WASP-33b is an extrasolar planet orbiting the star HD 15082. It was the first planet discovered to orbit a Delta Scuti variable star. With a semimajor axis of 0.026AU (3.9millionkm; 2.4millionmi) and a mass likely greater than Jupiter's,[1] it belongs to the hot Jupiter class of planets.
In 2010, the SuperWASP project announced the discovery of an extrasolar planet orbiting the star HD 15082. The discovery was made by detecting the transit of the planet as it passes in front of its star, an event that occurs every 1.22days.
Orbit
A study in 2012, utilizing the Rossiter–McLaughlin effect, determined the planetary orbit is strongly misaligned with the equatorial plane of the star, misalignment equal to −107.7±1.6°, making the orbit of WASP-33b retrograde.[4] The periastron node is precessing with the period of 709+33 −34 years.[5]
Physical characteristics
Limits from radial velocity measurements imply it has less than 4.1 times the mass of Jupiter.[1] The exoplanet orbits so close to its star that its surface temperature is about 3,200°C (5,790°F).[6] The transit was later recovered in Hipparcos data.[7]
Atmosphere
In June 2015, NASA reported the exoplanet has a stratosphere, and the atmosphere contains titanium monoxide, which creates the stratosphere. Titanium oxide is one of only a few compounds that is a strong absorber of visible and ultraviolet radiation, which heats the atmosphere, and is able to exist in a gas state in a hot atmosphere.[8][9] The detection of temperature inversion (stratosphere), water and titanium oxide was disproved with the higher quality data obtained by 2020. Only upper limit of titanium oxide volume mixing rate equal to 1 ppb can be obtained.[10] Later research reconfirmed the existence of titanium oxide in the atmosphere of WASP-33b, although in concentrations not detectable by HARPS-N. The neutral iron[11] and silicon[12] were also detected.
Atmosphere of WASP-33b was detected by monitoring light as the planet passed behind its star (top)—higher temperatures result in the low stratosphere due to molecules absorbing radiation from the star (right)—lower temperatures at higher altitudes would result if there were no stratosphere (left)[8]
In 2020, with the detection of secondary eclipses (when the planet is blocked by its star), the mass of the planet along with temperature profile across its surface was measured. WASP-33b has strong winds in its atmosphere, similar to Venus, shifting the hottest spot 28.7±7.1 degrees to the west. The averaged wind speed is 8.5+2.1 −1.9 km/s in the thermosphere.[13] The illuminated side brightness temperature is 3,014±60K (2,740.8±60.0°C; 4,965.5±108.0°F), while the nightside brightness temperature is 1,605±45K (1,331.8±45.0°C; 2,429.3±81.0°F).[3]
The atmospheric escape driven by hydrogen Balmer line absorption is relatively modest, totaling about one to ten Earth masses per billion years.[14]
The water in dayside atmosphere of WASP-33b is mostly dissociated to hydroxyl radicals due to high temperature, as planetary emission spectra indicated.[15]
Non-Keplerian features of motion for WASP-33b
In view of the high rotational speed of its parent star, the orbital motion of WASP-33b may be affected in a measurable way by the huge oblateness of the star and effects of general relativity.
First, the distorted shape of the star makes its gravitational field deviate from the usual Newtonian inverse-square law. The same is true for the Sun, and part of the precession of the orbit of Mercury is due to this effect. However, it is estimated to be greater for WASP-33b.[16]
Other effects will also be greater for WASP-33b. In particular, precession due to general relativistic frame-dragging should be greater for WASP-33b than for Mercury, where it is so far too small to have been observed. It has been argued that the oblateness of HD 15082 could be measured at a percent accuracy from a 10-year analysis of the time variations of the planet's transits.[16] Effects due to the planet's oblateness are smaller by at least one order of magnitude, and they depend on the unknown angle between the planet's equator and the orbital plane, perhaps making them undetectable. The effects of frame-dragging are slightly too small to be measured by such an experiment.
Nodal precession of WASP-33b, caused by oblateness of the parent star, was measured by 2021. The gravitational quadrupole moment of the HD 15082 was found to be equal to 6.73±0.22×10−5. The non-Keplerian precession is expected to be 500 times smaller, yet to be detected.[17]
Zhang, Michael; etal. (2017). "Phase curves of WASP-33b and HD 149026b and a New Correlation Between Phase Curve Offset and Irradiation Temperature". The Astronomical Journal. 155 (2): 83. arXiv:1710.07642. Bibcode:2018AJ....155...83Z. doi:10.3847/1538-3881/aaa458. S2CID54755276.
von Essen, C.; Mallonn, M.; Borre, C. C.; Antoci, V.; Stassun, K. G.; Khalafinejad, S.; Tautvaivsiene, G. (2020). "TESS unveils the phase curve of WASP-33b. Characterization of the planetary atmosphere and the pulsations from the star". Astronomy & Astrophysics. A34: 639. arXiv:2004.10767. Bibcode:2020A&A...639A..34V. doi:10.1051/0004-6361/202037905. S2CID216080995.
Watanabe, Noriharu; Narita, Norio; Palle, Enric (March 3, 2022). "Nodal Precession of WASP-33b for Eleven Years by Doppler Tomographic and Transit Photometric Observations". Monthly Notices of the Royal Astronomical Society. arXiv:2203.02003. doi:10.1093/mnras/stac620.
McDonald, I.; Kerins, E. (2018). "Pre-discovery transits of the exoplanets WASP-18b and WASP-33b from Hipparcos". Monthly Notices of the Royal Astronomical Society. 477 (1): L21. arXiv:1803.06187. Bibcode:2018MNRAS.477L..21M. doi:10.1093/mnrasl/sly045. S2CID49547292.
Nugroho, Stevanus K.; Kawahara, Hajime; Gibson, Neale P.; De Mooij, Ernst J. W.; Hirano, Teruyuki; Kotani, Takayuki; Kawashima, Yui; Masuda, Kento; Brogi, Matteo; Birkby, Jayne L.; Watson, Chris A.; Tamura, Motohide; Zwintz, Konstanze; Harakawa, Hiroki; Kudo, Tomoyuki; Kuzuhara, Masayuki; Hodapp, Klaus; Ishizuka, Masato; Jacobson, Shane; Konishi, Mihoko; Kurokawa, Takashi; Nishikawa, Jun; Omiya, Masashi; Serizawa, Takuma; Ueda, Akitoshi; Vievard, Sébastien (2021), "First Detection of Hydroxyl Radical Emission from an Exoplanet Atmosphere: High-dispersion Characterization of WASP-33b Using Subaru/IRD", The Astrophysical Journal Letters, 910 (1): L9, arXiv:2103.03094, Bibcode:2021ApJ...910L...9N, doi:10.3847/2041-8213/abec71, S2CID232110452
Iorio, Lorenzo (2010-07-25), "Classical and relativistic node precessional effects in WASP-33b and perspectives for detecting them", Astrophysics and Space Science, 331 (2): 485–496, arXiv:1006.2707, Bibcode:2011Ap&SS.331..485I, doi:10.1007/s10509-010-0468-x, S2CID119253639
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