WASP-17b is thought to have a retrograde orbit (with a sky-projected inclination of the orbit normal against the stellar spin axis of about 149°,[11] not to be confused with the line-of-sight inclination of the orbit, given in the table, which is near 90° for all transiting planets), which would make it the first planet discovered to have such an orbital motion. It was found by measuring the Rossiter–McLaughlin effect of the planet on the star's Doppler signal as it transited, in which whichever of the star's hemispheres is turning toward or away from Earth will show a slight blueshift or redshift which is dampened by the transiting planet. Scientists are not yet sure why the planet orbits opposite to the star's rotation. Theories include a gravitational slingshot resulting from a near-collision with another planet, or the intervention of a smaller planet-like body working to gradually change WASP-17b's orbit by tilting it via the Kozai mechanism.[12]
Spin-orbit angle measurement was updated in 2012 to −148.7+7.7 −6.7°.[13]
Physical properties
WASP-17b has a radius between 1.5 and 2 times that of Jupiter and about half the mass.[1] Thus its mean density is between 0.08 and 0.19 g/cm3,[1] compared with Jupiter's 1.326 g/cm3[14] and Earth's 5.515 g/cm3 (the density of water is 1 g/cm3). The unusually low density is thought to be a consequence of a combination of the planet's orbital eccentricity and its proximity to its parent star (less than one seventh of the distance between Mercury and the Sun), leading to tidal flexing and heating of its interior.[1] The same mechanism is behind the intense volcanic activity of Jupiter's moon Io. WASP-39b has a similarly low estimated density.
Comparison of "hot Jupiter" exoplanets (artist concept)
From top left to lower right: WASP-12b, WASP-6b, WASP-31b, WASP-39b, HD 189733 b, HAT-P-12b, WASP-17b, WASP-19b, HAT-P-1b and HD 209458 bThis is a transmission spectrum of the hot gas giant exoplanet WASP-17 b captured by Webb's Mid-Infrared Instrument (MIRI) on 12–13 March 2023. It reveals the first evidence for quartz (crystalline silica, SiO2) in the clouds of an exoplanet.[19]
See also
HAT-P-7b, another exoplanet announced to have a retrograde orbit the day after the WASP-17b announcement
TrES-4b, another large exoplanet with a low density
^Albrecht, Simon; Winn, Joshua N.; Johnson, John A.; Howard, Andrew W.; Marcy, Geoffrey W.; Butler, R. Paul; Arriagada, Pamela; Crane, Jeffrey D.; Shectman, Stephen A.; Thompson, Ian B.; Hirano, Teruyuki; Bakos, Gaspar; Hartman, Joel D. (2012), "Obliquities of Hot Jupiter Host Stars: Evidence for Tidal Interactions and Primordial Misalignments", The Astrophysical Journal, 757 (1): 18, arXiv:1206.6105, Bibcode:2012ApJ...757...18A, doi:10.1088/0004-637X/757/1/18, S2CID17174530
^Saba, Arianna; Tsiaras, Angelos; Morvan, Mario; Thompson, Alexandra; Changeat, Quentin; Edwards, Billy; Jolly, Andrew; Waldmann, Ingo; Tinetti, Giovanna (2022), "The Transmission Spectrum of WASP-17 b from the Optical to the Near-infrared Wavelengths: Combining STIS, WFC3, and IRAC Data Sets", The Astronomical Journal, 164 (1): 2, arXiv:2108.13721, Bibcode:2022AJ....164....2S, doi:10.3847/1538-3881/ac6c01, S2CID237363318
^Alderson, L.; Wakeford, H. R.; MacDonald, R. J.; Lewis, N. K.; May, E. M.; Grant, D.; Sing, D. K.; Stevenson, K. B.; Fowler, J.; Goyal, J.; Batalha, N. E.; Kataria, T. (2022), "A comprehensive analysis of WASP-17b's transmission spectrum from space-based observations", Monthly Notices of the Royal Astronomical Society, 512 (3): 4185–4209, arXiv:2203.02434, doi:10.1093/mnras/stac661