Abstract
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.
This is a preview of subscription content, access via your institution
Access options
Access to this article via Institution of Civil Engineers Library is not available.
Subscribe to this journal
Receive 51 print issues and online access
$199.00 per year
only $3.90 per issue
Buy this article
- Purchase on SpringerLink
- Instant access to full article PDF
Prices may be subject to local taxes which are calculated during checkout
Similar content being viewed by others
References
Wolszczan, A. & Frail, D. A. A planetary system around the millisecond pulsar PSR1257+12. Nature 355, 145–147 (1992)
Mayor, M. & Queloz, D. A Jupiter-mass companion to a solar-type star. Nature 378, 355–359 (1995)
Döllinger, M. P. et al. Discovery of a planet around the K giant star 4 U Ma. Astron. Astrophys. (in the press); preprint at 〈http://arxiv.org/astro-ph/0703672〉
Rasio, F. A., Tout, C. A., Lubow, S. H. & Livio, M. Tidal decay of close planetary orbits. Astrophys. J. 470, 1187–1191 (1996)
Rybicki, K. R. & Denis, C. On the final destiny of the Earth and the Solar System. Icarus 151, 130–137 (2001)
Østensen, R. et al. Detection of pulsations in three subdwarf B stars. Astron. Astrophys. 368, 175–182 (2001)
Kilkenny, D. Pulsating hot subdwarfs—an observational review. Commun. Asteroseismol. 150, 234–240 (2007)
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)
Kepler, S. O. et al. Measuring the evolution of the most stable optical clock G 117–B15A. Astrophys. J. 634, 1311–1318 (2005)
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)
Winget, D. E. et al. The search for planets around pulsating white dwarf stars. ASP Conf. Ser. 294, 59–64 (2003)
Sterken, C. The O–C diagram: basic procedures. ASP Conf. Ser. 335, 3–23 (2005)
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)
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)
Mukadam, A. S. et al. Constraining the evolution of ZZ Ceti. Astrophys. J. 594, 961–970 (2003)
Koen, C. Statistics of O–C diagrams and period changes. ASP Conf. Ser. 335, 25–35 (2005)
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)
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)
Villaver, E. & Livio, M. Can planets survive stellar evolution? Astrophys. J. 661, 1192–1201 (2007)
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)
Livio, M. & Soker, N. Star-planet systems as progenitors of cataclysmic binaries: tidal effects. Astron. Astrophys. 125, L12–L15 (1983)
Sweigart, A. V. & Gross, P. G. Evolutionary sequences for red giant stars. Astrophys. J. 36 (Suppl.). 405–437 (1978)
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)
Zahn, J. P. Tidal friction in close binary stars. Astron. Astrophys. 57, 383–394 (1977)
Eggleton, P. P. Approximations to the radii of Roche lobes. Astrophys. J. 268, 368–369 (1983)
Soker, N. Can planets influence the horizontal branch morphology? Astron. J. 116, 1308–1313 (1998)
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)
Stumpff, P. Two self-consistent FORTRAN subroutines for the computation of the Earth's motion. Astron. Astrophys. Suppl. Ser. 41, 1–8 (1980)
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)
Acknowledgements
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.
Author information
Authors and Affiliations
Corresponding author
Ethics declarations
Competing interests
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
Supplementary information
Supplementary Information
The file contains Supplementary Notes, Supplementary Tables 1-2 and additional references. (PDF 128 kb)
Rights and permissions
About this article
Cite this article
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
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1038/nature06143
