200 planets around 170 stars

Thus far, the main candidate for life elsewhere than in our own solar system has been Gliese 581 C, a world around the red giant star Gliese 581 about 20.4 light years distant. It has a diameter about 1.4 times that of Earth but since its orbit isn't transitory that is just an estimation. Its orbit lies in what is called the green zone, where the average surface temperature of a planet like Earth is not cold enough to freeze water nor hot enough to boil it. As a matter of fact, if it were Earth-like its mean temperature would be around 23°C but astronomers think that it is subject to a runaway greenhouse effect because of what we know thus far about its atmospheric makeup. However, we must consider the unique nature of its red sun so that the planetary composition and the intensity of solar radiation is still mostly speculative and that these factors should be considered only outside of our own relative context. Additionally, there could be a moon orbiting it which is large enough and which contains all the necessary ingredients for life much as we know it. It is a compelling and tempting notion. Once must consider the odds to be better than even in light of the Anthropocentric Universe theory.

Now another planet has been discovered orbiting well inside of the green zone of its parent sun. The star 55 Cancri is a sol type yellow sun 41 light-years away in the constellation Cancer. The planet is one of five orbiting the star and lies in a large gap between the third and fifth planets. The solar system seems stable, and all of its planets' orbits are fairly circular. 55 Cancri D lies about four-fifths of the distance between our sun and Earth from its own sun. Because it has a mass estimated to be equivalent to about 46 Earth masses the possibility of life on it is remote since it would be like a small Saturn. However, this does not preclude the presence of a moon or moons capable of harboring life.

Astronomers have been studying 55 Cancri for eighteen years in their search for planets. It takes a long time to compile all the Doppler shift light data of a star in order to make definitive determinations about the possible existence of extrasolar planets around another star. Planets cause this shift to change as their gravitational effects cause the star to wobble slightly as they orbit around a star. The discovery of the fifth planet around 55 Cancri makes this star the most populated of all the known planetary systems.

It is becoming more apparent that where a planet exists around another star there are usually more than one and there are probably more likely to be quite a complement of them, if our own solar system is any indication. To date, well over 200 planets are known to exist around 170 stars other than our own. It seems that planets are more common than the stars they orbit and a sufficiently mature sun is probably guaranteed to have them.