Letter

Nature 449, 189-191 (13 September 2007) | doi:10.1038/nature06143; Received 6 April 2007; Accepted 26 July 2007

A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi

R. Silvotti1, S. Schuh2, R. Janulis3, J.-E. Solheim4, S. Bernabei5, R. Østensen6, T. D. Oswalt7, I. Bruni5, R. Gualandi5, A. Bonanno8, G. Vauclair9, M. Reed10, C.-W. Chen11, E. Leibowitz12, M. Paparo13, A. Baran14, S. Charpinet9, N. Dolez9, S. Kawaler15, D. Kurtz16, P. Moskalik17, R. Riddle18 & S. Zola14,19

  1. INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
  2. Institut für Astrophysik, Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
  3. Institute of Theoretical Physics and Astronomy, Vilnius University, 12 A. Gostauto Street, 01108 Vilnius, Lithuania
  4. Institutt for Teoretisk Astrofysikk, Universitetet i Oslo, PB 1029 Blindern, 0315, Norway
  5. INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
  6. K. U. Leuven, Institute of Astronomy, Celestijnenlaan 200D, 3001 Leuven, Belgium
  7. Department of Physics and Space Sciences and the SARA Observatory, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA
  8. INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
  9. CNRS-UMR5572, Observatoire Midi-Pyrénées, Université Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France
  10. Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, Missouri 65897, USA
  11. Institute of Astronomy, National Central University, 300 Jhongda Road, Chung-Li 32054, Taiwan
  12. Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel
  13. Konkoly Observatory, P O Box 67, H-1525 Budapest XII, Hungary
  14. Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland
  15. Department of Physics and Astronomy, 12 Physics Hall, Iowa State University, Ames, Iowa 50011, USA
  16. Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
  17. Copernicus Astronomical Centre, ul. Bartycka 18, 00-716 Warsaw, Poland
  18. Thirty Meter Telescope Project, 2632 E. Washington Blvd, Pasadena, California 91107, USA
  19. Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Cracow, Poland

Correspondence to: R. Silvotti1 Correspondence and requests for materials should be addressed to R.S. (Email: silvotti@na.astro.it).

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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