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WASP-17b: an ultra-low density planet in a probable retrograde orbit
D. R. Anderson, C. Hellier, M. Gillon, A. H. M. J. Triaud, B. Smalley, L. Hebb, A. Collier Cameron, P. F. L. Maxted, D. Queloz, R. G. West, S. J. Bentley, B. Enoch, K. Horne, T. A. Lister, M. Mayor, N. R. Parley, F. Pepe, D. Pollacco, D. Ségransan, S. Udry, D. M. Wilson
TL;DR
The paper addresses the origin of WASP-17b's extreme low density, retrograde orbit, and inflated radius. It combines transit photometry, radial velocities, stellar modeling, and Rossiter-McLaughlin measurements, finding an exceptionally low density and suggesting tidal heating after dynamical scattering. More precise eccentricity and Rossiter-McLaughlin constraints are needed to test this interpretation.
Problem
The paper investigates how a short-orbit giant planet can have an exceptionally large radius and a probable retrograde orbit.
Method
The study combines transit photometry and radial velocities with stellar-evolution modeling, Rossiter-McLaughlin analysis, and tidal-heating models.
Results
WASP-17b has density 0.06–0.14ρJ and radius 1.5-2 RJup, while tidal-heating models can produce a radius near 2 RJup from a highly eccentric orbit.
Takeaways & Limitations
WASP-17b supports a formation history involving scattering, the Kozai mechanism, and tidal circularisation, and is a strong target for transmission spectroscopy.
Takeaways & Limitations
The Rossiter-McLaughlin amplitude may be overestimated, and more precise orbital eccentricity measurements are needed to distinguish the models.
Abstract
from arXiv · showhide
We report the discovery of the transiting giant planet WASP-17b, the least-dense planet currently known. It is 1.6 Saturn masses but 1.5-2 Jupiter radii, giving a density of 6-14 per cent that of Jupiter. WASP-17b is in a 3.7-day orbit around a sub-solar metallicity, V = 11.6, F6 star. Preliminary detection of the Rossiter-McLaughlin effect suggests that WASP-17b is in a retrograde orbit (lambda ~ -150 deg), indicative of a violent history involving planet-planet or star-planet scattering. WASP-17b's bloated radius could be due to tidal heating resulting from recent or ongoing tidal circularisation of an eccentric orbit, such as the highly eccentric orbits that typically result from scattering interactions. It will thus be important to determine more precisely the current orbital eccentricity by further high-precision radial velocity measurements or by timing the secondary eclipse, both to reduce the uncertainty on the planet's radius and to test tidal-heating models. Owing to its low surface gravity, WASP-17b's atmosphere has the largest scale height of any known planet, making it a good target for transmission spectroscopy.
1. INTRODUCTION
WASP-17b is presented against the background of unexplained inflated radii in short-orbit giant planets and competing migration mechanisms. The paper introduces it as the least-dense known planet and the first probable retrograde-orbit planet.
- Earlier transit discoveries showed that some giant planets have radii too large for standard evolutionary models, with TrES-4 previously the most bloated.
- Tidal circularisation of eccentric orbits is described as the leading explanation for some inflated planetary radii.
- The Rossiter-McLaughlin effect measures the sky-projected angle between stellar spin and planetary orbital axes, helping distinguish migration scenarios.
- WASP-17b is the least-dense planet currently known and the first planet found to be in a probable retrograde orbit.
2. OBSERVATIONS
Photometric and spectroscopic observations establish WASP-17's 3.7-day transit and planetary-mass radial-velocity signal. Bisector analysis supports interpreting the signal as arising from the transiting planet rather than a blended eclipsing binary.
- A 3.7-day periodicity was found from 15,509 usable WASP-South measurements spanning two years.
- The EulerCAM Ic-band transit was observed over six hours using 181 frames with exposure times of 32–98 seconds.
- The discovery photometry assigned a 0.74 probability that WASP-17 was a main-sequence star and rejected the usual small-radius follow-up criterion.
- Radial-velocity variations at the photometric period have a semi-amplitude of approximately 50 m s−1, consistent with a planetary-mass companion.
- The absence of correlation between bisector span and radial velocity supports identifying the transiting body as a planet rather than a blended eclipsing binary.
3. STELLAR PARAMETERS
The observations characterize WASP-17 as an F6 star and use photometric, spectroscopic, and rotational information to constrain its stellar properties. The stellar rotation analysis yields a tentative 24.7-day modulation, while the expected aligned rotation period is 8.5–11 days.
- The spectra provide a lithium-abundance upper limit of log n(Li/H) + 12 < 1.3, but the star's lithium-gap temperature prevents using lithium to determine its age.
- Assuming spin-orbit alignment, the measured v sin i = 9.0 km s−1 and stellar radius imply an expected rotation period of 8.5–11 days.
4. SYSTEM PARAMETERS
WASP-17 system parameters are inferred by jointly modeling photometry, radial velocities, stellar evolution, and the Rossiter–McLaughlin effect. Different stellar and orbital assumptions produce substantially different eccentricities and planetary radii, while the fitted RM signal favors a strongly misaligned orbit.
- Analysis method: The analysis combines WASP-South and EulerCAM photometry with CORALIE and HARPS radial velocities in a simultaneous MCMC fit using transit, orbital, stellar, and RM parameters.The proposal parameters include transit timing and duration, impact parameter, RV semi-amplitude, stellar mass, eccentricity components, and projected spin–orbit parameters.
- Orbital and planetary parameters: The preferred Case 1 models WASP-17 as evolved off the zero-age main sequence, with mass 1.20+0.10 −0.11 M⊙, age 3.0+0.9 −2.6 Gyr, and e = 0.129+0.106 −0.068.The stellar mass estimate initializes the MCMC solution, whose best-fitting eccentricity is non-zero at approximately the 2-σ level.
- Orbital and planetary parameters: RP = 1.74+0.26 −0.23 RJup is large but uncertain because poorly constrained e and ω make the stellar radius, and therefore the transit-derived planet radius, uncertain.The first MCMC solution gives a non-zero eccentricity at the 2-σ level, but the radius uncertainty remains substantial.
- Orbital and planetary parameters: A main-sequence prior gives RP = 1.51 ± 0.10 RJup and e = 0.237+0.068 −0.069, whereas imposing a circular orbit gives RP = 1.97 ± 0.10 RJup.The main-sequence prior pushes stellar density toward higher main-sequence values, while the circular solution increases the inferred stellar and planetary radii.
- Model discrimination: Further high-precision radial velocities, stellar-radius information from parallax, or secondary-eclipse timing are needed to distinguish the three solutions and better constrain orbital eccentricity.RV measurements constrain e sin ω most strongly, while secondary-eclipse timing constrains e cos ω.
- 4.1. A retrograde orbit?: λ ≈ −150 deg favors a retrograde spin–orbit configuration, although the fitted RM amplitude implies v sin i ≈ 20 km s−1, higher than the spectroscopic value.Relative to fixed aligned and perpendicular models, the preferred fit has lower RV RMS; the authors caution that Gaussian fitting of broadened cross-correlation profiles may overestimate the RM amplitude.
5. DISCUSSION
WASP-17b is an exceptionally low-density, highly inflated planet whose probable retrograde orbit supports a violent dynamical history and possible tidal-heating origin. Its atmospheric scale height makes it a strong target for transmission spectroscopy.
- 0.06–0.14ρJ makes WASP-17b the least-dense known planet, with a 1.5–2 RJup radius exceeding standard evolution-model predictions of at most 1.3 RJup.The comparison model assumes a 1-Gyr-old, coreless 0.41 MJup planet receiving high stellar flux at 0.02 AU.
- Tidal dissipation during circularisation of a highly eccentric, close orbit could inflate WASP-17b by depositing orbital energy in its interior.Higher eccentricity strengthens tides, while atmospheric opacity can retard contraction and enhance the retained heat.
- A model applied to HD 209458b suggests WASP-17b could reach RP ≈2 RJup after evolving from e ≈0.79 and a ≈0.085 AU with moderate tidal dissipation.The modeled final semimajor axis was within 10% of WASP-17b’s current value.
- The probable retrograde orbit supports a history involving scattering, the Kozai mechanism, and tidal circularisation, a pathway producing short-orbit giants in ∼30% of modeled cases.More Rossiter-McLaughlin measurements are required to determine how much this pathway contributes.
- Testing a scattering history requires searching for companions, but no significant radial-velocity drift was detected over 622 days.Outer planets may remain difficult to detect if they occupy long orbits.
- WASP-17b has the largest known atmospheric scale height, 1100–2100 km, and its projected atmospheric area is 1.9–2.7 times that of HD 209458b.Its low surface gravity and moderately high equilibrium temperature make the system a good transmission-spectroscopy prospect.