Vol 449 | 13 September 2007 | doi:10.1038/nature06143 LETTERS 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 After the initial discoveries fifteen years ago1,2, over 200 extrasolar Fig. 1), implying that the main pulsation period of the star is increa- 212 planets have now been detected. Most of them orbit main- sing at a rate of P_ 1 5 (1.46 6 0.07) 3 10 (or 1 s in 22,000 years). sequence stars similar to our Sun, although a few planets orbiting For V 391 Peg a simple second-order polynomial does not give a red giant stars have been recently found3. When the hydrogen in satisfactory fit and the sinusoidal residuals require further interpreta- their cores runs out, main-sequence stars undergo an expansion tion (lower panel of Fig. 1). An oscillating P_ is not compatible with into red-giant stars. This expansion can modify the orbits of pla- any evolutionary or pulsational model: it would require that the star nets and can easily reach and engulf the inner planets. The same expands and contracts periodically every 3 years, a time much larger will happen to the planets of our Solar System in about five billion than the dynamical timescale, which is of the order of 500 s for a years and the fate of the Earth is matter of debate4,5. Here we report subdwarf B star. Nor can the sinusoidal residuals be explained by any 16 the discovery of a planetary-mass body (Msini 5 3.2MJupiter) orbit- known pulsational effect. Random period variations are also not a ing the star V 391 Pegasi at a distance of about 1.7 astronomical possibility because these variations would be cancelled by the large units (AU), with a period of 3.2 years. This star is on the extreme number of pulsation cycles (.28 for each point in Fig. 1). horizontal branch of the Hertzsprung–Russell diagram, burning The simplest explanation for the sinusoidal component of the O–C helium in its core and pulsating. The maximum radius of the red- diagram in Fig. 1 is a wobble of the star’s barycentre due to the presence giant precursor of V 391 Pegasi may have reached 0.7 AU, while the of a low-mass companion. Depending on its position around the orbital distance of the planet during the stellar main-sequence barycentre of the system, the subdwarf B star is periodically closer phase is estimated to be about 1 AU. This detection of a planet to, or more distant from us by 5.3 6 0.6 light seconds and the timing orbiting a post-red-giant star demonstrates that planets with of the pulsation is cyclically advanced or delayed. From our best fit and orbital distances of less than 2 AU can survive the red-giant expan- Kepler’s third law (assuming a circular orbit, M1 5 0.5MSun, where sion of their parent stars. MSun is the mass of the Sun, and M2 = M1), we obtain: Porb 5 With an effective temperature close to 30,000 K and a surface 1,170 6 44 days (or 3.20 6 0.12 years), a 5 1.7 AU and M2sini 5 6 gravity ten times that of the Sun , V 391 Pegasi (or HS 220112610 3.2MJupiter, where a is the planet–star separation and 1 AU is the from the original Hamburg Schmidt survey name) is one of about 40 mean distance between the Earth and the Sun. The orbital parameters hot subdwarf B stars showing short-period p-mode pulsations7. of the system are listed in Table 1. This interpretation is robust: the Its pulsational spectrum exhibits four or five pulsation periods6,8 between 342 and 354 s (see Supplementary Information for more Table 1 | Orbital parameters details on the star’s properties). Parameter Value Because of their compact structure, subdwarf B pulsators have 1 170 44 extremely stable oscillation periods, like white dwarf pulsators. It is Orbital period, Porb (d) , 6 therefore possible to register very small differences in the arrival times Epoch of maximum time delay, T0 (BJD) 2,452,418 6 96 9,10 Eccentricity, e (assumed) 0.0 of the photons , which in principle allows the detection of low- 1 600 000 190 000 11 Star projected orbital radius, assini (km) , , 6 , mass secondary bodies , through the use of the observed–calculated 21 Star projected orbital velocity, vssini (m s ) 99 6 12 12 27 (O–C) diagram (see Fig. 1 legend for more details). Functionally, Mass function*, f(M1, M2)(MSun)(1.19 6 0.43) 3 10 it is equivalent to the timing method used to find planets around Distance from the star{, a (AU) 1.7 6 0.1 1,13 Maximum elongation{ (milliarcsec) 1.2 6 0.1 pulsars . 21 Planet orbital velocity{, vp (km s ) 16 6 1 When a pulsation period changes linearly in time, the O–C dia- Planet mass{, M2sini (M ) 3.2 6 0.7 . Jupiter . gram has a parabolic shape, as confirmed by all the previous mea- 3 3 2 * fMðÞ,M ~4p2ðÞa sini GP2 ~ðÞM sini ðÞM zM . surements of dP/dt (or P_ ) in compact pulsators9,14,15. The same 1 2 s orb 2 1 2 { These numbers are obtained assuming M1 5 0.5 6 0.05MSun (suggested from astero- behaviour is found in the O–C plot of V 391 Peg (upper panel of seismology) and M2 = M1. 1INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy. 2Institut fu¨r Astrophysik, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, 37077 Go¨ttingen, Germany. 3Institute of Theoretical Physics and Astronomy, Vilnius University, 12 A. Gostauto Street, 01108 Vilnius, Lithuania. 4Institutt for Teoretisk Astrofysikk, Universitetet i Oslo, PB 1029 Blindern, 0315, Norway. 5INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy. 6K. U. Leuven, Institute of Astronomy, Celestijnenlaan 200D, 3001 Leuven, Belgium. 7Department of Physics and Space Sciences and the SARA Observatory, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA. 8INAF-Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy. 9CNRS-UMR5572, Observatoire Midi-Pyre´ne´es, Universite´ Paul Sabatier, 14 avenue Edouard Belin, 31400 Toulouse, France. 10Department of Physics, Astronomy and Materials Science, Missouri State University, 901 S. National, Springfield, Missouri 65897, USA. 11Institute of Astronomy, National Central University, 300 Jhongda Road, Chung-Li 32054, Taiwan. 12Wise Observatory, Tel Aviv University, Tel Aviv 69978, Israel. 13Konkoly Observatory, P O Box 67, H-1525 Budapest XII, Hungary. 14Cracow Pedagogical University, ul. Podchorazych 2, 30-084 Cracow, Poland. 15Department of Physics and Astronomy, 12 Physics Hall, Iowa State University, Ames, Iowa 50011, USA. 16Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK. 17Copernicus Astronomical Centre, ul. Bartycka 18, 00-716 Warsaw, Poland. 18Thirty Meter Telescope Project, 2632 E. Washington Blvd, Pasadena, California 91107, USA. 19Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30- 244 Cracow, Poland. 189 © 2007 Nature Publishing Group LETTERS NATURE | Vol 449 | 13 September 2007 same sinusoidal component is also found in the O–C diagram of 2,000 2,500 3,000 3,500 4,000 the secondary pulsation frequency of the star (see Figs 2 and 3 for 20 20 more details). Any alternative interpretation of our results would have 10 10 to be compatible with this fact. The sinusoids in the lower panels of 0 0 Figs 1, 2 and 3 suggest a circular orbit. From our observations we –10 –10 cannot yet set a precise upper limit to the eccentricity, but it must –20 –20 be close to zero. 5 5 Using the known characteristics of the V 391 Peg system, we can determine a first estimate of the planet’s effective temperature by 0 0 balancing the flux received from the star with the blackbody flux Observed–calculated (s) –5 –5 re-radiated by the planet (see Supplementary Information for more details). Assuming a Bond albedo of 0.343 (similar to that of 2,000 2,500 3,000 3,500 4,000 t (BJD – 2,450,000) Jupiter17), we obtain an effective temperature for the planet of about 470 K, corresponding to a maximum of the blackbody radiation near Figure 2 | The O–Cdiagramoff1. In this version of the O–C diagram, all the 6.2 mm from Wien’s law. runs of each observing season were combined and the phases were recalculated With a projected radius of about five light seconds, the wobble of on these larger data sets. This reduces the noise (but also reduces the time the barycentre of V 391 Peg points towards a planet (for comparison, resolution), so that in principle O–C diagrams can be built for each pulsation frequency of a multiperiodic pulsator. In this way, if the pulsating star has a the amplitude of the solar displacement around the barycentre of our companion, each pulsation mode can supply an independent confirmation of Solar System is almost three light seconds). However, depending on the periodic motion around the centre of mass. V 391 Peg has four or five the unknown inclination i of the system, a brown dwarf or even a low- pulsation periods; for the two that have sufficiently large amplitudes of 1% and mass stellar companion cannot be totally excluded.