Saturday, April 28, 2007

An Earth-Like Planet

An important astronomical discovery was a big enough deal to make the news this week. A team of Portuguese, Swiss, and French scientists reported the first Earth-like planet ever discovered orbiting another star. In fact, this article describes the discovery of two more planets around a star in the constellation Libra, a star already known to have a large (Neptune-sized) planet. But it is the smallest of this sun's three (known) planets that warrants excitement. Of the 200+ planets now identified outside our own solar system, this is the first to meet the first criterion for life-support.

For astronomers and physicists in search of life-suitable planets, the first axiom is "follow the water." It is unanimously recognized that liquid water is required for life. Researchers can establish--based on the size and type of a star--a habitable zone, an orbitable range in which liquid water might be present. And this planet is the first to be located within its star's habitable zone. Moreover, its size is such that it might meet another necessary criterion; it may (assuming that it turns out to be a rocky planet) have the sort of long-term vulcanism and plate tectonics believed essential for life.

So here's the skinny... The smaller of the two newly-discovered planets has a mass five times that of Earth and a radius that might be as small as 1.5 times Earth's. It orbits 14 times closer to its star (Gl 581) than Earth orbits our sun. Nonetheless (and assuming an atmosphere as thin as ours), its surface temperature is estimated to be between 0 and 40 degrees Centigrade. And that's because its star is an M-type (red) dwarf, which is much smaller and less luminous than Old Sol. The system in question is 20 light years away, meaning that a journey there--or here--would require between 200 or 2000 years (estimates of the maximum possible travel speed vary between 0.1 and 0.01 the speed of light).

This really is exciting stuff, and I'm particularly impressed by astronomers' increasing ability to find stars of such small size and to measure important characteristics of them. But does this latest find provide support for the "principle of mediocrity"--the idea that the earth is nothing special and that there are probably many planets out there supporting life? Does this discovery undermine the anthropic principle, the idea that Earth (and our galaxy and solar system) were exquisitely designed for life? Some of the media hype would lead one to think so.

The answer is, well, no, not at all. Perhaps a little perspective is in order.

For one thing, a number of interested experts believe that the radiation emitted by this type of star would prevent life on any planets orbiting one. The planet's proximity to its sun likely means that it is tidally-locked (as our moon is by Earth), so that it does not rotate but always has the same side facing the star. This presents a number of problems for life. No rotation means no plate tectonics. It also means that one side of the planet would always be extremely hot while the other side would always be extremely cold. Moreover, the Earth's rotation is essential for maintaining the magnetic fields that protect us from solar activity. It is unclear how the planet in question would maintain a magnetic field, and its need for one is great since M-dwarf stars are much more active than stars like our sun.

The greater size of this planet would suggest a very thick atmosphere, which would negate the liquid water hypothesis. In addition, the proximity of the other two planets likely prevents this one from having the stable orbit necessary for life maintenance.

This planet is the first to satisfy the very first criterion for life-suitability--it lies (at least currently) within its star's habitable zone. Relative to the other 200+ planets so far identified, it is unique. But even if we ignore the problems for life (mentioned above) already being discussed by researchers, existence within the star's habitable zone is only one of more than 150 characteristics of our galaxy, solar system, and planet that lie precisely within extremely narrow limits that make life's existence possible here.

The evidence overwhelmingly supports the design hypothesis--this newest discovery notwithstanding--and leaves the principle of mediocrity without any evidential or reasonable basis.

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