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What lurks at the center of it all

Gamma Bursts, X-rays
and Black Holes
by Bruce Moomaw
Cameron Park - May 2, 2000 - The inner parts of our Galaxy, on the other hand, have such a high density of stars that any habitable planet will be much more likely to be exposed at some point in its history to dangerous waves of intense X-ray, gamma-ray or particle radiation from a nearby supernova or some other violent astrophysical event.

Globular clusters -- made up largely of first-generation stars -- combine the problems of crowded stars and sparse "metals".

(Ward and Brownlee have also said in their book that the gravitational tuggings of such nearby stars would greatly increase the rate at which the comets in the Oort clouds of solar systems would be diverted inwards to bombard inner planets -- but this set off a debate at the Conference, since some astronomers think the same effect would keep large Oort clouds from forming around stars in the first place, and that most of their comets would instead be pulled completely away from the star into the interstellar void. Ward finally relented to the point of saying that stars in the inner Galaxy may indeed have smaller Oort clouds -- but that the same increased tuggings by nearby stars would increase the speed with which those comets that did enter their inner systems would strike the planets there, so that "the greater force of impacts may make up for their lesser frequency".

It might be wise for me to call this particular issue a wash at this point.)

Indeed, at the Conference, Ward and Guillermo Gonzalez announced their concept of the "Galactic Habitable Zone" -- a relatively narrow band at a certain intermediate distance from a spiral galaxy's center in which "metals" are plentiful enough for planets to be common, but stellar density is sparse enough that high bombardment rates by radiation and comets doesn't render those planets uninhabitable.

Gonzalez said that his very preliminary calculations indicate that this Zone is quite narrow in our own Galaxy (and may be limited even there to those parts of the spiral arms where stellar density is acceptable), and that in some other spiral galaxies it may be so narrow as to be nonexistent.

All these "deserts", though, are really irrevelvant to our own search for life on other worlds, simply because they are at such a tremendous distance from us -- tens of thousands of light-years or more, even if our own Galaxy's Habitable Zone belt is (relatively) narrow.

Unless we somehow manage to achieve faster-than-light travel or communication, we'll never be able to explore those realms anyway.

What about the millions of relatively close stars which our future technology might conceivably allow us to probe for life or intelligence in the future? How unlikely is metazoan life on their planets?

It seems to this reporter that -- even giving the benefit of the doubt to their opponents -- Brownlee and Ward have made a strong case that planets carrying metazoan life are quite rare.

The growing evidence that a solar system's gas-giant planets must be properly sized and placed in very restricted orbital patterns -- both to avoid destroying habitable inner planets themselves and to serve as their protectors against lethal bombardment rates by comets (and perhaps also to divert enough water-rich asteroids to them during the system's formative days) -- is especially strong.

It's possible that evolution may be ingenious enough to overcome some of the climate problems produced by a planet having an excessive axial tilt, being on the fringes of its sun's Habitable Zone, having a borderline rate of crustal tectonics, or having an inappropriate amount of water to allow its air to build up oxygen -- especially since, as a star ages and gradually grows into a red giant, its H.Z. will simply move farther from it, so that planets that were previously too cold for life will become potentially habitable for a while.

(Several billion years from now, the moons of Jupiter and Saturn may very well become warm enough for surface life -- although they are all so small that at that time it will be impossible for them to hold onto any substantial atmosphere.)

And there are other problems that have been recognized for a long time, including the fact that it's considerably harder (though by no means impossible) for planets to establish stable orbits around binary stars, and the fact that the star itself must be in a restricted size range.

Stars more than about 1.5 times more massive than our Sun burn their hydrogen fuel, and turn into red giants and then white dwarfs, so quickly that there is no time for metazoans to evolve on their planets.

On the other side, "M" class red dwarfs -- which make up the vast majority of stars -- hve stable lifetimes much longer than our Sun, but they have another problem.

They're so dim, and their HZs are thus so close to them, that the solar tides on any planets in those Zones will slow the planet's rotation in just a few hundred million years until it keeps one face permamently toward the star, as Mercury was once thought to do.

And its nightside will then become so cold that the planet's water supply will tend to freeze into a big permanent icecap on any nightside continents, leaving the dayside seriously dry.

If that planet's oceans are big enough, though -- or it is on the outer fringes of its star's HZ and thus has a dense temperature-equalizing CO2 atmosphere -- this problem may be overcome. But there are other problems: such a planet would rotate so slowly that it would have little or no magnetic field, greatly increasing the amount of particle radiation that would hit it (although its atmosphere might itself be an adequte shield).

Also, as Ramon Wolstencroft pointed out in a Conference poster, chlorophyll is very inefficient at utilizing the red light of such suns -- and biologist Simon Conway Morris, in a witty speech on possible limitations in the flexibility of evolution, pointed out that every photosynthetic pigment ever evolved by any organism on Earth makes inefficient use of even the Sun's yellow light, suggesting that there may be serious biochemical limits on whether organisms anywhere can evolve other, more efficient kinds of photosynthesis.

Once again, though, it should be pointed out that about one star out of every ten is either a "G" type like our Sun, or a somewhat smaller, dimmer and longer-lived "K" type that will thus have longer-lived Habitable Zones than our own Sun.

That's still a great many stars that are the right size.

  • Click For Part One - Part Two - Part Three - Part Four
  • Astrobiology at Ames


    My favourite rock
    A Rocky Debate
    by Bruce Moomaw
    Cameron Park - April 28, 2000 - Meanwhile, the seemingly endless debate continued over whether Mars meteorite ALH84001 does or does not contain microfossil and chemical evidence of ancient Martian microbes.

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