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A new equation for life

 What factors go into a planet's "habitability index"?

Astrobiologists are trying to work out a mathematical equation to quantify how suitable other planets are for life, similar to the famous Drake Equation for judging the chances of contacting extraterrestrial civilizations.

The exercise could help future generations figure out where to look for aliens - or where to settle down. But coming up with a new "habitability index" isn't just a matter of arithmetic.

"To be honest, it's really difficult to find a way forward here," said Axel Hagermann, a planetary scientist at The Open University in Britain who is raising the habitability issue at this week's European Planetary Science Congress in Potsdam, Germany.

Hagermann and a university colleague of his, Charles Cockell, are aiming to develop a single indicator that combines all the factors thought to make life as we know it possible. "What we're looking at is, 'If you've got this, and that, and the other, you've got life. Otherwise, you can't have life,'" Hagermann told me.

Based on their study of earthly examples, scientists generally list three factors: the presence of liquid water, chemical compounds that can be combined in organic reactions, and an energy source to fuel those reactions. But is it possible to quantify the factors behind habitability to such an extent that you can give Mars a habitability index of 0.5, the ice-covered moons of Jupiter and Saturn a 0.2, or the faraway planet called CoRoT-7b a 0.001?

Hagermann said the problem of measuring habitability is "getting more and more complicated, and more and more interesting."

Life on Earth ... and beyond?

The more researchers learn about life on Earth, the harder it is to draw a line between habitable and non-habitable zones. Organisms can be found in places that seem absolutely inimical to life - for example, the Antarctic sandstone outcroppings where microbes lurk or the deep-sea volcanic vents where weird creatures thrive.

Looking beyond Earth, Hagermann is finding that the questions become more complicated, even when he focuses exclusively on how the light from an alien star could help or hinder the development of life.

"For instance, while visible and infrared wavelengths are important for life and processes such as photosynthesis, ultraviolet and X-rays are harmful," he said in a news release. "If you can imagine a planet with a thin atmosphere that lets through some of this harmful radiation, there must be a certain depth in the soil where the 'bad' radiation has been absorbed but the 'good' radiation can penetrate."

Some astrobiologists hold out hope that may be the case on Mars, where a few inches of soil and a trickle of subsurface water might yet provide a haven for Red Planet life. But how do you quantify that?

"I feel like we're looking at a toolbox here," Hagermann told me. "We've got a problem: 'Put nail in wall.' Now we've got the toolbox, and we're trying to figure out which tool to use to solve that nail-in-wall problem ... but we don't know what the nail looks like."

One possibility would be to factor in the characteristics of an alien star's radiation, the distance from that star to a planet, measurements of the planet's atmospheric filtering ability, the composition of the surface, the chemical potential for transforming energy inputs into organic outputs, and ... well, you now see how complicated the calculations can get.

Hagermann hopes his presentation will generate more discussion - and eventually help astrobiologists nail down exactly what it is they're looking for when they look for alien life. "In a way, it's not about maths, it's about methods," he said. "A cry for help? That might be a way of putting it."

Sympathy from SETI

Seth Shostak, senior astronomer at the SETI Institute in California, sympathizes with the British researchers. "It's a good thing to try to do, and if nothing else, it confronts you with the difficulty of doing it. Which tells you something," he said.

He pointed out that "there's no definition of life that really works very well." Even if you were able to define life as we know it, you might be missing out on life as we don't know it.

Shostak and others involved in SETI (the search for extraterrestrial intelligence) are primarily interested in the complex kind of life that broadcasts its existence. In fact, some of those broadcasters may not be life forms at all, but spacefaring machines sent out by alien civilizations, Shostak said.

The way Shostak sees it, not-so-intelligent life should be much more common in the universe than intelligent life is.

"If you're willing to settle for microbes, then there are lots and lots of habitats," he noted. There could be as many as seven such habitats in our own solar system, not counting Earth. (The list includes Mars, Europa, Ganymede, Callisto, Enceladus, Titan and maybe Venus. In fact, water's disappearance from Venus was the subject of another presentation at the European science meeting.)

Veteran radio astronomer Frank Drake, the author of the Drake Equation and the director of the SETI Institute's Carl Sagan Center for the Study of Life in the Universe, agrees with the view that primitive life is probably widespread in the universe.

"Any planet that's like Earth is going to produce it," Drake told me. "There are so many pathways to the origin of life that it's going to happen. ... If you knew a system had planets with bodies of water on them, that would be a habitability index of 1."

Drake's equation takes initial assumptions about the prevalence of habitable planets in our galaxy, and multiplies that number by other factors to come up with a smaller number for the prevalence of intelligent civilizations. But when it comes to rating the potential habitability of specific alien planets, Drake thinks we have to learn more about those planets first.

"Once we learn more, we can start to do this seriously," he said. "Right now, our information is so incomplete that we can't do a good job of coming up with something like a habitability index."

What do you think? Check out our Drake Equation calculator, then see if you can develop your own formula for life in the universe. Leave a comment below to let the rest of us know what you come up with.

More on the habitability question:

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