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


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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Figure 1: The O–C diagram of the main pulsation frequencyf1 of V 391 Peg.
Figure 2: The O–C diagram of f1.
Figure 3: The O–C diagram of f2.


  1. Wolszczan, A. & Frail, D. A. A planetary system around the millisecond pulsar PSR1257+12. Nature 355, 145–147 (1992)

    ADS  Article  Google Scholar 

  2. Mayor, M. & Queloz, D. A Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995)

    ADS  Article  Google Scholar 

  3. Döllinger, M. P. et al. Discovery of a planet around the K giant star 4 U Ma. Astron. Astrophys. (in the press); preprint at 〈

  4. Rasio, F. A., Tout, C. A., Lubow, S. H. & Livio, M. Tidal decay of close planetary orbits. Astrophys. J. 470, 1187–1191 (1996)

    ADS  Article  Google Scholar 

  5. Rybicki, K. R. & Denis, C. On the final destiny of the Earth and the Solar System. Icarus 151, 130–137 (2001)

    ADS  Article  Google Scholar 

  6. Østensen, R. et al. Detection of pulsations in three subdwarf B stars. Astron. Astrophys. 368, 175–182 (2001)

    ADS  Article  Google Scholar 

  7. Kilkenny, D. Pulsating hot subdwarfs—an observational review. Commun. Asteroseismol. 150, 234–240 (2007)

    ADS  Article  Google Scholar 

  8. Silvotti, R. et al. The temporal spectrum of the sdB pulsating star HS 2201+2610 at 2 ms resolution. Astron. Astrophys. 389, 180–190 (2002)

    ADS  Article  Google Scholar 

  9. Kepler, S. O. et al. Measuring the evolution of the most stable optical clock G 117–B15A. Astrophys. J. 634, 1311–1318 (2005)

    ADS  CAS  Article  Google Scholar 

  10. Reed, M. et al. Observations of the pulsating subdwarf B star Feige 48: constraints on evolution and companions. Mon. Not. R. Astron. Soc. 348, 1164–1174 (2004)

    ADS  Article  Google Scholar 

  11. Winget, D. E. et al. The search for planets around pulsating white dwarf stars. ASP Conf. Ser. 294, 59–64 (2003)

    ADS  Google Scholar 

  12. Sterken, C. The O–C diagram: basic procedures. ASP Conf. Ser. 335, 3–23 (2005)

    ADS  Google Scholar 

  13. Thorsett, S. E., Arzoumanian, Z. & Taylor, J. H. PSR B1620–26—A binary radio pulsar with a planetary companion? Astrophys. J. 412, L33–L36 (1993)

    ADS  Article  Google Scholar 

  14. Costa, J. E. S., Kepler, S. O. & Winget, D. E. Direct measurement of a secular pulsation period change in the pulsating hot pre-white dwarf PG 1159–035. Astrophys. J. 522, 973–982 (1999)

    ADS  Article  Google Scholar 

  15. Mukadam, A. S. et al. Constraining the evolution of ZZ Ceti. Astrophys. J. 594, 961–970 (2003)

    ADS  Article  Google Scholar 

  16. Koen, C. Statistics of O–C diagrams and period changes. ASP Conf. Ser. 335, 25–35 (2005)

    ADS  Google Scholar 

  17. Hanel, R., Conrath, B., Herath, L., Kunde, V. & Pirraglia, J. Albedo, internal heat, and energy balance of Jupiter—preliminary results of the Voyager infrared investigation. J. Geophys. Res. 86, 8705–8712 (1981)

    ADS  Article  Google Scholar 

  18. Maxted, P. F. L., Napiwotzki, R., Dobbie, P. D. & Burleigh, M. R. Survival of a brown dwarf after engulfment by a red giant star. Nature 442, 543–545 (2006)

    ADS  CAS  Article  Google Scholar 

  19. Villaver, E. & Livio, M. Can planets survive stellar evolution? Astrophys. J. 661, 1192–1201 (2007)

    ADS  Article  Google Scholar 

  20. Duncan, M. J. & Lissauer, J. J. The effects of post-main-sequence solar mass loss on the stability of our planetary system. Icarus 134, 303–310 (1998)

    ADS  Article  Google Scholar 

  21. Livio, M. & Soker, N. Star-planet systems as progenitors of cataclysmic binaries: tidal effects. Astron. Astrophys. 125, L12–L15 (1983)

    ADS  Google Scholar 

  22. Sweigart, A. V. & Gross, P. G. Evolutionary sequences for red giant stars. Astrophys. J. 36 (Suppl.). 405–437 (1978)

    CAS  Article  Google Scholar 

  23. Han, Z., Podsiadlowski, Maxted, P. F. L., Marsh, T. R. & Ivanova, N. The origin of subdwarf B stars—I. The formation channels. Mon. Not. R. Astron. Soc. 336, 449–466 (2002)

    ADS  Article  Google Scholar 

  24. Zahn, J. P. Tidal friction in close binary stars. Astron. Astrophys. 57, 383–394 (1977)

    ADS  Google Scholar 

  25. Eggleton, P. P. Approximations to the radii of Roche lobes. Astrophys. J. 268, 368–369 (1983)

    ADS  Article  Google Scholar 

  26. Soker, N. Can planets influence the horizontal branch morphology? Astron. J. 116, 1308–1313 (1998)

    ADS  Article  Google Scholar 

  27. Silvotti, R. et al. The rapidly pulsating subdwarf B star PG 1325+101. I. Oscillation modes from multisite observations. Astron. Astrophys. 459, 557–564 (2006)

    ADS  CAS  Article  Google Scholar 

  28. Stumpff, P. Two self-consistent FORTRAN subroutines for the computation of the Earth's motion. Astron. Astrophys. Suppl. Ser. 41, 1–8 (1980)

    ADS  Google Scholar 

  29. Charpinet, S., Fontaine, G., Brassard, P. & Dorman, B. Adiabatic survey of subdwarf B star oscillations. III. Effects of extreme horizontal branch stellar evolution on pulsation modes. Astrophys. J. 140 (Suppl.). 469–561 (2002)

    Article  Google Scholar 

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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).

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