Planets suitable for life

Recently astronomers have discovered planets in orbit around most stars close enough to determine whether they have planets. But of all the billions of planetary systems, which of them have planets capable of supporting life? At present we do not have the technology to look closely at those systems. Of our own solar system, as far as we can tell, earth is the only one capable of supporting life. There are two other planets that might support life. Mars may have supported life in the past, but so far no concrete evidence of this has discovered. Europa, a moon of Jupiter, has a liquid ocean. Perhaps life developed there (if the ocean is water).

So what makes earth unique in the solar system. First, it is not too hot nor too cold to sustain life. Most of the planets and moons further from the sun than Mars are too cold. Mercury and Venus are too hot. Although Venus is not so close to the sun that a temperature within a range to support life is ruled out for that reason. Venus is sometimes regarded as earth's sister planet. Venus is only slightly smaller than Earth (95% of Earth's diameter, 80% of Earth's mass). Because of these similarities, it was thought that below its dense clouds Venus might be very Earth like and might even have life. Unfortunately, we now know that Venus may be the least hospitable place for life in the solar system.

The pressure of Venus' atmosphere at the surface is about the same as the pressure at a depth of one km in Earth's oceans. It is composed mostly of carbon dioxide. There are several layers of clouds many kilometers thick composed of sulfuric acid. These clouds completely obscure our view of the surface. This dense atmosphere produces a runaway greenhouse effect that raises Venus' surface temperature hot enough to melt lead. Venus' surface is actually hotter than Mercury's despite being nearly twice as far from the Sun.

Another factor to support life as we know it is the presence of water. Apparently Mars once had liquid water on its surface, but it no longer does. Another problem with Mars is its thin atmosphere. One possible reason that it no longer has water and air is its small mass. Any smaller planet would likely be completely airless like our own moon.

Hence, it can be seen that although three planets are in a zone capable of supporting life as far as distance from our star (the sun), only one can support life. The other two are too dry. One has an atmosphere that is too thick, the other an atmosphere that is too thin. Thus, it can be seen that planets capable of supporting life are relatively rare. The question is how rare. If ten percent of planets in the galaxy are capable of supporting life, there may be nine billion planet capable of supporting life. If this number is one percent there would nine hundred million; one tenth of one percent , ninety million. That's still a lot of planets capable of supporting life.