30.3 Searching for life beyond earth  (Page 5/10)

Habitable planets orbiting other stars

One of the most exciting developments in astronomy during the last two decades is the ability to detect exoplanets—planets orbiting other stars. As we saw in the chapter on the formation of stars and planets, since the discovery of the first exoplanet in 1995, there have been thousands of confirmed detections, and many more candidates that are not yet confirmed. These include several dozen possibly habitable exoplanets. Such numbers finally allow us to make some predictions about exoplanets and their life-hosting potential. The majority of stars with mass similar to the Sun appear to host at least one planet, with multi-planet systems like our own not unusual. How many of these planets might be habitable, and how could we search for life there?

In evaluating the prospect for life in distant planetary systems, astrobiologists have developed the idea of a habitable zone    —a region around a star where suitable conditions might exist for life. This concept focuses on life’s requirement for liquid water, and the habitable zone is generally thought of as the range of distances from the central star in which water could be present in liquid form at a planet’s surface. In our own solar system, for example, Venus has surface temperatures far above the boiling point of water and Mars has surface temperatures that are almost always below the freezing point of water. Earth, which orbits between the two, has a surface temperature that is “just right” to keep much of our surface water in liquid form.

Whether surface temperatures are suitable for maintaining liquid water depends on a planet’s “radiation budget” —how much starlight energy it absorbs and retains—and whether or how processes like winds and ocean circulation distribute that energy around the planet. How much stellar energy a planet receives, in turn, depends on how much and what sort of light the star emits and how far the planet is from that star, The amount of starlight received per unit area of a planet’s surface (per square meter, for example) decreases with the square of the distance from the star. Thus, when the orbital distance doubles, the illumination decreases by 4 times (2 ² ), and when the orbital distance increases tenfold, the illumination decreases by 100 times (10 ² ). Venus and Mars orbit the sun at about 72% and 152% of Earth’s orbital distance, respectively, so Venus receives about 1/(0.72) ² = 1.92 (about twice) and Mars about 1/(1.52) ² = 0.43 (about half) as much light per square meter of planet surface as Earth does. how much it reflects back to space, and how effectively the planet’s atmosphere can retain heat through the greenhouse effect (see Earth as a Planet ). All of these can vary substantially, and all matter a lot. For example, Venus receives about twice as much starlight per square meter as Earth but, because of its dense cloud cover, also reflects about twice as much of that light back to space as Earth does. Mars receives only about half as much starlight as Earth, but also reflects only about half as much. Thus, despite their differing orbital distances, the three planets actually absorb comparable amounts of sunlight energy. Why, then, are they so dramatically different?