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What is the Gravitational Microlensing Method?

This formidable-sounding method makes use of the fact that light bends when it passes near a massive object. Not only does light bend, but the apparent brightness of the image received by a distant observer is affected disproportionately. The light bending and amplification is an effect of relativity caused by the gravity of the massive objects involved, in this case two stars. The two stars involved are not necessarily connected with each other, and one star could be much further away than the other. However, the light from one star passing closely to the other star on its way to telescopes on or near Earth carries signatures of the close passage that can in principle be calculated. The reason why this is relevant for exoplanet detection is that the signature contained in the observed power from the stars versus time can be profoundly affected if one of the stars is hosting an exoplanet. The exoplanet shows up as a very narrow but relatively strong spike in the brightness versus time domain. The method is also in principle sensitive to “free-floating” planets (i.e., those that are not associated with a host star).

As might be expected, so-called “microlensing events” are even rarer than transits. There are many possible sources of confusion and error that must be taken into consideration, and the observing campaigns require huge amounts of observing time on multiple telescopes. It is a very labor-intensive effort. In August 2012, the Extrasolar Planets Encyclopedia listed only 16 exoplanets discovered by the gravitational lensing method.

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With such a low yield, and so many caveats, you may wonder whether it is worth all the effort. The unique thing about the microlensing method is that it is more sensitive to low-mass, Earthlike planets than any other method. It is sensitive to low-mass planets that are far enough away from the host star that water can exist in liquid and solid form (i.e. exoplanets that are beyond the so-called “snow line” or “ice line”). This gives considerable motivation for continuing to pursue the method because the other methods are not the best in this regime. Another advantage of the microlensing method is that it is very sensitive to multiplanet systems and it is sensitive to detecting moons around an exoplanet. The method is also sensitive to systems that are fainter than the ones that could potentially be detected by other methods because it does not rely directly on the light intensity of the host star. Moreover, it is not required to observe a complete orbit of the exoplanet with the microlensing method because the detection is virtually instantaneous. This is especially important for exoplanets that have orbital periods of many years. For example, for a planet like Jupiter that has a period of about 12 years, it would take that long to validate data using a method that required a complete orbit. On the other hand, microlensing events are one-time events. There is no possibility of repeating the experiment in order to validate any questionable findings in the data.

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File under: Explanation of the gravitational microlensing method of detection of exoplanets.

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