After the initial discoveries fifteen years ago1,2, over 200 extrasolar planets have now been detected. Most of them orbit main-sequence stars similar to our Sun, although a few planets orbiting red giant stars have been recently found3. When the hydrogen in their cores runs out, main-sequence stars undergo an expansion into red-giant stars. This expansion can modify the orbits of planets and can easily reach and engulf the inner planets. The same will happen to the planets of our Solar System in about five billion years and the fate of the Earth is matter of debate4,5. Here we report the discovery of a planetary-mass body (Msini = 3.2MJupiter) orbiting the star V 391 Pegasi at a distance of about 1.7 astronomical units (au), with a period of 3.2 years. This star is on the extreme horizontal branch of the Hertzsprung–Russell diagram, burning helium in its core and pulsating. The maximum radius of the red-giant precursor of V 391 Pegasi may have reached 0.7 au, while the orbital distance of the planet during the stellar main-sequence phase is estimated to be about 1 au. This detection of a planet orbiting a post-red-giant star demonstrates that planets with orbital distances of less than 2 au can survive the red-giant expansion of their parent stars.
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R.S. thanks M. Capaccioli, J. M. Alcalá, E. Covino and S. O. Kepler for discussions and suggestions, S. Marinoni and S. Galleti for their contribution to the observations, and the MiUR for financial support. S.S. thanks T. Nagel, E. Goehler, T. Stahn, S. D. Huegelmeyer, R. Lutz, U. Thiele and A. Guijarro for their help in data acquisition, and the DFG for travel grants. R.Ø. is supported by the Research Council of the University of Leuven and by the FP6 Coordination Action HELAS of the EU. T.D.O. acknowledges support from the US National Science Foundation. P.M. acknowledges support from the Polish MNiSW.
Author Contributions R.S. analysed and interpreted the data from which the presence of the planet was inferred. R.S., S.S., R.J., J.-E.S., S.B., R.Ø., T.D.O., I.B., R.G., A. Bonanno., G.V., M.R., C.-W.C., E.L. and M.P. contributed to the large amount of observations and/or data reduction. A. Baran., S.C., N.D., S.K., D.K., P.M., R.R. and S.Z. contributed to the organization and/or on-line data reduction/analysis during the XCov23 Whole Earth Telescope campaign of August-September 2003, in which V 391 Peg was observed as a secondary target. S.K. performed some tests on theoretical . S.K. and S.Z. did independent checks of the O–C fits. E.L. made statistical tests on the significance level of the O–C fits. All authors discussed and interpreted the results and commented on the manuscript. D.K. and R.Ø. in particular helped to improve the text.
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Silvotti, R., Schuh, S., Janulis, R. et al. A giant planet orbiting the ‘extreme horizontal branch’ star V 391 Pegasi. Nature 449, 189–191 (2007). https://doi.org/10.1038/nature06143
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