Thursday, September 30, 2010

Life with a smaller star

kw: musings, astronomy, extrasolar planets

Late yesterday it was announced that extrasolar planet Gliese 581g, the sixth planet discovered circling dwarf star Gliese 581, is in a "Goldilocks" orbit: not too close, not too far away, but just right, smack in the middle of the habitable zone around the star. The planetary particulars:
  • Mass: 3x Earth or more (most likely: 4x)
  • Distance to Star: 0.146 AU or 14 million miles
  • Equilibrium temperature of an airless body: 228K = -45C (A watery atmosphere's greenhouse effect adds about 35C → -10C average, but can range much higher and lower depending on latitude)
The star's particulars:
  • Mass: 0.3 Sun
  • Diameter: 0.3 Sun
  • Surface Temperature: 3200K (Sun is 6500K)
  • Stellar type: M3V (Sun is G2V)
  • Visible brightness: 0.002 Sun (see below)
  • Total luminosity: 0.012 Sun (lots of infrared)
This image, from this Wikipedia article (recommended reading!), shows how Gliese 581 would look were it in the vicinity of our Sun. However, if there are inhabitants about the new planet, they are almost seven times as close to a star which is 0.3x the diameter of our Sun, so it would appear twice the diameter in their sky, having an angular width of more than one degree.

That is the first thing that would be different about life on this new super-Earth, so-called because it is larger, probably about 1.5x the diameter of Earth. What else would be different, and what would seem the same?

Firstly, just because astronomers call an M star a "red dwarf" doesn't mean they are all that red. An incandescent bulb's filament has a temperature near 2800K, while a carbon arc (think searchlight or old-fashioned movie projector) has a temperature near 3400K. Both of those look pretty white unless you compare them to sunlight on a clear day, then they look a little yellowish. The Sun is the standard of "white" to our eyes because all the eyes on this planet evolved to take maximum advantage of the Sun's light. Similarly, any creatures on the new planet (which I propose naming Goldilocks) will have eyes adapted to take advantage of its light, which will look white to them, even as light from our Sun would appear a little bluish to them.

So, being closer to the star means it covers a lot more sky, about 4.5x compared to the Sun's apparent area. 4.5×0.012 = .054, or 1/18th and 4.5×0.002 = 0.009, or 1/111. The different sensitivity of the eyes of a resident of Goldilocks would cause the apparent brightness to be closer to 1/20th than to 1/100th. While the ambient lighting will be brighter than the inside of an office building, it won't be by much. And what color would that sky appear? It would be blue, even for one of us. The Rayleigh scattering in a clear atmosphere scatters blue light nine times as well as red light. In fact, it takes a very, very cool star, less than 2000K, to have light that would scatter to look whitish or yellowish rather than blue (to us).

One nice thing about an M3 star compared to a redder star of M5-M9: stability. While more variable than the Sun, an M3 star is not a flare star, so it won't periodically blast the planet's surface with x-rays. The star is considered to be about twice the age of the Sun, but its stellar evolution is so much slower that is is probably only a few percent brighter than it was eight or nine billion years ago (the Sun is 40% brighter than four billion years ago). So the stability of any ecosystem on Goldilocks depends on the stability of its orbit, which we know very little about yet.

One consequence of having much less of the incident light in the "energetic" wavelengths that we humans can see is that there is a lot less energy available for photosynthesis of the kinds we know. Both C3 and C4 photosystems use blue photons with energies greater than 2.6 eV and red-orange photons with energies close to 2 eV. They don't need the green ones in between, so the green light is rejected (reflected). There are precious few 2.6 eV photons that reach Goldilocks, so a different system is needed. Plants there might look black; they have to absorb everything effectively. It'll take some interesting chemistry to utilize infrared photons, if it is possible at all.

This means the energy available to drive a biosphere is correspondingly small, maybe 1/50th to 1/20th of the productivity per acre compared to Earth plants. That corresponds to the productivity on the floor of a thick forest or rain forest. Ferns and other dimness-tolerant plants do well enough there, but life runs at a slower pace. The forest canopy is where the action is. Goldilocks will probably have few places where a canopied forest is even possible, being more of a tundra planet.

And all that is if, big IF, there is life there. While the temperature range is right for having liquid water on parts of Goldilocks, it will be some time before we will be able to determine whether there really is any water there.

My speculation? That it is an ocean planet, and may not have any land that emerges to the air. Look at our solar system. There's lots of ice on the moons of the cooler planets (Jupiter and outward). Mars lost most of its water because it is to small, with low gravity. Goldilocks is cooler than Earth, more like Mars, but is quite a bit heavier. It will have lost very little of any water that came its way, or was part of its formation. I do hope there is some permanent, solid ground somewhere on Goldilocks. It is hard to imagine smart dolphins developing effective telescopes or radio transmitters and thus finding out about the rest of the universe. A radio message has been sent their way, to arrive about 2029. If they receive it, we could get a reply in another twenty years, about 2050. Long before that, maybe we'll have a telescope system that can image the planet directly, so we can see whether it has continents or any weather.

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