Follow-up Photometry
greg posted in exoplanet detection on May 19th, 2007
Image Source.
I’m still astounded by the dramatic detection of the transit of Gl 436b, and I’m working on some posts that sort through the scientific results and implications that this discovery is generating.
GJ 436b was found using the same basic strategy that led to the detection of the transits of HD 209458b, HD 149026b, and HD 189733b. First, the planet is located with the radial velocity technique. Doppler velocities, of course, do not give the inclination of the planetary orbit, but they do give a prediction of when transits would occur if the line of sight to the system lies within a small enough angle of the planet’s orbital plane.
Short-period planets have higher a-priori chances of being observed in transit (a 12% probability is typical for a hot Jupiter on a short-period orbit) and so in general, most of the RV-detected planets with orbits of less than a week are checked photometrically for transits by members of the discovery team before the planet is publicly announced. The discovery teams found the transits of HD 209458b, HD 1409026b, and HD 189733b. Dramatically not so, however, with Gl 436b.
Note: In the initial version of this post, I jumped to some incorrect conclusions about how the Gl 436 discovery was made. This article on swissinfo caused me to infer that the initial April 2nd detection of the transit was a postcard-perfect story of an independent small-observatory follow-up of the variety encouraged by transitsearch.org. It turns out, however, that the OFXB telescope is tightly linked to the Geneva program. The Gl 436 detection was made in the course of an ongoing systematic survey of the known planet-bearing M-stars and K-stars, and of as-yet unannounced new candidates discovered by HARPS and SOPHIE. Michael Gillon, lead author on the Gl 436 paper, and the lead scientist for the photometric follow-up effort was kind enough to correct my facts.
May 20th, 2007 at 10:29 pm
Hi Greg,
After reading the swissinfo article, I can see how you inferred from it that the initial April 2nd detection of the transit was a postcard-perfect story of an independent small-observatory follow-up. I’m glad to hear that at least a group of independent transit observers, participating in transitsearch.org, weren’t cheated out of credit for a major discovery, as I’m sure are you.
Eric
May 21st, 2007 at 2:00 am
[…] First, I squandered literally years of opportunity to coordinate a photometric follow-up transit search on Gl 436. Then I managed to incorrectly report the circumstances of the detection on the initial version of this (now corrected) oklo post! Naturally, I’m feeling sheepish, and my situation bears a distant echo to that of John Herschel (son of William), who was partly to blame for the inadequate coordination of an observational follow-up to John Couch Adams’ predictions of Neptune’s location. […]
July 20th, 2007 at 5:15 am
[…] Regular oklo readers will recall Gillon et al.’s discovery that the Neptune-mass planet orbiting the red dwarf star Gl 436 can be observed in transit. Transitsearch got scooped, and the whole eposide got me all worked up enough to neglect the exigencies of everyday academic life and reel off three straight posts on the detection and its consequences (see here, here, here, and also here). The transits of Gl 436 b are a big deal because they indicate that the planet is composed largely of water. It’s not a rock and it’s not a Jupiter-like gas giant. Rather, it’s a fully Neptune-like object, hauled in for inspection on a 2.64385 day orbit. […]