During 2006 two important microlensing exoplanet discoveries were announced. One of the papers and the press release based on it indicate that low mass planets are very common in the Milky Way. Here is a quote from the press release URL: (http://www.astrobio.net/news/modules...er=0&thold=0):

And second," he added, "these icy super-Earths are pretty common. Roughly 35 percent of all stars have them."

If this is the case, then why aren't more of these "super-Earths" turning up? How can planets that are supposedly very common have discovery rate of 1-2 per year? I have a feeling it has something to do with the discovery technique, but even so. I find it paradoxical that the microlensing technique seems to be telling us that a higher fraction of stars have planets than the radial velocity technique and yet, within the space of 2006, more planets were found via the radial velocity technique.

Sorry, where's the paradox? Isn't one technique with better resolution being applied much more rarely than the other?

So, why shouldn't the first report a higher success sample rate for smaller rocky bodies, yet only be able to report fewer in quantity?

Or is it that you think both trechniques actually are as often applied? Or that the techniques have the same resolution ability?

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1 - 2 per year doesn't seem that unusual. Doesn't the planet have to be passing between the earth and it's star in order to be detected this way? Any number of different orbit angles could mean that this never happens for a great many planets. Also - depending on their distance from their star and orbital speed - orbits longer than a year would give us only one opportunity a year to catch these planets in the act, and we'd have to be looking at that particular star when the event took place.

Or am I completely unaware of what gravitational microlensing is?

More planets are found with radial velocity, but they aren't "super-earths". They are gas giants with rapid orbits close to their star. Faster orbit - more opportunities for detection.

Have any "super-earths" been discovered using radial velocity?

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Some words, anyway:

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Your skepticism is well founded. The full paper can be found at

http://xxx.lanl.gov/abs/astro-ph/0603276

In it, you can see that the rate calculations are based on measurements of (get this) 4 systems. Yup, four. The authors attempt to place limits on the fraction of systems which should contain these large icy planets -- they quote a fraction f = 0.37 +30 -21, which means that they have a range of f = 0.16 to f = 0.67.

However, I wouldn't make an estimate of this sort based on so few systems. It would be like walking down the street and noticing that 2 of the first 4 people you meet are wearing red sweatshirts. "Wow," you might claim, "that means that 50 percent of all people wear red sweatshirts."

(I'm being slightly unfair to the authors here, but only slightly).

So, the estimate is based on only a small number of examples, but isn't that reflected in the large error bars? It would be strange that we just happened to stumble upon these planets if they are any rarer than what the numbers suggest.

I guess part of what I am wondering is when the pace of microlensing discoveries pick up?

You are asking far to much of this discovery of exo planets.
The distances we are dealing with are very large. It is enough that a second star just happens across your field of view near to your line of sight star of interest. Micro lensing is not such a precision science, but a accidental accoltation of a distant star. The method has been prov-en to be sound and reliable information can be attained. Just not as often as we might like.
The previously prov-en methods of direct observation and the detection of brightness variations due to the passage of a planet across the visible disk is also just a little hap hazard. Direct observation of exosolar planets is still on our wish list of thing yet to do. Bigger and more technically correct telescopes will eventually allow this advancement.

A paper just came out yesterday on the astrophysics preprint archive which discusses the past, present, and future of planetary microlensing searches. Here is the title and author: Microlensing Searches for Planets: Results and Future Prospects B. Scott Gaudi Department of Astronomy, The Ohio State University

And, low and behold, it provides an answer to the question upon which this thread is based. Specifically, it says: "With the recent MOA upgrade, the rate of planet detections has increased substantially.
From 2003-2006, six planets were detected (four have been published).
From the 2007 bulge season alone, there are four fairly secure planetary events. This rate can be expected to increase modestly as analysis techniques improve, and so the next several years should bring of order a dozen planet detections."

Never thought that I would one day be reading a conversation like this.

[old man's voice] Only 1 or 2 per year? When I was young we discovered planets at the rate of 1 per century. [/old man's voice]

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When I was young we discovered planets at the rate of 1 per century.



And then had them taken off us.....

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