The Spectrum of 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 three of this six-part edited series, she pieces together the lines of evidence that will indicate whether a planet is able to support life.
The Spectrum of Life
A lecture by Vikki Meadows
We also look at the mass and orbital parameters of the planet itself. Is it the right size; is it a terrestrial mass? Is it in the habitable zone? Then we look at what we call its photometric characteristics. “Photometric” just means to measure light. So we look at the light coming from that planet, we determine its brightness compared with its distance, and try to work out how much light it’s reflecting to tell us whether it’s covered with clouds or not. We look at its color and whether it changes with time. If the brightness or the color changes with time, that could indicate clouds going around, or vegetation growing over one side of the hemisphere, anything that would give us further evidence that this planet is terrestrial, that it has clouds, that it has a surface that perhaps changes.
And then we pull out the big guns -- the spectra. If the starlight reflecting off the planet is bright enough, we’ll be able to break that light up into its constituent colors to look for molecules like carbon dioxide and water vapor. The carbon dioxide tells us that we probably have a terrestrial planet. Jovian planets don’t tend to build up large amounts of carbon dioxide because it reacts with of all the hydrogen in them. So if you see CO2 on a planet, chances are it is terrestrial. Water vapor would tell us it’s got liquid water on its surface. We can also see ozone in the spectra, and that would tell us whether or not a planet has a UV shield, which is very important for a habitable planet. Does it have something in its upper atmosphere like an ozone layer that stops damaging UV from getting to the surface?
We take the temperature of planets in our own solar system all the time by looking at the infrared radiation coming from those planets. But you still might get the temperature wrong, because the only temperature you can take is the temperature the planet is willing to let you take, which is the temperature of the emitting layer. So we’ll look for clouds and other greenhouse gases.
A little greenhouse warming is a wonderful thing. The Earth has about 32 degrees Celsius of greenhouse warming, which takes us from frozen to very nice, thank you very much. Venus, on the other hand, as I said, if you look at its effective temperature, it’s very cold, but its surface temperature is mind-bogglingly hot. It undergoes about 513 degrees Celsius of greenhouse warming. Mars, on the other hand, tries really hard, but it can only manage 5 degrees Celsius of greenhouse warming. So it just goes from really really frozen to just really frozen. It tries, but having a very thin atmosphere doesn’t help. That’s another thing about habitability, your atmosphere has to be thick enough to be able to get your surface temperature up to something decent.
The point is that a planet’s greenhouse effect is at least as important in determining the planet’s surface temperature as its distance from the star. So you determine the distance first, but you also better take a census of the gases in the atmosphere and how dense that atmosphere is to work out what the surface temperature is.
We have so many planets out there. There are well over 200 extrasolar planets now known. But there’s one problem with most of these planets that we have found so far. They’re too big! The planets we have found are mostly gas giant planets. Small rocky Earth-like planets, the terrestrial planets that we want to find around good parent stars, are still very difficult to find.
But now, in the last couple of years, thanks in part to something called the microlensing technique, we’ve found a handful of planets that are less than 10 Earth masses. These are our best candidates for being habitable rocky worlds. Some of you may have heard of the recent discovery of Gliese 581c. That planet is only about 5 Earth masses; it’s one of the smallest planets we’ve found so far. At the time of the press release they were saying that it was in the habitable zone of its parent star, and that was hugely exciting. This was the first potentially habitable planet that we had found, and in my own lifetime.
So how can we tell if a planet is inhabited, especially from a huge distance? If it’s got aliens on it, it’s easy because they can say hello to the universe and tell us that they are there. But you have to remember that our planet has been able to say hello to the universe only for the last 100 years or so. For the previous 4.6 billion years, we didn’t say a whole lot to the universe. Life on Earth for most of its history was comprised of things like stromatolites, which are microbial mat communities. Statistically, when we look for life on other planets, those may be what we’re more likely to see. So when you’re looking for life on most planets out there, you’re going to have to be cleverer than just simply listening.
Our best chance then is to look for global changes in the atmosphere and surface of the planet thanks to life. Life leaves its footprint on this planet in a big way, and we’re looking for similar astronomical biosignatures. Not the in situ, “I have the rock in my hand” biosignatures, but biosignatures that can only be seen by looking out at a distant planet using an Earth-based telescope, or a space-based telescope in our solar system.
On the Earth, methane is a biosignature. It’s produced in rice paddies, it’s produced by cows, it has a seasonal cycle, and it’s seen in the presence of huge amounts of oxygen. Chemically, that shouldn’t happen. Oxygen and methane like to destroy each other by various circuitous pathways. So the fact that they’re both here means that they are both being produced at the surface of the planet, and the most likely source of both of them is life.
There’s methane in Titan’s atmosphere, but it’s just part of its atmosphere. And you can tell that by looking at the other constituents in Titan’s atmosphere. So in that case, Titan’s methane is just a part of its atmosphere, but on the Earth, it’s actually a sign of life -- oxygen and methane together are a sign of life. Ozone is produced from oxygen, and if you see a lot of ozone chances are you have a lot of oxygen, and so that’s also a proxy for life.
Changes in gas concentration as a function of time would also tell us if a planet is inhabited. Methane on Earth has a seasonal cycling tied to vegetation growth cycles on our planet. So for a planet around another star, we could watch it for awhile to see if similar cycling occurred.
That leap in reflectivity can be seen from space. In fact, Landsat uses it to determine vegetation coverage on the planet, to map how much forest we have in Brazil, for example. They do that by measuring light in two different bands, looking for this thing called the red edge, because that’s where it occurs in the spectra.”