Answer 1:
there are about 100 billion ( 100,000,000,000.) stars in the MIlky Way galaxy which is only one of about 3000,000,000,000. galaxies in the known UNIVERSE !!!!
It is presently believed that solar systems form around many stars. It is only within the last few years that we have detected EXTRA SOLAR planets...that is, planets around other stars.
but the models we have for star formation seem to indicate that many stars will develop a family of planets.
even if only 0.1% of stars in the MILKY WAY have planets, that means that there are still ten million solar systems in the MILKY WAY.
so the best estimates we have for the number of solar systems is : at least 1 million and maybe 100 billion. this is a big range . it represents our uncertainty regarding this issue. probably 100 years from now we will have a better answer to this question. this is how Science works. 100 years ago we didnt even know that most stars in the UNIVERSE are located in specific structures (clumps) called Galaxies. and 100 years before that we had no idea of what made stars burn .
Science is a process of discovery; we slowly lift the shroud of fog and finally see clearly the way the universe has been unfolding.
Answer 2:
Our galaxy has approximately 2x10^11 stars, of which many are binary (two or more stars in orbit around each other). Binary stars can have planets so long as the stars are very close compared to the planets orbital radius, or very far, so let's make a guess that only 1x10^11 stars are able to have planets around them.
So far, astronomers have detected 19 planets around 17 other stars, which is about 4% of the stars that have been studied. However, only planets that are the size of Jupiter or larger can be detected, so there may be others. Four percent of 1x10^11 is 4x10^9 or four billion.
Many of the planets that have been found are very massive and have highly eccentric orbits, which implies that there are no other planets around the star. These planets would likely be inhospitable to life.
Answer 3:
To determine this, it is necessary to know what fraction of the stars in a galaxy have any sort of a solar system. Multiplying this fraction by the total number of stars would give a rough estimate of the number of solar systems. The difficulty with this is that even our most powerful telescopes (like the Hubble space telescope) are only barely able to see indications of planets around other nearby stars and those planets need to be very large ones like Jupiter or Saturn. Large planets such as these suggest that there are smaller planets like the Earth closer to the star but we have no way of knowing if this is the case with our current technology. If 10% of the stars in the galaxy have some sort of solar system and there are 300 billion solar masses worth of stars in the Milky Way (assume that means 300 billion stars), how many solar systems would there be? Of course, if you're interested in solar systems that might be suitable for life, this number drops significantly.
Answer 4:
There are actually HUNDREDS of billions of stars in our galaxy, and it seems likely that the vast majority might have planets orbiting around them, given the frequency with which we are currently detecting planets around other stars. So there could be literally hundreds of billions of solar systems. How many of these planets would provided suitable conditions for life to evolve is, however, a completely open question.
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