 90-95% of all main sequence stars are dimmer and less massive than the Sun.

1 comment:

  1. K type stars (orange dwarfs) are longer lived than the sun but release a quite higher dose of radiation. If I may, I'd like to point out my recent article on the Fermi Paradox which touches on the subject: https://medium.com/predict/9cd470be1687?source=friends_link&sk=74c06e2049baeca702c27a702e3f40cf


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