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Old 18-04-2010, 08:30 AM
binofied
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Join Date: May 2005
Location: Auckland New Zealand
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Microlensing vs other methods

Just thought I would start a bit of a thread about why Gravitational Microlensing is a bit different than some of the other methods. This is from my understanding after being involved for only a few years and may not be 100% perfect.

Each method being doppler, transits or microlensing has its limitations. This is a link to a graph of the sensitivity of several methods. It has been shamelessly stolen from a slideshow so I can't explain all the detail.

http://www.binoscope.co.nz/planetmethods.png

It shows the planets in our solar system and their position on a graph of Mass vs Distance from the parent star. The other crosses and dots show actual discoveries of extra solar planets. This one only shows three (purple crosses) by Microlensing so is a bit out of date.

The doppler methods for example seem to be able to detect planets above about 5 earth masses very close to the star or at least 30 earth masses at our distance from the sun.

Kepler will be sensitive to much smaller planets but they are going to be close and damn hot at one extreme to just being able to see planets at our distance and mass from a star.

The limiting factor in mircolensing is the distance of the planet from the star in terms of the Einstien radius, with a planet at the Einstien radius being the most sensitive. At this point the caustics or distortions caused by the planet in the pure stellar lens are resonant and the easiest to detect. Luckily for us the Einstien ring diameter is also often just above 1AU thus the maximun sensitivity is around 1AU from the star. As the mass of the planet goes down the size of the pertibations decrease. This means we reach a point where noise dominates the planetary signature and we can't detect it. However there is still some sensitivity to earth mass planets all be it low. Also the planets discovered by most other methods are very close to us. A microlense can happen to stars hundreds of times farther away. So Gravitational Microlensing probes a different part of parameter space than other methods and will throw up new discoveries.

The reason we care about detecting planets is that up to a decade or just over ago, all solar system formations theories were just that. Now we are starting to provide real information of how other solar systems look to test those ideas and in a few cases totally blow theories out of the water. The ultimate goal is to answer, with certainty, the question "are there other rocky planets, like ours, near the ice line, that can actually support life".
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