Home > detection, worlds > GL 581. Flat, unfortunately.

GL 581. Flat, unfortunately.

April 17th, 2006


Regular visitors to oklo.org are familiar with GL 581 b, a Neptune-mass planet in a 5.366 day orbit around a nearby M-dwarf star. I’ve developed a fascination with this planet, because if it can be observed in transit across the disk of its parent star, then we will learn an incredible amount about the planet’s interior structure. In a nutshell, if the planet has a small transit depth then we’ll know it’s made of rock and metal, and if it has a larger transit depth, then we’ll know it’s made mostly of water.

The a-priori geometric probability that transits by GL 581 b occur is 3.6%. Because the planetary orbit is fairly well known, the time windows during which transits can occur are fairly narrow. The expected transit depth for the planet (if it’s made of water) is a respectable 1.6%, which means that observers with small telescopes will be able to detect the transits if they are occurring.

For more details on the GL 581 campaign, please read (1) this oklo post, “clouds”, and then (2) this oklo post, “two for the show”. For information on how amateur astronomers and small-telescope observers can participate in the search for transiting extrasolar planets, see our website for transitsearch.org. Over the coming months, we’ll be integrating transitsearch.org much more tightly into the oklo site. The systemic project and the transitsearch project both have a common goal of facilitating meaningful public participation in cutting-edge extrasolar planet research.

Every 5.366 days, I’ve been peppering the transitsearch.org observers mailing list with exhortations to observe GL 581 during the transit windows. The weather has not been very cooperative, and many opportunities worldwide were thwarted by clouds, but we now have two data sets that indicate that transits by GL 581 b are unlikely to be occurring:

gl581 photometry

The top data set (from April 2nd) was obtained by David Blank and Graeme White (of James Cook University) using a robotic Celestron C14 stationed at the Perth Observatory. The observations were made through an uncalibrated R filter. The operation of the telescope is made possible by the Perth Observatory staff Jamie Biggs and Arie Verveer, with Carl Pennypacker participating remotely from UC Berkeley. The bottom data set, from April 12th, was obtained by Kent Richardson, using the transitsearch.org robotic telescope, which was set up by Tim Castellano, and which is located in San Diego.

Sadly, neither data set shows any hint of a transit. In addition, David Blank has another data set in hand from March 28th, which also shows no sign of transit. I’ll update the post shortly to include that set as well. Several more observations will be required to really scratch GL 581 b off the list, but at this point it doesn’t look good for transits.

So yeah, I’m a little bummed out. But look at the bright side. A worldwide network of small-telescope observers has obtained an important astronomical result, demonstrating the feasibility of the transitsearch.org approach. If we keep observing the candidates, eventually we’ll hit pay dirt.

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