Very nice explanation of flora colors.

Very nice explanation of flora colors. I sort of knew most of this before, but had never put it all together. I liked this consequence the most:

“And since our eyes have evolved to see green at high resolution, not red, Keplerian fields would look very strange to us — almost uniform in color, with motion hard to see, because our eyes aren’t adapted to seeing fine shades of red.”

Originally shared by Yonatan Zunger

As we continue to search for planets beyond our Solar System, we are starting to find worlds that we might actually be able to stand upon. And in honor of this, the folks at JPL have produced some travel posters for these brave new worlds, available as free high-res downloads from their site.

Kepler-186f, the one pictured below, is my favorite because it captures some interesting physics. It orbits a red dwarf about 500 light-years from Earth, and it was the first planet discovered which is potentially suitable (in terms of things like temperature) for life as we know it. But life would be different in some interesting ways.

One of the reasons is that photosynthesis would be a bit different. Plants on Earth are green because their leaves contain chlorophyll, a chemical which absorbs sunlight and turns that energy into excited electrons. Those energetic electrons are then fed into the entire photosynthesis system, and ultimately that energy is stored in the form of sugars and used to sustain the plant’s life. The reason chlorophyll is green, though, comes down to three diagrams.

The first is the solar spectrum, that is, the color of light the Sun shines.

The X-axis of this graph shows the wavelength of light, from ultra-violet on the left to infra-red on the right; the Y-axis shows how bright the Sun is in each of those colors. As you can see, the Sun shines fairly evenly in the entire band between about 500 and 700nm, which is exactly the set of colors that the human eye can see. (No coincidence! Our eyes have evolved to see sunlight, not x-rays, because there aren’t that many x-rays around to see by) 

There are two graphs here: the yellow curve shows the color of sunlight itself, and the red curve shows the color of the light we see at sea level. The difference is that the atmosphere absorbs some colors of light but not others. For example, the fact that the red curve is way below the yellow curve at the far left is because ozone in the upper atmosphere is very good at absorbing UV light — the reason why it protects us from skin cancer.

The second diagram is the absorption spectrum of chlorophyll:

This graph uses the same X-axis, but the Y-axis shows how effective chlorophyll is at absorbing light of each color. There are two curves because there are actually two different kinds of chlorophyll: the green kind (chlorophyll-A) which is most prevalent, and the red kind (chlorophyll-B) which often stays behind after the green one has left, giving autumn trees their color. The bumps on the left actually aren’t very interesting, since the Sun doesn’t produce much light in those colors — they’re there because it’s hard to design a chemical which doesn’t absorb those colors. (For various technical quantum mechanics reasons) The sharp spikes on the right are what makes chlorophyll so important to photosynthesis, and for chlorophyll-A, that spike happens at a wavelength of 680nm, smack in the middle of where sunlight is the brightest. For comparison, sunlight is the brightest at 665nm.

As it turns out, the chlorophyll molecule is fairly flexible and complex, and small modifications to it would likely lead to “pseudo-chlorophyll” molecules with their peaks in different places, which we’ll come back to in a moment.

So chlorophyll has evolved (or rather, creatures have evolved to produce this one particular molecule) to very efficiently absorb light of exactly the color that the Sun produces the most of. Why does this make chlorophyll green?

Imagine that you shine sunlight on some chlorophyll. The chlorophyll absorbs the 680nm light; in fact, if you want to be precise about it, you can flip the chlorophyll graph upside-down (that is, replace it with 1-absorption, to instead show how much light it lets through) and multiply it by the sunlight curve, to see what color of sunlight bounces off of it. Light bouncing on chlorophyll would look just like the incident sunlight, but with another gap in it, corresponding to the colors that chlorophyll absorbs away for its own purposes. 

Because chlorophyll’s absorption peak is so sharp, you can basically imagine this as light with the 680nm part of it removed. What color is 680nm? It’s a bright red. And that brings us to the third diagram, namely how the human eye sees color:

Color vision works by having three kinds of “cone” receptor in the eye: one which sees red, one green, and one blue. (These are called L, M, and S in the diagram for obscure reasons) This diagram has the same X-axis again, and now the Y-axis shows how sensitive each cone type is to each color. So for example, if you shine pure 680nm light onto an eye, that stimulates the red cone some, and the blue and green cones not at all, which the eye reads as “red.” If you shone 580nm (yellow) light instead, that would stimulate both the red and green cones a lot, but the blue cone not at all, which our brain interprets as “oh, that must be yellow.”

(Incidentally, that’s also why color-combination tricks work. If you shine both red and green light on a point, it looks yellow to your eye. If you look at the monitor you’re reading right now with a magnifying glass, you’ll see that each pixel is actually three little pixels — one red, one green, and one blue — and that a yellow pixel has red and green lit but not blue, taking advantage of the same illusion to show you all the colors)

So back to plants. Sunlight on its own tends to stimulate your red and green cones a lot, but not much blue. (Take a look at the steep drop-off on the left of the sunlight diagram, and how that overlaps with what the blue cone sees) That’s why the Sun normally looks yellow. But sunlight bouncing off chlorophyll — i.e., what you see when you stare at a plant — is missing a bunch of its red light, so it only stimulates green. And that’s why plants look green.

(Incidentally, when you look at the eye-sensitivity chart, you might notice that the red and green cones are right next to each other, but the blue cone is off by its lonesome. This isn’t a coincidence: many species only have red and blue. The green cone only shows up in some species, and because it’s just like red but a little off, small differences in color in the range that they both hit therefore look very different to us. That gives us tremendously high frequency sensitivity in the greens: 490nm and 500nm light look really different, while 650 and 660nm are nearly indistinguishable. That’s really useful when you need to recognize different kinds of plant!)

So back to Kepler 186f: its star is a red dwarf, which is smaller, dimmer, and redder than our own sun. We could repeat the entire calculation above using its color of starlight, and what you discover in this case is that efficient Keplerian chlorophyll would be absorbing light off in the infra-red. Doing the same subtraction of reflected light, we find that Keplerian chlorophyll under Keplerian skies would look deep red to our eyes.

And since our eyes have evolved to see green at high resolution, not red, Keplerian fields would look very strange to us — almost uniform in color, with motion hard to see, because our eyes aren’t adapted to seeing fine shades of red.

You can actually do this calculation for any kind of star, and you’ll find that the color of “local chlorophyll” will range from red (for red dwarfs), through green (for stars like our own), out to yellow (for slightly blue stars). It never gets beyond that, because stars beyond “slightly blue” have a very short lifespan, and would never be around for long enough to develop their own native flora anyway.

So when you’re going out traveling among the stars, expect a fairly wild color show.

If you want to play with what different wavelengths of light look like, this site has a simple slider:

To read more about color vision, start at:

And for photosynthesis, start at:

Incidentally, the planet’s name simply means that it’s the sixth (f) planet out from the 186th star studied by the Kepler planetary survey.