The Light of Alien Life
The Voyager 1 spacecraft, after traveling about 4 billion miles into space, turned around and looked back home. From such a distance, the Earth appeared as a pale blue dot, a single point of light suspended in the vast blackness of space. If aliens from much more distant worlds were to look at our solar system, the Earth, if it could be seen at all, would seem even more tiny and faint. How could they know that dot of light represents a world teeming with life?
We face this problem when we search for life in other solar systems. As yet, we have no pictures of extrasolar planets; the evidence for their existence comes from the gravitational and spectral effects they exert on their host star. Over the next decade, however, space telescopes may begin to search for and provide images of Earth-sized planets orbiting distant stars. These telescopes include the European Space Agency’s COROT and Darwin missions, and NASA’s Space Interferometry Mission (SIM), Kepler, and Terrestrial Planet Finders. These missions may be able to tell us about the geology, chemistry, and atmosphere of terrestrial worlds in alien solar systems. Such information could help determine if planets are rich with life like the Earth, or dead, barren worlds where life never took hold.
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 one of this six-part edited series, she discusses the challenges facing those who plan to search for habitable worlds.
The Light of Alien Life
A lecture by Vikki Meadows
“Tonight I’m going to talk about the search for life beyond the solar system. Essentially we’re trying to answer that age-old question, “Are we alone?” This question has been asked for thousands of years, and probably was one of the first questions asked by early tribe people when, sitting around their campfires, they looked up at the stars and imagined that they were also campfires with tribes around them.
More recently, Giordano Bruno in 1584 wrote that “there are countless suns and countless Earths all rotating around their suns in exactly the same way as the seven planets in our solar system.” And he was correct. He wrote, “We see only the suns because they are the largest bodies and are luminous, but their planets remain invisible to us because they are smaller, and non-luminous.” Also correct. He wrote, “The countless worlds in the universe are no worse and no less inhabited than our Earth.” We don’t know if he’s right about that, but we’re trying to see whether we can prove him right once again.
By the way, he was called a heretic and burned at the stake later on, so he came to a bad end. I felt sorry for him initially but after reading about his history I learned that he liked provoking people, so he wasn’t entirely blameless.
To answer this question of whether we’re alone, or whether there is life beyond the solar system, we turn to a new science that’s been around only for about 20 years, the science of astrobiology.
Astrobiology is the scientific study of life in the universe — its past, its present, and its future. And that past – present – future motive translates into three major questions: How does life begin and develop? Does life exist elsewhere in the universe right now? And what is life’s future on Earth and beyond; what is the future of our society as our planet and our sun evolve?
To answer these enormous questions, you need more than one discipline. These are not the sort of questions that can be answered by physicists alone, or biologists alone, or chemists alone. You need a combination of almost all of the sciences in order to address them. People who are astrobiologists can come from almost any walk of science. They can be biologists, chemists, geologists, astronomers, planetary scientists, paleontologists, oceanographers, physicists and mathematicians, and they’re all required to work together to answer these questions.
Today we’re going to concentrate on one question: Does life exist elsewhere in the universe? So where will we start our search for life outside our solar system? First you have to find a habitable world, a world where life can be sustained on its surface. The technical definition for a habitable world is a world that can maintain liquid water on its surface.
I know some people may be jumping up and down in their seats and saying, “Yes, but aren’t we going to look for life on Mars and Europa?” Because there could be life in the subsurface of Mars, and there could be life in the oceans of Europa underneath a very thick ice cap.
But remember we’re going to look for life around distant stars, so we do have to give ourselves a break. Our definition that it must have liquid water on its surface is so that there will be lots of biomass, lots of life on its surface, which makes that life easier to detect. Trying to detect life beneath 100 kilometers of ice 10 parsecs away is extremely difficult. So when we’re searching for life outside our solar system, we tend to stick to this slightly limited definition of a habitable planet as one that has liquid water on its surface.
There are many challenges in searching for these habitable worlds. First of all, we believe habitable worlds are probably going to be terrestrial planets, that is, rocky planets roughly the size of the Earth. These planets don’t give off their own visible light. We must see them by the reflection of their star’s light upon them. They are also very far away, which makes them very faint. That makes finding them hard enough, but they are also lost in the glare of their parent star.
So when we’re looking for these terrestrial planets around other stars, we have to do two things. We have to suppress the light coming from the star so it isn’t blinding us to the faint little planet sitting next to it, and we also have to be able to separate the planet from the star so that we can see them as two distinct points of light, not just one glob.
But even when you manage to do that, and you need absolutely huge telescopes to separate the planet and the star, you’re still going to see the planet as just a point of light. When we find our planets around other stars in the next twenty years or so, we won’t have the technology to be able to spatially resolve that planet, that is, to see details on its surface. So if you imagine a little blue dot, that will be the average of everything that planet really is. A wonderfully complex world with continents and oceans and life and clouds – everything will be in that dot. We won’t be able to hone in on something that looks more interesting than anything else. Everything we learn about it will be disc-averaged.
So the signs of life that we would look for on this planet must be a global phenomenon. They must cover a large fraction of the surface of the planet or be visible in the atmosphere of that planet. And remember too that our ability to tell if that planet is habitable, that it can support life, or if it’s already inhabited, will only be as good as how far down into the atmosphere we can see. So that’s another challenge. We may not see all the way to the surface if a planet is completely covered in clouds.
Even if that sounds discouraging, we will be able to take the light from that dot and break it up into its constituent colors, into what we call a spectrum. The finer that we can break up that spectrum, the better off we are, because we’ll then be able to look for different amounts of particular colors in the spectrum, and these will be signatures of what the planet’s surface and atmosphere are like. There will be all these different processes contributing to that spectrum of the planet, like the chemistry of the atmosphere, the temperature and pressure structure, whether there are volcanic gases and aerosols, whether the atmosphere escapes over time, whether it’s been enriched by impacts, and then finally any impact of life on that planetary environment.
We can learn about the planet this way because as the light travels down through the atmosphere and then back to us, it interacts with the planet on the way down, it interacts with the atmosphere, it interacts with the surface. When it comes back to us, it tells us what material it passed through, what material it bounced off of.
The spectral lines look different according to what the light has interacted with. A reflection of light that you get off clouds, which are very highly reflecting, looks different from light reflecting off forests or deserts or oceans. It’s a tough task to disentangle all of these different contributions to the planet’s spectrum.
When we’re looking at spectra from different planets, we’re often seeing the effect of molecules in their atmospheres. Each type of molecule has its own signature. So carbon dioxide, for instance, has a characteristic sign at one position in the spectrum. Earth is interesting because its spectrum is so complex. You see water vapor in the atmosphere, and also abundant oxygen in our atmosphere, which we’re very grateful for.”