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 two of this six-part edited series, she describes the clues that tell us if an extrasolar planet would be a good place to call home. A lecture by Vikki Meadows

"What makes a world habitable? There are many worlds out there. Why are some habitable and others not? If you look at planets in our solar system, there are several clues.

The first quality we look for is planet mass. We don't want a planet to be too big because then it just accretes a huge atmosphere and becomes a giant planet. We don't think giant planets are likely to harbor life because of the huge convection cells they have in their atmospheres. If you were a little molecule trying to form life, you'd be going up and down, and you'd be frozen up here and overheated and pulled apart down there. So, at least in our initial search for life, we think it'll be found on the terrestrial planets, the Earth-sized planets. It doesn't have to be exactly the same size as Earth; perhaps it can be up to 10 times the size of Earth.

A planet of that size can hold onto an atmosphere, and an atmosphere is really important for making sure your ocean doesn't boil off. It also means we can get plate tectonics, which is the circling of the crust of the planet. Plate tectonics helps us buffer, or control the concentration of, carbon dioxide in our atmosphere.

I'm sure you've all heard of global warming. If you don't have plate tectonics on your planet, over very long periods of time the carbon dioxide builds up, and that leads to global warming. So it's always nice to have enough mass to have plate tectonics. Mars, for example, doesn't have plate tectonics -- it's too small. You need to be about a third of the size of the Earth to have plate tectonics that function over a reasonable amount of time.

In our search for habitable planets, we also look at atmospheric composition, what the atmosphere is made of. We'll look at how well the atmosphere reflects light, how well it absorbs radiation and warms the surface of the planet.

You also want the planet to be in a circular orbit. When it goes around its parent star, if it's in a circular orbit it gets about the same amount of radiation all the time. But if it's in an elliptical orbit it gets hotter and cooler depending on when it comes close to or far away from its parent star. We think if the planet has an atmosphere we can tolerate a little bit of that, but you don't want the orbit to go too far from circular.

And then, finally, as in real estate, location, location, location. The location is important for knowing whether your planet is habitable or not. First, what kind of a parent star is it orbiting around? Is this a well-behaved parent star or is this a psychopathic parent star that's going to be a problem? Second, you need to know if the planet is close enough to the star to be warm enough to have liquid water, but not so close that the water will boil away. Or is it too far away so it doesn't get enough radiation, enough heat, and so it's too cold for water to remain liquid?

What does it take to be a suitable parent star? At least for the case of looking for life on other planets, we'd like the star to live a reasonably long amount of time. The bigger stars live fast and die young. They go off in a hundred million years, up to a billion years or so. So if you're looking for life to have climbed out of the primordial ocean and finally worked its way up to having a global effect on the planet, you really need more than a billion years. Stars that are significantly hotter than our sun die so fast that we don't think they're good places to look for planets with life.

We also want the star to be stable, that is, to not have a lot of flares and basically be a nuisance. The younger stars tend to be that way. So that's another reason to want your stars to be at least a billion years old or so.

It's also preferred that your star be bigger than half the mass of our sun. If it's any smaller, the planet has to get so close to the star in order to get enough radiation to be warm enough that it ends up being tidally locked, with the same side of the planet always facing the star. That can create problems for trying to maintain an equal surface temperature on the planet.

We've learned that stars that tend to have planets also tend to have what we call higher metallicity. To an astronomer, a metal is anything heavier than helium. Stars are made predominately of hydrogen and helium, but they have other elements like lithium, carbon, nitrogen, oxygen, and so on. These elements are called metals by astronomers even though you and I breathe them, for example. Because stars that have higher metallicity are more likely to have planets, we think high metallicity stars are good targets for finding potentially habitable planets.

So we tend to favor looking around what are called F, G, K, and M stars. Our sun is a G star. An F star is hotter than our sun, and the K and M stars are cooler. We start with stars like the sun and we go a little bit hotter and a little bit cooler. For a planet to be habitable, it's absolutely crucial for the planet to be in what we call the habitable zone. The habitable zone is the distance from the parent star that allows a planet to maintain liquid water on its surface, and that's determined by how big and how bright the star is. If you're a very bright star, the habitable zone is much further away from the star. If you're a faint cool star the habitable zone is much closer. In about 1993, Jim Kasting did calculations determining habitable zones based on the type of star. Luckily for us, the Earth falls right in the habitable zone for our star, although it's pretty close to the inner edge right now.

You also have to consider the continuously habitable zone for a star. That is, what region around the star stays habitable for a very long period of time? For our solar system, that continuously habitable zone has a tiny span. It's about 5 percent closer to the sun than we are right now, and about 15 percent further away from the sun than we are right now. Because our sun will get bigger and hotter with time, that habitable zone will move outwards. We're already at the edge of it, so it's gaining on us.

You may have read in basic astronomy textbooks that the sun is middle aged, it's lived for 5 billion years, it'll live for another 5 billion years, so don't worry about it. But the bad news is the habitable zone will run out faster than that. The sun may become about 10 percent brighter in the next billion years or so, and the climate modelers say, not even counting what's happening with carbon dioxide in our atmosphere right now, but just based on what the sun is doing, Earth may be uninhabitable in another 500 to 900 million years. So the end is coming much sooner than you thought."

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Read Part 1 of this lecture: "The Light of Alien Life."
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