How Many Habitable Exoplanets Exist
November 26, 2013

Just How Many Habitable Exoplanets Are There?

John P. Millis, PhD for - Your Universe Online

In the last decade, exoplanet scientists have been finding new worlds at a brisk pace. But as the number of planetary bodies in our galaxy rises, questions about how many of these may, in fact, be habitable have been reaching a crescendo.

The debate centers on the criteria for what it takes for a planet to be considered habitable. It seems the presence of liquid water on the surface is necessary, though some researchers are now suggesting subterranean flows could also be a breeding ground for primitive life. In either case, water is a critical component in the metabolic process of carbon-based life. (Why life should be expected to be carbon based is a question for another time.)

Additionally, the planet should have a solid surface, in contrast to Jupiter-like gas giant worlds. And finally, the planet should possess an atmosphere. Should that blanket of gas be found to contain substantial molecular oxygen, it is likely life has proliferated on the surface. (Other molecules quickly absorb oxygen; it would need to be replenished by a biological process in order for high levels of it to be sustained in a planetary atmosphere.)

With these criteria in mind, astronomers have created an equation for calculating the habitable zones around stars. Based on the radiant energy of the star – which is directly related to its mass – scientists can determine a range of distances around the star that will warm the planet enough to maintain liquid water, but not so much that the atmosphere boils off.

While this seems fairly straight forward, debate continues. The particular variables used to make the calculations are open for interpretation, with some arguing for more broad ranges of parameter values and others proposing narrower ranges in their calculations. Some recent work has even proposed certain star types – such as the smallest red dwarf stars – might be altogether poor incubators for planets, as the calculated habitable zone would be quite near to the star, and therefore interactions with the solar magnetic field would impede formation.

Of course, we could be wrong about all of it. According to James Kasting, Evan Pugh Professor of Geosciences at Penn State, "Maybe every planet out there that has the right conditions develops life. We don't really know the answer to that. But, it could be. If you're an optimist, you think it just takes the right conditions. It happened on Earth, why wouldn't it happen somewhere else?"

Ravi Kopparapu, a member of Kasting’s research team, has calculated that perhaps 40 to 50 percent of M-dwarf stars could contain habitable planets. While stars more similar to our Sun may have habitable planets about 10 percent of the time. Both of these estimates are more conservative than previous works.

Of course, the challenge is even if habitable worlds are found, determining the presence of life is more difficult. Unless the planetary atmosphere can be analyzed, conclusive evidence may prove elusive a long time into the future.

Image 2: (below): This chart shows star temperatures vs. stellar flux showing various zones including Earth. Credit: Chester Harmon