Sunday, September 25, 2016

Life Around Hot Stars

This is not about really hot stars, the O’s and B’s and A’s as everyone knows that their lifetimes are too short to provide enough duration for life to originate and evolve. However, there seems to be a common misconception about F’s and G’s, and it affects the origination of life.

When you hear someone talking facetiously about life on Earth, they often say we have five billion years left before the sun turns into a red giant and engulfs the planet. Unfortunately, F’s and G’s vary their output a lot over their lifetimes, and that affects this number in two major ways. As an example of the variation, consider the sun, which has been modeled much more than any other star. The sun started out at zero power, but very quickly ramped up to about 70% of current output. From there to 100% of current output took about 4.5 billion years, and projections are that the output will go to about 200% of current output, after about another 5.5 billion years. This is when it leaves the main sequence and begins its evolution into a red giant, for the first time.

There is no need to worry about red giants such as the sun and other main sequence stars will become, as their lifetimes are much too short for a planet to originate life around them, even if the planet happened to be at the right orbital radius and had all the other correct conditions for life to originate. The red giant phase is followed by a collapse back to a much smaller phase, again moving the liquid water zone (LWZ) around.

The evolution of F’s and G’s appears to be fairly well understood, even if there is little data available. The changes all happen in the inner onion layers of these stars, being the hydrogen-burning core, the radiative transport layer, and the convective layer about that. The sizes of these layers change as the amount of unburned hydrogen changes, as it fuses into helium. These different sizes affect the output, and there is only one way it goes: hotter.

The liquid water zone starts in closer to the star, and moves out as the star grows hotter. The length of time a particular planetary radius stays within the LWZ might be thought of as the time during which life can evolve, and so it needs to be perhaps 4 to 5 billion years. This would be wrong. The LWZ is usually calculated as the bare rocky planet temperature, but atmospheres change this, if they possess greenhouse gases. Back when life was trying to evolve on Earth, 4 billion years ago, the Earth was not in the bare planet LWZ, but it was in the greenhouse LWZ. The planet would have been too cold for liquid water, if it had had, say, an atmosphere of pure nitrogen, which is not a greenhouse gas. However, back then the atmosphere had methane and carbon dioxide, and these raised the surface temperature, allowing liquid water to exist and the origination of life to begin.

The other end is less promising. As the output of a star increases, it will make the planet hotter, and this includes the atmosphere as well. A hotter atmosphere escapes more readily, so the devastation begins with the atmosphere becoming thinner and thinner as time progresses. Greenhouse gases make the temperature hotter on the surface, and as the star’s output increases, in order to keep the temperature in the liquid water range for as long as possible, it would be best to eliminate them, and go to an atmosphere with no greenhouse gases. Whether that can happen is uncertain, as it depends on what life exists. Carbon dioxide is produced by volcanic eruptions, photosynthesis, and fire or combustion of any sort of carbon-containing materials. It is a potent greenhouse gas, and only a small amount would be enough to shorten the time the planet dwells in the LWZ. Even if there are no greenhouse gases present in the atmosphere, the increased output of the star would be enough to move the bare planet LWZ out past the planetary orbit. For our own G2 star, we have about a half billion years left, which is not much compared to the five billion years that the sun will stay on the main sequence.

What this means is that our G2 star is pretty much near the top of the heap for stars that can originate life. A five billion year duration in the LWZ, aided by as much greenhouse gas effect as possible during the early cooler initiation of this period, means that if life, from the first membrane to a starship launch, takes four and a half billion, we just fit in. Maybe a G1 or G0 could also, with even less slack time. By the time a F9 is reached just about the G0, there may be none at all. So, for convenience in calculating the population of stars that could originate life, G’s and K’s are about the only good choices.

There are two numbers that could easily be calculated from this conclusion. One is the number of stars that might be have originated life all the way up to an alien civilization, no matter if it is still there or has left or gone extinct. Another is the number of stars that could originate life. They differ in the way they treat time. The second one is just the number of G and K stars in the galactic disk, which is about 11.5% by current counts of all stars in the disk, which is of the order of 10 billion. The result might be 1.1 billion, but that is too precise for the quality of data that is available, so 1 billion is a good rough estimate. The first one only looks at stars older than 4 or 5 billion years, which reduces the number by about half, if the rate of star-making has been approximately constant over the life of the Milky Way. In other words, about half of the G and K stars were formed in the last 5 billion years, and they haven’t had time yet to generate an alien civilization. This gets the number of stars that could potentially have hosted an alien civilization in the Milky Way down to a half billion.

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