Planets orbiting other stars could in principle be found through the periodic dimming of starlight as a planet moves across—or ‘transits’—the line of sight between the observer and the star. Depending on the size of the planet relative to the star, the dimming could reach a few per cent of the apparent brightness of the star. Despite many searches, no transiting planet has been discovered in this way; the one known1,2 transiting planet—HD209458b—was first discovered using precise measurements2,3 of the parent star's radial velocity and only subsequently detected photometrically. Here we report radial velocity measurements of the star OGLE-TR-56, which was previously found to exhibit a 1.2-day transit-like light curve4,5 in a survey looking for gravitational microlensing events. The velocity changes that we detect correlate with the light curve, from which we conclude that they are probably induced by an object of around 0.9 Jupiter masses in an orbit only 0.023 au from its star. We estimate the planetary radius to be around 1.3 Jupiter radii and its density to be about 0.5 g cm-3. This object is hotter than any known planet (∼ 1,900 K), but is still stable against long-term evaporation or tidal disruption.
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We thank A. Udalski and the OGLE team for generous contributions to this project. We are very grateful to S. Kulkarni for support, to R. Noyes and D. Latham for comments, to T. Barlow for assistance with the spectroscopic reductions using the MAKEE Keck Observatory HIRES Data Reduction Software (http://spider.ipac.caltech.edu/staff/tab/makee) and to K. Stanek for his encouragement. The data presented here were obtained at the W.M. Keck Observatory (operated by Caltech, the University of California, and NASA), which was made possible by the generous financial support of the W. M. Keck Foundation. M.K. gratefully acknowledges the support of NASA through the Michelson Fellowship programme. G.T. acknowledges support from NASA's Kepler Mission. S.J. thanks the Miller Institute for Basic Research in Science at UC Berkeley for support via a research fellowship.
The authors declare that they have no competing financial interests.
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Konacki, M., Torres, G., Jha, S. et al. An extrasolar planet that transits the disk of its parent star. Nature 421, 507–509 (2003). https://doi.org/10.1038/nature01379