Hi.
I'd like to know how many stars are believed to have a planetary system? I know that as of now there has been found like 350 exoplanets but how many do the scientists believe might prove to have planets?
Do stars tend to have planets or is it a more special type of stars? Is it fair to assume that when you look at a star at night, it has planets?

Thanks in advance.

Good question. Right now, the answer is indeterminate, but recent reports suggest that most Sun-like planets (as many as 60% of them) in the Milky Way may have planetary systems.

And we may not yet have a really good handle on our own Solar System (read this).

However, according to the currently accepted model for planetary and Solar System formation, any star that forms from a cloud of gas and dust that is more massive than the star which eventually forms, probably also has some range of planetary bodies in the same cloud.

Very massive stars would tend to blow away most of their pre-stellar nebula shortly after lighting up, and extremely massive stars or stars that are born as multiple-star systems would tend to eject planetary bodies.

This model might work in favor of the development of life, by ensuring that only the "just right" sized stars develop planetary systems. But, thus far, the vast majority of exoplanets discovered have been too big, too gaseous, too hot, and too near their host stars for life as we know it to exist. As our methods for detecting exoplanets improve, we're beginning to find smaller planets, so this picture is changing virtually as we speak.

 That's good news. Glad to hear it

Thanks

   The numbers are constantly changing based on several factors, one of which is that we don't really know how big the MIlky Way is!  A recent estimates puts the MW at 400 billion stars.  Extrapolated from this, the number of terrestrial planets was pegged at 100-120 BILLION by Marcy.   The total number of planets would obviously be much higher, perhaps in the 300-400 billion range, or more.

  Obviously, not every star can support a planetary system.  The scenarios that Jeff painted, 1st generation stars in metal/silicate poor regions, close binaries.  Then  again, we could assume, (via ye old Copernican Mediocrity Principle) that many systems are like ours with multiple planets--and this has been observed in several instances.

  The Kepler mission will be returning some significant data in the 2-4 year range.  It should be possible to get some numbers with a lower % or error at that time.  For now, it seems that we can only assume that planets are a normal part of our galaxy, which bodes well/better for terrestrials, habitable zones, and life itself.

  The next ten years will bring us gigantic terrestrial scopes and very sophisticated programs for observing stars/planets and even planetary atmospheres.  Along with Kepler, the Webb scope, and maybe the Terrestrial Planet Finder or some derivitive, I think that we are currently living in the most exciting decade since Galileo first raised his telescope...

   The momentous discoveries are out there, and we are SO close to answers.  Exciting times to be an amateur astronomer!

 I see. Interesting.
So, if we can safely assume that planetary systems are a usual thing in our galaxy, then the question is: how regular is our galaxy? Can we take this type of assumption to other galaxies?

We can apply the principle, but we can't be sure of the accuracy of our estimates.

Currently we are not certain of the evolutionary model of galaxies, but we do observe galaxy mergers in various stages and it seems that a general trend from spiral toward elliptical or irregular shapes is the norm.

There is a project called the Galaxy Zoo (now in its second generation) where you can download ultra-deep survey images and participate in categorizing the galaxies represented among them. One goal of this project is to get a better estimate not only of the total number of galaxies in the observable universe but also of the distribution of types. If we have a better idea of the distribution of types, then we'll also have a better idea of where the observable universe lies along its evolutionary path.

At present, about all we can say is that there seems to be an evolutionary curve for galaxies and that spirals are an early or midpoint form along that curve ... but we can't be sure what percentage of known galaxies are at that point. This rather limits our ability to say how "typical" a galaxy like the Milky Way might be.

All that is by way of background for the supposition that a great many--perhaps most--of the galaxies out there are too chaotic for stable planetary systems to be expected.

If the goal of the questioning is to determine the possibilities of life elsewhere in the universe, then we have to take into account the length of time that planetary systems are capable of supporting life (as we know it), and that would seem to be a small fraction of their overall lifetime--if our own system's evolution is any guide. It's a question complicated by the dynamics of evolving planetary and galactic systems, and the math is quite chaotic. The Drake Equation is an attempt to simplify the question, but note that several of its terms require rather large leaps of logic and "solutions" are at the moment more speculative than quantitative. For example, the equation generally ignores the impact (!) of catastrophism during a planetary system's "habitable" period ...