It has been news in the first half of 2007 that a "somewhat earthlike" planet appears to exist in orbit around the star Gliese 581, at a distance from that star that may favor temperatures suitable for liquid water and life.
UPDATE 1/2/2011: Since 2007, additional planets that may have temperatures suitable for liquid water and life have been found to orbit Gliese 581.
Photometrics
Color
Planet Surface Temperature
Size Appearance of the Star
Ultraviolet and Blue Light and Photosynthesis Issues - ozone, plant growth,
related issues
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 41% 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. This is based on the absolute balometric magnitude of each star (Gliese 581 has .013 times as much "bolometric luminosity" as the sun has), and the luminous efficacies of 5780K (sun, 92 lumens/watt) and 3350K (Gliese 581, see below, 32.2 lumens/watt) blackbody radiators.
As a result, illumination by Gliese 581 at a distance of .07 A.U. is photometrically to human vision about 90% of that by the sun at 1 A.U. (earth orbit radius). This is reduced slightly to 84.5% if the sun's luminous efficacy is the 98 lumens/watt mentioned by Wikipedia.
UPDATE 1/2/2011: Although I am basing what I say here on Gliese 581 having a surface temperature of 3350 K, Wikipedia says that star has a higher surface temperature of 3480 +/- 48 K.
Based on interpolation between Barnard's Star (spectral class M4) 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 3200-3500 K (updated 11/28/2019). 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 4500K. This is a "mired shift" of about 50 micro reciprocal degrees. Although the principle of "mired shift" is only approximate, it predicts 3350K being shifted by the same atmosphere to 2870 K correlated color temperature. This is slightly on the whitish side of most incandescent lamps.
UPDATE 1/2/2011: Even the most extremely small and cool red dwarf stars are now thought to have surface temperature of approx. 2600 K. One example is the star VB 10. (Spectral class of VB 10 is mentioned as being M8, while borderline between "red dwarf" and "brown dwarf" has also been mentioned as M6.5.) Even after light of 2600 K color temp. passes through Earth's atmosphere, on average its color temperature would be around 2300 K. Most 120V incandescent nightlights of 4 to 7.5 watts achieve around 2300 K.
Then again, Gliese 581 C may have a very different atmosphere, and the color of "direct sunlight" there may be significantly different.
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 "4th root law" applied to 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 94 degrees C! (Corrected 11/28/2019)
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 about 4.8 times as large in the sky of its planet C as the sun appears in Earth's sky. (Make that only 4.5 times as large if Gliese 581 has surface temperature of 3480 K.)
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 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).
Erythemal ultraviolet - which some animals have some need for. Erythemal UV is longer wavelength UVC, UVB, and shorter wavelength UVA. This I would represent with 310 nm, a longish UVB wavelength. Shorter UVB wavelengths and UVC wavelengths are largely blocked by Earth's ozone layer.
Many reptiles have a need for this. Humans without intake of dietary Vitamin D or supplements thereof have some need for this. However, humans 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 can cause.
Now for intenisty of 310 nm UV radiated towards Gliese 581 planet C, relative to
such intensity radiated towards Earth:
Ratio of Gliese 581's content of this to that of the Sun is fairly low, roughly
4%. Multiply by the 2.65 ratio of total radiation intensity onto Gliese 581 C to
that of earth, and this means 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: Gliese 581 C may get 13-14% as much erythemal UV as Earth gets, assuming atmosphere having same filtering effect at this wavelength that Earth's atmosphere has.
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% as great at 3350 K as at 5780
K. 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 mean Gliese 581 C gets about .5-.6% as much
ozone-forming UV in its upper atmosphere as Earth does.
I don't expect Gliese 581 C to have much of a stratospheric 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 without a stratospheric ozone layer, despite total "solar radiation intensity" being 2.65 times as great on Gliese 581 C as on earth.
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