Gliese 581 - The "Red Dwarf"

And implications for its "earthlike" planet Gliese 581c

It has been news in the first half of 2007 that a "somewhat earthlike" planet appears to exist in orbit around the star Gliese 581c, at a distance from that star that may favor temperatures suitable for liquid water and life.

Now, what would life be like there with a "red dwarf" as a sun?

Contents:

Photometrics
Color
Planet Surface Temperature
Size Appearance of the Star
Ultraviolet and Blue Light and Photosynthesis Issues - ozone, plant growth, related issues

Photometrics

There has been some talk that this star glows with ember-like dimness and color, and that this planet receives little light.

The "red dwarf" Gliese 581 is widely said to have light output about .002 times that of Earth's sun.

Keep in mind that Gliese 581's planet c is only 7% as far from that star as earth is from its sun. Using inverse square law, illumination at Gliese 581c would be about 40% as bright as that of sunlight on Earth.

Furthermore, this is based on "visual magnitude", which is determined with instruments having a spectral response more favorable to shorter wavelengths and less favorable to longer wavelengths than human photopic vision has. This is reasonable for approximating perceived brightness of stars in the nighttime sky, when human vision relies in significant part on scotopic vision. However, visual magnitude underestimates the photopic-defined photometrics of stars of color like that of Gliese 581.

Gliese 581's "photometric sunpower" is more than .002 times that of the sun - more like .0045, based on the absolute balometric magnitude of each star and the luminous efficacies of 5780K (sun) and 3350K (Gliese 581, see below) blackbody radiators.

As a result, illumination by Gliese 581 at a distance of .07 A.U. is about 90% of that by the sun at 1 A.U. (earth orbit radius).

Color

The best-studied red dwarf in terms of radiating properties and temperature is Barnard's Star. An impressive "determination" of its effective surface temperature is 3134 +/- 102 degrees K. See Here.

Based on interpolation between Barnard's Star and the sun from an assumed logarhythmic relation between effective surface temperature and B-V color index, Gliese 581 has an effective surface temperature of approx. 3350 K.

Keep in mind that the filaments of photographic and projection lamps are typically in the range of 3200-3400 K. The color here is not an ember-like reddish color, but somewhat whiter than that of most incandescent lamps.

Even after passing through Earth's atmosphere, the color of direct starlight from Gliese 581 would be incandescent-like. The earth's atmosphere typically shifts direct sunlight from 5780K to around 4800K. This is a "mired shift" of 35 micro reciprocal degrees. Although the principle of "mired shift" is only approximate, it predicts 3350K being shifted by the same atmosphere to 2996 K correlated color temperature. This is still slightly whiter than most incandescent lamps.

For an example of a fairly extreme red dwarf, there is Wolf 359. Most better determinations of its effective surface temperature are 2230 to 2600 K. Assuming 2300 K and a 35 mired decrease for typical effect of earth's atmosphere, this works out to 2127 K. This is still slightly whiter and less red than a candle flame.

Then again, Gliese 581 c may have a very different atmosphere, and the color of "direct sunlight" there may be significantly different.

Planet Surface Temperature

Gliese 581 has a widely-published bolometric luminosity (output in radiometric units) .013 times that of the sun. With its planet c being 7% as far from it as Earth is from its sun, inverse square law predicts intensity of UV, visible and IR radiation combined to be approx. 2.65 times as intense as that of the sun at Earth's orbit.

One published estimate for average surface temperature of Gliese 581 c is 40 degrees C. I suspect this is for without an atmosphere containing much water vapor, CO2 or other greenhouse gases.

Assuming ratio of outgoing radiation emissivity to incoming radiation absorption same as that of Earth, average surface temperature in Kelvin would be Earth's 288 K times the fourth root of 2.65, which is 367 K, which works out to 95 degrees C!

Size Appearance of the Star

Assuming bolometric luminosity .013 times that of the sun and an effective surface temperature of 3350 K, Gliese 581 would have a diameter about .34 times that of the sun.

On the planet Gliese 581 C which is only 7% as far from its star as the Earth is from its sun, Gliese 581 would appear nearly 5 times as large in the sky of its planet c as the sun appears in Earth's sky.

Ultraviolet and Blue Light and Photosynthesis Issues - ozone, plant growth, related issues

Once concern raised about suitability of Gliese 581 for life is ability of plants to grow under the light of a star that has much lower temperature than the sun has. Many plants on Earth have some significant requirement for blue light, and chlorophyll works well with both blue and red light.

As things turn out, given Gliese 581 providing 2.65 times as much radiation to its planet c as the Sun provides to Earth:

One set of assumptions:

"Average Daylight" on Earth has a color temperature of 5500 K and the Sun is 5780 K, and the "mired shift" of that is 8.8 "mireds" (micro reciprocal degrees). Apply the same 8.8 mired shift to 3350 K and so if Gliese 581 c has an Earth-like atmosphere, then "average daylight" there would have a color temperature of about 3250 K.

Red visible light - which chlorophyll utilizes well, and chlorophyll even has a major utilization peak in the "mid-red" region of the spectrum, for both the A and B types of chlorophyll. Ratio of mid-red to total radiation (I select 640 nm as a "representative example") is about 69% as great at 3250 K as at 5500 K.

Multiply this by Gliese 581 c getting 2.65 times as much radiation as Earth does, and mid-red irradiation is increased by 83%.

Blue Visible Light - which chlorophyll utilizes well and has a major utilization peak at, also needed for proper flowering and fruiting of some plants. Ratio of mid-blue to total radiation (I select 455 nm as a "representative example") for 3250 K is about 26% of that of 5500 K. Multiply that by the 2.65 factor of Gliese 581 c getting more radiation than Earth gets, and Gliese 581 c gets about 68% as much of this wavelength as Earth gets.

Violet Blue - for live coral! - I select 430 nm as a "representative wavelength". Ratio of radiation content at this wavelength to total radiation content is 23% as great at 3250 K as it is at 5500 K. Multiply this by the 2.65 ratio of total radiation intensity onto Gliese 581 c to that onto Earth, and the result is that any Earth-like live coral on Gliese 581 c gets about 61% as much violet-blue light there as such coral would get on Earth (assuming optically similar atmospheres).

Now, how about erythemal ultraviolet - which some animals have some need for: This I would represent with 310 nm, a longish UVB wavelength. Many reptiles have a need for this. Humans without intake of dietary Vitamin D or supplements there of have some need for this, although tend to do best with exposure to such UV content in sunlight to an extent less than is available in the tropics due to sunburn, skin aging, and skin cancer that erythemal ultraviolet (whether longer wavelength UVC, UVB, or shorter wavelength UVA) can cause.
Now for intenisty of 310 nm UV radiated towards Gliese 581 planet c, relative to such intensity radiated towards Earth:
That gets fairly low, roughly 4%. Multiply by the 2.65 ratio of total radiation intensity onto Gliese 581 c to that of earth, and this still means maybe 11% as much intensity of that wavelength on Gliese 581 c as on Earth.

I try again at 320 nm, borderline-barely UVA, longish wavelength erythemal ultraviolet: Ratio of intensity at that wavelength to total radiation intensity is about (probably a bit under) 5% as great at 3250 K as at 5500 K. Multiply by 2.65, and Gliese 581 c may get 13-14% as much erythemal UV as Earth gets, assuming atmosphere having same filtering effect at this wavelength.

Ozone-producing UV: Here, I see ratio of this wavelength range (below 200 nm) to total radiation (using 5780 and 3350 K as opposed to 5500 and 3250 K for surface-reaching radiation) to be about .06%. If wavelengths a little longer than 200 nm significantly contribute to stratospheric ozone formation, this increases - I suspect this ratio gets to more like .2%. Adjust for the 2.65 factor of more radiation at Gliese 581 c than at Earth, and this may get to the .5-.6% range.
I don't expect Gliese 581 c to have much of a startospheric ozone layer.

As for UVC being worse due to lack of an ozone layer: Back to 3250 K vs. 5500 K for analysis at what occurs at the surface, assuming optically similar atmospheres on both planets: Gliese 581 c would have only about 2.1% as much UVC at its surface as Earth would have with lack of a stratospheric ozone layer, despite total "solar radiation intensity" being 2.65 times as great on Gliese 581 c as on earth.

Written by Don Klipstein.

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