The Solar System
In our search for life or intelligence on other planets, our familiarity with the Solar System inevitably guides our concepts of what other planetary systems around stars will be like. It was disturbing for us to find that early planets discovered were giant planets orbiting very close to their star. However, it was quickly pointed out that large planets that circled their star quickly (because in tight orbits) were the ones easiest to discover. As our techniques improved and the length of our observations of a star became longer, smaller planets far enough away from their star to allow liquid water to exist were detected. And we continue to find our own planetary system as a convenient yardstick to against which to measure other stellar systems.
Our Sun is a type G main sequence star. It has the characteristic yellow light of this type, which gives us the quality of our sunshine. It is expected to have a life of about 10 billion years. This is enough to allow about 4.5 billion years for the appearance of civilization after the initial creation of planets, and to give room for some future development. Other star types may be bluer, more massive, more energetic, and burn up much faster. Redder stars ( types K and M) are frequently dwarfs, shorter lived, and radiating much less energy than the sun. In these last types, heavy elements make up a much smaller fraction of their mass, making it difficult for them to form rocky planets like Earth. Often, the masses of stars are expressed as ratios of the solar mass, with the main sequence stars ranging from 30 times the solar mass to 0.3 times. The accompanying variability in temperature, stability, radiation, and lifetime all have to be taken into account when trying to decide whether life and civilization might evolve around another star than the Sun.
The location of known inner Solar System objects on July 20, 2002. Light blue lines indicate orbits of planets. Green dots indicate asteroids, officially known as minor planets. The red dots indicate asteroids that come within 1.3 Earth-Sun distances (AU ) of the Sun and so pose an increased (although small) collision risk with the Earth. Comets appear as dark blue squares, while dark blue points are Jupiter Trojan asteroids, which orbit just ahead of, or just behind, Jupiter. Most asteroids in the inner Solar System orbit between Mars and Jupiter in the main asteroid belt. Every day this plot shifts, with objects nearer the Sun typically shifting the most. (Credit: Minor Planet Center)
As far as planets are concerned, we find it convenient to express their characteristics relative to the Earth. In our system, rocky planets like Earth range in size from from 1.0 Earth mass to 0.055 Earth mass (Mercury). Planets as a whole range in density from about 5.5 (Earth) to 3.9 Mars, for the rocky planets, to 1.7 (Neptune) to 0.7 (Saturn) for the gas planets. We look first at rocky planets, or rocky satellies of gas giants, for signs of life.
If we find life evolved elsewhere in the solar system, we will want to know if it is related to terrestrial life; or did some other solar system body support an independent origin of life. Mars offers a complication in this respect because meteorite or asteroid impacts on Mars eject enough material for it to reach Earth as meteorites, or the reverse. Because of this there is the possibility that life first evolved on Mars and then seeded Earth via transfer meteorites. Alternatively, life may have evolved independently on the two planets. Even if life never developed elsewhere in our Solar System, we still wish to know whether there is a chemical record preserved from pre-biological times in ancient rocks on other worlds that might suggest how life began on Earth.
If the unique origination of life in the Solar System remains ambiguous due to the possibility of cross-contamination by meteorites, then the discovery of life on a planet around another star may be a means of resolving the question.