Artist's concept of what plants might look like on different planets. Image credit: Caltech/Doug Cummings
In May 2007, Victoria Meadows, Principal Investigator for the Virtual Planetary Laboratory at the California Institute of Technology's Spitzer Science Center, presented a lecture at NASA's Jet Propulsion Laboratory. In part five of this six-part edited series, she explores the impact plants have on planets, and how the type of star providing sunshine may affect the color of the alien equivalent of bushes, trees, and blades of grass.
"We're studying the co-evolution of photosynthesis with the atmosphere on extrasolar worlds. Our studies determined we had a good chance of predicting the range of colors of plants on other worlds that orbit different types of stars. Studying the co-evolution of photosynthesis with its atmosphere means we were trying to determine whether plants are smart enough to work out where the best places are to get radiation for photosynthesis.
If we can figure out that they are smart enough, and if we can figure out what rules they use to choose the pigments for photosynthesis, we wondered if we could apply those rules to other planets around other stars. Could we use them to predict where the photosynthetic pigments are going to be, and what color these plants are going to be?
A colleague of mine, Nancy Kiang, and a bunch of her biologist friends went through all of the literature and found every pigment life uses for photosynthesis on this planet. And by pigment, I mean a particular type of molecule that's specifically tuned to take in a particular type of light and turn that into food for the plant.
She discovered, as others have done, that plants seem to be spectacularly inefficient. They throw away a large fraction of that lovely green light - we get more green light coming in from the sun than any other color that's available to us.
In fact, if you look at the energy spectrum of light that hits the surface of the planet, there's also more green light in the energy spectrum. So botanists and others had wondered for quite some time, why are plants green? Because, by being green, it means they're reflecting the green light and not using it. But if there's more green light than anything else, why would they not use it and be as efficient as they possibly could be? Why do plants absorb red and blue light but not green?
She used our models to plot out the radiation that's coming down on the surface of our planet. When we model those spectra that show you what a planet would look like through a telescope, at the same time we model what the spectrum of the star would look like to a microbe sitting on the surface. So she used the spectrum of the star for the microbe sitting on the surface. But she plotted it in photons rather than energy, so she plotted it in particles of light. Photosynthesis is photon process - it uses particles of light.
It turns out that chlorophyll A absorbs light right at the peak of the most number of particles that come down to Earth, which is near the red part of the spectrum. So plants are using the light in the form that has the most number of particles. Now the question is, why does the peak move to the red once the light gets down to the surface of our planet? The reason is because of ozone absorption. Plants put out oxygen, which produces ozone, but in doing so they actually shift the usable region of their light more towards the red.
And so plants over time have changed the composition of our atmosphere, but in the process had to evolve to keep up with what they were doing to the planet. So back in the beginning of our history when we didn't have as much oxygen on the planet, plant pigments would have been much more towards the blue. And then over time they would have moved towards the red as they produced ozone.
She then used that to look at planets around other stars. For example, if you're orbiting an F star, the peak of the F radiation is towards the blue, and the ozone just makes it more towards the blue. So if you were a plant on a planet going around an F star, which is a star hotter than our sun, then your pigments are more likely to be in the blue, and so you as a plant are more likely to reflect orange or red radiation.
Another thing she did was look at safe ocean depths. M stars tend to flare quite a bit, so in the very early stages of planetary development -- when you haven't got enough photosynthesis to build up oxygen so you don't have your ozone shield yet -- you are susceptible to large bursts of UV radiation up to 30,000 times what we're used to in any given day. So if you're a life form you don't want to be out on the surface. What can you do? Well, you can go under the surface, or you can go down in the water.
So she asked the question, if you're a microbe who likes to do photosynthesis, can you go down into the ocean to escape the flares but still have enough energy to do photosynthesis?
She and her team found that yes, indeed you can. And even for the worst possible M star flare we could imagine, you could go underwater 9 meters, or about 10 yards, and still be able to escape the flares but have enough energy to do photosynthesis to create your food. That meant even for early planets around M stars, there are regions where it could be possible to have life down in the oceans.
Everything I've described so far is essentially background theory to support one particular mission called the Terrestrial Planet Finders. There's also a sister mission called Darwin, which is also under concept development by the European Space Agency but is potentially a joint US and European effort. We hope that these telescopes will be launched sometime in the next 20 years or so, but for now they've been mothballed. We hope to at least get the concept up and going as soon as possible, so that within many of our lifetimes we will be able to launch telescopes that will be able to take the spectra of Earth-like planets around other stars.
I find that enormously exciting for a number of reasons. First of all, we can look for other habitable worlds and we can see if there are signs of life there. If we find signs of life, that will be an absolutely momentous discovery. But also from a planetary point of view, when we look out at stars in our solar neighborhood, those stars are all very different ages. They range in age from just born to about 10 billion years old. The planetary systems around them are about the same age as the star. So if I'm looking at a 2 billion-year-old star, I'm also looking at a 2 billion-year-old planetary system.
By looking at these stars in our neighborhood, we can essentially look back in time to see how terrestrial planets evolve over time. I think it would be great to be able to get the spectrum of a 2 billion-year-old planet instead of our own 4.6 billion-year-old planet, to be able to look back in time and see what was happening.
So we're looking forward to these missions being able to fly. They are enormously technologically challenging, but well worth it. I invite you to visit our VPL team web site, or the Planet Quest web site to learn about these missions that are eventually going to do this kind of science."