Planet Hunter
Discovering life outside of Earth has long been considered the Holy Grail of science. At the University of Texas at Arlington, Manfred Cuntz, associate professor of physics and co-director of astronomy, hunts for habitable planets. Recently three planets were discovered orbiting the star Gliese 581. The planet Gliese 581c was at first considered to be a possible place to find life, but Cuntz helped explain why Gliese 581d was the more likely candidate.

Cuntz took a few moments to discuss his work and explain how planets could harbor life forms.

What is being done today to locate habitable extrasolar planets?
What we understand as habitability is a planet's principal chance of hosting life, which is measured by a planet's ability to have fluid water on the surface. And that depends on many factors - for instance, the size of the planet, whether that planet has an atmosphere and how thick the atmosphere is. Also, it depends on the distance of the planet from the host star and on the type of the host star - like our sun, which has less luminosity [than others].

But there are many other questions. Which conditions are acceptable for life as we know it? This type of work is being done by biologists who are studying extremeophiles [organisms that thrive in extreme environments, such as in ice or near hydrothermal vents on the sea floor].

How are planets observed?
Most planets are "observed" by their parent stars and studying their gravitational pull on that star. This results in a unique identification of a planet, which is good enough to satisfy the criterion that it is observed.

What's the acceptable range of temperatures on habitable planets? Does it need to be similar to Earth?
People studying habitability generally resort to the concepts of sophisticated forms of life, which require fluid water, which would imply a temperature range of zero to 100 degrees Celsius under an atmospheric pressure of one Earth atmosphere. However, we actually know there are life forms possible, which can tolerate higher [and] lower temperatures, which is actually augmenting the concept of habitability.

What's the most likely form of life to be discovered?
In the early 1990s, the first extrasolar planet was identified. What change allowed this to occur?

It was a technology change - the ability to have ultrasensitive spectra of stellar photospheres [the visible surface of a star]. Planets still are discovered most of the time by the gravitational pull on parent stars. This is measured by the so-called Doppler effect of spectra lines in the spectra of stars. To identify such an effect, in an effort to see the orbit of a planet around a star, you must be able to measure photospheric lines of a star with extreme accuracy. We call it ultrahigh spectroscopy. This change in observational methods made observation of planets around stars possible.

A planet in the habitable zone is in a narrow band of space just the right distance from a star, which allows a higher probability of life occurring. Can you talk about your work with Gliese 581c and Gliese 581d?
The earliest work [among those studying habitability] was not considering the planetary atmosphere in an appropriate way. They were treating planets like the moon, just having surface and no atmosphere. If you do that, you get the wrong temperature range. If you are not considering the atmosphere, you can underestimate the temperature.

[Gliese 581d] is hotter with the atmosphere included. This makes the planet effectively move from the outside edge of the habitable zone into the habitable zone. The result of