Where are the solar systems like our own?

[timken]

I have a question that someone out there might be able to help me with. We have currently detected around 140 extra solar planets, but most of these systems are a bit weird with gas giants orbiting close to their stars.
It’s fairly obvious that we are finding these systems first because they are the only ones detectable by our current methods. I believe we are close to being able to detect more “sensible” systems like our own with gas giants orbiting in Jupiter like periods, but we are not quite there yet.
In the absence of any cataclysmic event such as the influence of a binary companion, the gas cloud left over after star formation must surely form planets. After all, there must be gas left over when a star forms, and it isn’t just going to sit there for billions of years doing nothing. So if we assume that all single stars have planets, can we assume that those that we have looked at to date which have shown no evidence of planetary perturbations might have planetary systems more like ours.

So my question is how many stars have been looked at to yield the current 140+ extra solar planets. It would be nice if 95% of the stars studied so far have shown no evidence of associated planets.

 

[Chronos]

Greetings timken, welcome to PF. One of the more popular methods being developed to detect more earthlike planets is photometry. Techniques such as astrometry and doppler shifting can only detect large masses. These techniques are, however, a good discriminator. Having already found at least one planet orbiting a star greatly enhances the chances of finding others. The photometry method has greater sensitivity [largely due to technological advances] and Earth sized planets are potentially detectable by this method. It checks for changes in brightness due to planets passing across the stellar disc.

Not sure if anyone has published a list of stars checked vs exoplanets found. Most searches have focused on sun-like stars [some pulsar candidates have been found by accident]. Estimates of the probabability that sun-like stars possesses planetary systems range from 25% - 100%. The fact we have found so many good candidates to date [the great majority are within 20 parsecs] strongly suggests planetary systems are common, at least around stars similar to the sun. I would be quite surprised if the actual figure was less than 50%.

Here is an informative link:
http://cfa-www.harvard.edu/planets/

 

[Phobos]

Welcome to PF, timken!

There are also several examples of young stars with the centers missing from their protoplanetary disks (the gas cloud you mentioned). Presumably, the center of those disks are missing in part due to material being swept up by planets.

 

[Nereid]

Here's another resource: http://www.obspm.fr/encycl/encycl.html .

If I'm not mistaken, several of the papers reporting (new) extrasolar systems do give (some) data on the fraction of stars they're watching have given results consistent with planets. Also, IIRC, there was some work done by HST on (eclipsing) planets, in a globular cluster (and M32?), They found nothing, which is interesting because it sets some (weak) limits on planets around old stars.

 

[timken]

I had a look around the net to see if I could find info on the proportion of stars investigated for planets which had revealed planets. I couldn't find what I was looking for, but I did uncover one reference from a team using the transit method who have detected some planets to date. Sorry I've lost the reference, but apparently in one survey they checked something like 25000 stars over a few weeks in one star field and didn't find any planets. This isn't as bleak as it might appear. If there were lots of Jupiter+ planets orbiting close to their stars I think that this team would have detected a proportion of them.

If my original postulation that most stars have planets is true, then a zero detection rate in the above survey would only occur if most gas giants orbited further out as in our system. In this case orbital periods of greater than several years would be typical, and you wouldn’t expect to see any transits if observing for only a few weeks.

Any way, that’s enough speculation. I’m sure we’ll have some definitive answers in the next few years.

 

[Nereid]

If my original postulation that most stars have planets is true, then a zero detection rate in the above survey would only occur if most gas giants orbited further out as in our system. In this case orbital periods of greater than several years would be typical, and you wouldn’t expect to see any transits if observing for only a few weeks.

Don't forget that you can only observe transits if the orbital plane of the system is within a narrow range around 0⁰, wrt the line of sight to the Earth! IF such planes were randomly distributed (in angle), then only a quite small fraction would ever be observed