Sunday, December 16, 2007

Gravitational Microlensing for Detection of Extrasolar Planets

Einstein taught us mass curves space-time, and everything moving in it has to behave accordingly, even light. Obeying the laws of General Relativity, large masses bend light rays around them. Essentially a mass acts like a convex lens, and collects light rays that otherwise would have missed the focal point - here the observer, an Earth or space based detector. This effect is called gravitational lensing. One speaks of gravitational microlensing if the object causing the light bending is of solar size. (If you have the Windows Media player, check out this nice animation from the NSF.)

If the lensing object crosses our line of sight to a more distant source star, it will affect the light from that star, producing two or more close images whose total brightness and magnification is enhanced. If the lensing star is accompanied by a planet, one can potentially observe not only the lensing effect from the star, but also a smaller effect resulting from the presence of the planet.

The plot below shows a particularly nice example, the detection of an extrasolar planet with the poetic name OGLE-2005-BLG-390Lb. Observed in July 2005, it was estimated to have a mass about 5.5 times that of the Earth. The plot shows the magnification of the source object, a bright G2 giant, due to the crossing of the star OGLE-2005-BLG-390. The small bump is the effect of the planet. The data set consists of 650 data points from various observatories, the errorbars are 1 σ.

[Reprinted by permission from Macmillan Publishers Ltd: Nature 439 437-440 (26 January 2006) doi:10.1038/nature04441, Copyright 2005.]

The top left inset shows the light curve of the previous 4 years, and the top right one shows a zoom of the planetary deviation on a time interval of 1.5 days. The solid curve is the best fit with the star and planet system. The dashed grey curve is the fit with best binary source model (two independent lensing stars) that is rejected by the data, and the dashed orange line is the best single lens model. These results were published in Nature 439, 437-440 (26 January 2006).

News report from the Probing Lensing Anomalies NETwork (PLANET): Discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet.

Paper by J.-P. Beaulieu et al.: Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing, Nature 439, 437-440 (26 January 2006) doi:10.1038/nature04441

This post is part of our 2007 advent calendar A Plottl A Day.


  1. Nice post. The MOA telescope is atop Mt John here, in a scenic part of the country not far from Aoraki. A great place to visit.

  2. Dear Kea,

    so it is at this place? Looks awesome!

    To me, this microlensing business seems to be totally crayzy - I mean, it appears to be so improbable that one star, and then even one with a planet, passes exactly through the line of sight to another one. It's like looking for a needle in a haystack. OK, I understand that all these observation programs are fully automatic, but that also sounds crazy, to imagine that a telescope in say, New Zealand tells another telescope in the Andes to observe the light curve of some specific star...

    Best, Stefan

  3. It might appear more improbable than it is because if you look at the sky with mere eyes you see only very little of the stars that can be observed today. And it doesn't actually happen that often. What I find amazing though is given how faint these objects are, how incredibly well all the data from numerous different observatories fit together! Best,


  4. Yes, that's it, Stefan. The university have a deal with the company Earth and Sky, who run astronomy tours and a cafe on Mt John.


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