Researchers at Penn state have developed a new method for calculating the habitable zone around stars. The computer model based on new greenhouse gas databases provides a tool to better estimate which extrasolar planets with sufficient atmospheric pressure might be able to maintain liquid water on their surface. The new model indicates that some of the nearly 300 possible Earth-like planets previously identified might be too close to their stars to to be habitable.

So far, scientists have found some 18,000 extrasolar planet candidates with only a handful of these the right size, distance and having the proper orbital characteristics to be potentially habitable. “Habitable” is very broadly defined as being very approximately the right size and having a temperature where liquid water could exist on the surface of the planet. It’s a very generous definition, but it’s still one that leaves a very large margin of error.

Part of the reason is the variables the scientists use to calculate the habitable zone. One half of the equation is the star itself. Is it old? Is it young? Is it hot? Is it cool? Is it a variable? These determine how far the habitable zone is from the star and how wide it is. Then there is the planet itself, with characteristics such as size and temperature used to fine tune the estimates.

The Penn State model is based on previous work by James Kasting, Evan Pugh Professor of Geosciences also at Penn State. In the current study, the habitable zone is calculated based on stellar flux incident on a planet, that is, the amount of light falling on it, instead of its equilibrium temperature.

It is not, however, a way of coming up with a simple temperature reading. Instead, it’s a complex computer model based on assumptions about the atmosphere of the planet and how it absorbs and radiates heat under given conditions. Even though these calculations are so involved they need a supercomputer to carry them out, they are still very simplified compared to reality and operate on a number of assumptions. For example, this study assumes a one-dimensional, radiative-convective, cloud-free climate. The team themselves admit that some factors may have been under or overestimated and the results will reflect this.

The team used updated absorption databases of greenhouse gases, such as carbon dioxide and water vapor, that are more accurate than those used by Kasting 20 years ago. These were fed into supercomputers at Penn State and the University of Washington and from this the habitable zone was calculated for various classes of stars.

The habitable zone was calculated between the point where the planet would be so hot that water would be hopelessly lost (the inner limit) and the point where the greenhouse effect would be too weak to melt ice (the outer limit). The results of the Penn State study indicate that the habitable zones are farther away from their stars than previously thought. This means that some extoplanets previously thought to be potentially habitable might not be so.

One disturbing finding of the study was that the Solar System’s habitable zone lies between 0.99 AU (92 million mi, 148 million km) and 1.70 AU (158 million mi, 254 million km) from the Sun. Since the Earth orbits the Sun at an average distance of one AU, this puts us at the very edge of the habitable zone.

This may seem like a good argument for moving to Mars, which has an average distance from the Sun of 1.52 AU, but the team is careful to point out that their model doesn’t take into account feedback from clouds, which reflect radiation away from the Earth and stabilize the climate.

According to the team, the model can be used to investigate the over 2,000 potential systems found by the NASA Kepler mission. It could also help the Penn States Habitable Zone Planet Finder (a spectrograph designed to seek water-sustaining planets) as well as NASA’s proposed Terrestrial Planet Finder telescope network.

A paper describing the team’s results has been accepted for publication in the Astrophysical Journal, but a pre-publication copy is available for viewing (PDF).

Source: Penn State