There are currently several projects under way with the sole objective of identifying other planets outside our solar system. Since the discovery of the first extrasolar planet in 1995, teams of scientists across the world have worked to identify well-over 100 planets… and counting! However, planets detected so far are Jupiter-sized and are therefore not likely to be similar to Earth. In addition, many of these planets are orbiting extremely close to their central stars. At distances this close, temperatures would be too great for life as we know it to exist. This leads us to an important question, as we discover more and more planets outside of our solar system, how will we know if they are habitable places for life?
There are a slew of factors that dictate the habitability of a planet. Of course, we define habitability with respect to our own experiences with life. Consequently, we have a biased view of habitable planets as worlds that would permit the survival of carbon-based life forms dependent on the presence of liquid water. Nevertheless, we can guide our search for life in the universe by developing a concise definition for the limits of habitable planets.
Around any given star, there is a region that is optimal for the development of life. This region, termed the habitable zone, depends on the type of star a planet orbits. For example, our star is a G type star. The inner boundary of the habitable zone around the Sun has been estimated between 0.84 AU and 0.95 AU (remember that we orbit the Sun at a distance of 1.0 AU).
For high-mass stars on the main-sequence, the habitable zone around that star will be further
I really like this image as a concept for habitable zones. It shows how our solar system resides in a “good” place in the galaxy, and that Earth is in our Sun’s habitable zone.
Image taken from an article in Scientific American, and I do have the PDF so don’t go looking for it if you decide you need it! J
Gonzalez, G., Brownlee, D., & Ward, P.D. (2001). Refuges for life in a hostile universe. Scientific American, October issue, 60-67.