The search for other life in the universe begins with the search for other solar systems. Our definition of habitable planets describes Earth-like planets. But in a way, this bias is natural—the only example we have of a planet with life is Earth, and the only life we know of is terrestrial.
Life as we know it is based on water, the universal solvent. Water is a unique substance, and makes up a large percentage of every living creature on Earth. Water is also present on Earth in all three forms—solid ice, liquid water, and gaseous water vapor.
If we expect that life elsewhere would also be based on liquid water, then we can define a habitable zone surrounding a star as the distance from a star where liquid water can be present on the surface of a planet. Large stars will have a large habitable zone, and small stars a small habitable zone.
Large Star, More Habitable?
If larger stars have larger habitable zones, are they the best places to look for life? Not necessarily. As scientists understand it, life takes quite a while to develop on a planet, especially intelligent life. But remember that the larger the star, the shorter its lifetime. Large stars will likely have too short a lifetime to allow stable conditions on their planets to persist long enough for intelligent life to come about.
This situation holds true even if they do have a large habitable zone with many planets in it, since life most likely develops independently on each planet (with perhaps some transfer back and forth via meteorites). If we assume that intelligent life takes about 3 billion years to develop on a planet, only stars with a minimum lifetime of 3 billion years are suitable. Such stars would end up having masses of 1.5 solar masses or less.
FIGURE 18-1:Habitable zones
Small Star, Less Habitable?
There's also a problem if the star is too small. Less massive stars last much longer than big ones, but their habitable zones get much closer to the star since they give out much less energy. For a 0.5–solar-mass star, for example, the habitable zone is so close to the star that any planet in range probably would end up tidally locked to the star, with one side always facing the star and one side always facing away. The side facing the star would be extremely hot, and the side facing away extremely cold—not conditions too conducive to life! The minimum size for a star is therefore at least 0.5 solar masses. We now have a mass range of between 0.5 and 1.5 solar masses. Conveniently for us, our Sun is right in the middle!
Stellar Requirements for the Habitable Zone
Recall that some heavy elements are created in large stars, and even heavier ones must be created in supernovas. In order to have the necessary heavy elements for life, such as carbon, nitrogen, and iron, the star and planets must form from a gas cloud that is already enriched in these elements from previous stellar generations. Astronomers can actually detect metals in the spectra of faraway stars—if a star has metals in its spectrum, then any planets around that star probably will too. Therefore, stars that are more likely to have life on their planets can be identified.
For life as we know it to form, stars must be between 0.5 and 1.5 solar masses, sufficiently enriched in metals and other heavier elements, and in either single star systems or certain binary star systems. In other words, astronomers are looking for stars that are quite similar to our own Sun!
Planets must also be in stable orbits around their star. This requirement eliminates most complicated star systems with three or more stars orbiting each other, but stable orbits are feasible around closely separated binary star systems. Actually, if the stars in a binary (or greater) star system are separated widely enough, then planets that are relatively close to either star can have stable orbits as well.
One example is the Alpha Centauri system, which happens to be the closest star system to our solar system. The distance at which the binary stars Alpha Centauri A and B orbit each other would allow planets close to either star to be in stable orbits. The Alpha Centauri system also contains a third star, Alpha Centauri C (or Proxima Centauri), but it is a small red dwarf star that is too small and cold to sustain habitable planets around it.