As I mentioned, first we'll look at its neighborhood, what its parent star is like. We also will look to see if there other planets in the system that could cause a problem. For example, a large Jupiter-sized planet careening through the solar system on a highly elliptical orbit. Those occur more often then nice well-behaved circular Jovians, and having one of those would be very bad if you were trying to be a habitable planet in the inner solar system.

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. If your planet is completely covered with clouds, like Venus for example, you'll get a very cold temperature because you're reading a region above the clouds. But in actual fact Venus is extremely hot on the surface. So even though you can take the temperature of the planet, you may not get the surface temperature. But if you can do a census of the greenhouse gases in the atmosphere and get some idea of the composition and mass of that atmosphere, you can use our next tool in this battle, which are the models.

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. Unfortunately, my team recalculated the habitable zone and discovered that this planet is probably a little too close to its star. So it's not a Super Earth, it's a super Venus. But its brother, another planet in that system named Gliese 581d which is about 7 to 8 Earth masses, potentially is in the habitable zone, on the outer edge of it. And given that it's a very large planet, it may have a very dense atmosphere, one that could in fact be habitable. So what you lose on the swings you gain on the roundabouts.

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.

These biosignatures are global scale photometric, spectral or temporal features - that's counting the light, breaking it up into its constituent colors, or time-varying features indicative of life. And if you look at the Earth, there are three major types of biosignatures that I can think of. There are things in the planet's atmosphere, there are things on the planet's surface, and there are ways that the planet's appearance, both its surface and its atmosphere, change over time that tell us life is there. But we also have to remember before we go rushing off and saying "oh, we'll look for oxygen," for example, is that biosignatures must always be identified in the context of the planetary environment you find them in. Otherwise you'll get fooled. And the classic example is methane.

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.

Vegetation produces another signature of planetary life. Plants absorb light waves as they attempt to get energy for photosynthesis, but after about 0.7 microns, which is just at the edge of what your eye can see, plants start to become wildly reflective. They send more radiation back to the observer, and that means if you could see in the near-infrared plants would be very bright. They would look like they're foil-covered. Some of you may remember a car commercial that was filmed with an infrared camera, and it looked like the car was driving through a silver forest. That was an image of what plants look like in the infrared.

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."

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Read Part 1 of this lecture: "The Light of Alien Life."
Part 2: "Location, Location, Location."
Life Beyond Earth
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