Colors of Alien Plants
Below are shown the predicted colors of photosythesizing organisms (such as the plants of Earth) under other suns and with different blankets of atmospheres.
The color of the photopigments was predicted by assuming that the light-absorbing photosystems would adapt to absorb the part of the light spectrum where the intensity was higher, but also where the rate of change of the intensity with changing wavelength is high (Arp et al., Science 368, 1490 (2020) argue that this helps the photosystems to more reliably deliver optimal power without damage).
I chose a hueristic function that produced an absorption spectrum similar to that of chlorophyll for the light of a spectral class G2 sun like Earth's sun under an open sky on a clear, sunny day.
The particular details of a photosystem's absorption spectrum may vary somewhat from what is predicted by this model – the peculiarities of evolutionary biochemistry might give a few extra wiggles and bumps in the absorption spectrum that can change the color.
In addition, we see that the plants of Earth often use pigments that give them colors not associated with photosythesis; photosynthesizing organisms of other worlds are likely to do the same.
Thus, it is entirely reasonable to adjust the color, brightness, or saturation from the values given here – consider the color of a blue spruce, the purple leaves of spiderworts, and the red leaves of a Japanese maple to see how much plants can vary from the basic green shown here.
For each sun spectral type and atmospheric thickness, we show a range of colors that can be optained by varying the concentration of photoabsorbing molecules.
| Spectral Class
| 1 atmosphere pressure
| 3 atmosphere pressure
| 10 atmosphere pressure
|
| A0 |  |  |  |
| A2 |  |  |  |
| A5 |  |  |  |
| F0 |  |  |  |
| F2 |  |  |  |
| F4 |  |  |  |
| F6 |  |  |  |
| F8 |  |  |  |
| G0 |  |  |  |
| G2 |  |  |  |
| G4 |  |  |  |
| G6 |  |  |  |
| G8 |  |  |  |
| K0 |  |  |  |
| K2 |  |  |  |
| K4 |  |  |  |
| K6 |  |  |  |
| K8 |  |  |  |
| M0 |  |  |  |
| M2 |  |  |  |
| M4 |  |  |  |
| M6 |  |  |  |
| M8 |  |  |  |
For habitable planets around low mass stars, like the cooler K-type stars and the M-class red dwarfs, the planet is likely to be tide-locked, so that the sun is always in the same place in the sky.
The above table shows colors assuming the photosynthesizers have adapted to the light of a sun high in the sky.
But for a tide locked world, photosynthesizers in the never-changing strip of twilight between the daylit and night-time halves of the planet may adapt to their particular lighting conditions.
The predictions for fully twilight-adapted colors are shown below.
As you go toward places where the sun is higher in the sky, the photosynthesizers are likely to grade from the twilight to daytime colors.
| Spectral Class
| 1 atmosphere pressure
| 3 atmosphere pressure
| 10 atmosphere pressure
|
| K0 |  |  |  |
| K2 |  |  |  |
| K4 |  |  |  |
| K6 |  |  |  |
| K8 |  |  |  |
| M0 |  |  |  |
| M2 |  |  |  |
| M4 |  |  |  |
| M6 |  |  |  |
| M8 |  |  |  |
The tan color seen on many leaves for cool, low mass stars is just my assumption for the color of the underlying leaf. The absorption spectrum in my model is shifted so far into the infrared that it does not affect the visible color.