An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b

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Abstract

The ‘hot Jupiters’ that abound in lists of known extrasolar planets are thought to have formed far from their host stars, but migrate inwards through interactions with the proto-planetary disk from which they were born1,2, or by an alternative mechanism such as planet–planet scattering3. The hot Jupiters closest to their parent stars, at orbital distances of only 0.02 astronomical units, have strong tidal interactions4,5, and systems such as OGLE-TR-56 have been suggested as tests of tidal dissipation theory6,7. Here we report the discovery of planet WASP-18b with an orbital period of 0.94 days and a mass of ten Jupiter masses (10 MJup), resulting in a tidal interaction an order of magnitude stronger than that of planet OGLE-TR-56b. Under the assumption that the tidal-dissipation parameter Q of the host star is of the order of 106, as measured for Solar System bodies and binary stars and as often applied to extrasolar planets, WASP-18b will be spiralling inwards on a timescale less than a thousandth that of the lifetime of its host star. Therefore either WASP-18 is in a rare, exceptionally short-lived state, or the tidal dissipation in this system (and possibly other hot-Jupiter systems) must be much weaker than in the Solar System.

Main

Through monitoring by the WASP-South transit survey8, coupled with radial-velocity observations from the Coralie spectrograph, we have discovered a 10-MJup planet transiting the star WASP-18 ( = HD 10069) every 0.94 days (Fig. 1, Table 1, Table 2). WASP-18b is the first confirmed hot-Jupiter planet that has a period of less than one day (candidates with periods of less than a day have previously been announced based on photometry alone9, though experience shows that less than 10% of such candidates are actual planets10).

Figure 1: Discovery data for WASP-18b.
figure1

a, The WASP-South lightcurve folded on the 0.94-day transit period, together with the model curve from the parameters of Table 1. Monitoring from May–December in 2006 and 2007 resulted in 8,235 photometric data points. b, Coralie radial-velocity measurements, again with the best-fitting model. The parameters of the system, derived from26 the radial-velocity data, the WASP photometry, and additional transit photometry from the Euler telescope, are given in Table 1. The parameters of the host star in Table 2 were derived independently27 from the stellar spectra. The stellar rotation rate (vsini) is 11.0 ± 1.5 km s–1, which (assuming that the spin and orbit are aligned) implies a rotation period of 5.6 days, typical for a young F star.

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Table 1 System parameters for WASP-18
Table 2 Stellar parameters of WASP-18

From comparison of the host star to stellar evolutionary tracks11,12 (see the Supplementary Information) we find a stellar mass of 1.24 ± 0.04 solar masses, , and an age of  Myr, which is short compared to the approximately 5-Gyr main-sequence lifetime of a star of this mass. A further age constraint is that the observed lithium abundance of WASP-18 is below that typical of F6 stars in the Pleiades (age 120 Myr) but comparable to that in the Hyades13 (age 600 Myr). Thus we conclude that WASP-18 has an age of 0.5–1.5 Gyr, making it one of the youngest known planet-hosting stars.

The theory of tidal interaction for hot Jupiters in close orbits4,5,14 predicts that the tidal bulge on the star, raised by the planet, exerts a torque that drains angular momentum from the planet’s orbit, causing it to spiral inwards (this arises when the planetary orbit is shorter than the stellar rotation, and contrasts with the Earth–Moon system where the longer orbit of the Moon compared to Earth’s spin causes it to move away over time). The spiral infall timescale is determined by the mass and orbital distance of the planet, and by the tidal dissipation parameter of the host star, Q. This quality factor is the ratio of the available energy to the amount dissipated by frictional losses during each orbital forcing cycle.

Q is found to be of the order of 105 to 106 from studies of binary stars15 and the gas-giant planets in our Solar System16,17 (in which it is often supposed that the Q value of stars and gas-giant planets will be similar). Thus values of Q = 105–106 are often applied to the star–planet tides of hot Jupiters18,19,20. However, for Q ≈ 106 the future lifetime of WASP-18b is only 0.65 Myr (Fig. 2), which is 10-4 of the estimated lifetime (5 Gyr) of the host star WASP-18. Thus, either WASP-18 is in an exceptionally shortlived state, or its Q value is much higher. Matching the infall timescale to the current age of WASP-18 gives a Q as high as 109, in line with some previous indications7,21,22.

Figure 2: Future evolution of WASP-18b.
figure2

The decrease in the orbital semi-major axis for different values of the tidal quality factor Q. The spiral infall timescale is given by (see equation (29) of ref. 14 and equation (5) of ref. 18):where n is the orbital angular frequency, Ms/Mp is the ratio of stellar to planet masses, a/Rs is the ratio of the orbital semi-major axis to the stellar radius, and the star is assumed to rotate slowly. The Q parameter measures the inverse fraction of the available energy dissipated in the star by frictional processes per tidal forcing cycle. It is usually expressed as a dimensionless ratio of the tidal quality factor of the star to the tidal Love number14,19.

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For comparison, WASP-18b’s infall timescale is an order of magnitude shorter than that of the much-discussed OGLE-TR-56b6,7 (assuming that Q is the same for both), and gives a current rate of period change of –0.00073 (106/Q) s yr–1. For low values of Q this would accumulate to a detectable change in transit epoch in less than a decade (for Q = 106 the transit time shifts by 28 s after 10 yr, which compares with a currently achievable timing accuracy23 of 5 s). Thus WASP-18b is a diagnostic planet, either (for a low Q) being an exceptionally rare object in which the tidal decay is directly measurable, or forcing a reappraisal to much higher Q values; either way it will help establish the dynamical ages of the class of hot-Jupiter planets. WASP-18 will also help constrain our understanding of stellar interiors, given that the Q value depends on the dissipation of interior waves excited by the tidal forcing7.

For all values of Q up to about 1010 the planet will spiral inwards to destruction within the star’s main-sequence lifetime. The trajectories in Fig. 2 continue until the planet reaches its Roche limit, at which point it will be tidally disrupted, and its material will likely feed onto the star through the Lagrangian point (this ignores radiative evaporation, which will be hindered by the relatively high surface gravity of this massive planet). Assuming that the angular momentum is assimilated by the star, it would be spun up from a rotation rate of 5.6 days to about 0.7 days, and thus be reborn as a rapidly rotating star. Heavier elements from the planetary core could contaminate the stellar atmosphere, where, owing to the relatively small convection layer of an F6 star, they might be readily visible. The planet-hosting star HD 82943 shows evidence of planet engulfment24, although it is currently rotating slowly, and thus any engulfment must have occurred long enough ago for magnetic braking to have since slowed the star.

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Acknowledgements

We thank the South African Astronomical Observatory for hosting WASP-South and the UK’s Science and Technology Facilities Council for funding.

Author Contributions WASP-S construction, operation and candidate selection (C.H., D.R.A., D.M.W., P.F.L.M., B.S., S.J.B.); WASP-S design (D.L.P); WASP observatory software (J.I., D.R.A., P.F.L.M.); WASP-S data processing (D.R.A., D.M.W., B.S.); WASP data pipeline (A.C.C., T.A.L., N.P., K.H.); transit-search code (A.C.C., L.H., B.E.); WASP data archive (R.G.W., P.J.W.); Coralie/EulerCAM data (M.G., A.H.M.J.T., D.S., D.Q.); Euler/Coralie construction and upgrade (D.Q., M.M., S.U., F.P.); planet characterization (A.C.C., D.R.A., M.G.); host star characterization (B.S., L.H.); paper writing (C.H., A.C.C.).

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Correspondence to Coel Hellier.

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This file contains Supplementary Table 1 and Supplementary Figures 1-4 with Legends. (PDF 259 kb)

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Hellier, C., Anderson, D., Cameron, A. et al. An orbital period of 0.94 days for the hot-Jupiter planet WASP-18b. Nature 460, 1098–1100 (2009) doi:10.1038/nature08245

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