Living Around an M-Star: the Red Light District

June 7th, 2008

NOTE: I’m keeping the below post as originally written. It will be superseded by a post to come as the issue is more complicated than I appreciated.

A friend of mine has a story coming out and wanted to doublecheck some issues of colors on a planet orbiting an M-star. My friend supplied me with a technical source:

http://www.giss.nasa.gov/research/briefs/kiang_01/ and the press release is at http://www.giss.nasa.gov/research/news/20070411/.

The thing is, these are terrible! I mean, technically they seem correct and very detailed, based on a paper in Astrobiology last year, but I have a PhD in astronomy and understand the science but I can’t tell what they’re claiming in any specifics. The pictures would be great if they actually specified what sort of planet/star they were talking about.

Well, let’s consider the case of an M-star at any rate, and I’ll give you my take. Stellar classifications depend on the mass and age of a star. The temperature/mass sequence is OBAFGKM (“Oh be a fine girl, kiss me!” is the mnemonic). An M-star is a very low mass star, a few tenths solar, that is much cooler. How cool? Maybe a couple of thousand degrees Kelvin, about 1/3 of the surface temperature of the Sun. A star this cool puts out most of its light in the near-infrared part of the spectrum, and almost no blue or ultraviolet light whatsoever. It doesn’t look white like the Sun. It looks red. This makes a big difference in what things would look like on the surface of a planet in the water zone of an M-star, vegetation or not.

Fred Pohl wrote an interesting novel called JEM I remember reading in my college days. It takes place on a planet orbiting Barnard’s Star, an M-star and things are red, red, red!

The atmosphere will scatter shorter wavelength light preferentially, like on Earth, but a late enough type M-star won’t have much. Compared to the star (which will look big compared to the Sun, if you’re in the water zone where temperatures will be higher), the sky will be more orange/yellow.

The planets can be of all sorts, including like those here on Earth, which do a lot of absorption at red wavelengths. They wouldn’t, I don’t believe, look very green. I suggest they’d look black.

This is a very complicated multidisciplinary problem to simulate, as we not only need to know about the spectrum of the local sun, the atmosphere and what it absorbs, how bright the light is and how dilated the eye, but also to incorporate the response of the eye. The eye sensitivity is peaked at green wavelengths. This is why green lasers, which usually have the same power as red lasers, look so much brighter.

I submit that the color pallet is going to be red, orange, maybe yellow, and black. Something that looks blue under a white light is going to look black under the red light of an M-star. For an experiment, get some red/orange party lights (easier to find at Halloween) and then go look at different colored items under that light. Try different light levels, too, as the eye is less sensitive to color at low light levels.

Under an M-star, the spectrum is limited. It’s like living in a red light district, literally.

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