WASP-17b
WASP-17b is a transiting hot Jupiter exoplanet orbiting the F-type star WASP-17, notable for its exceptionally large radius, low density, and retrograde orbital motion.[1][2] Discovered in 2009 via the transit method by the Wide Angle Search for Planets (WASP) consortium, it completes an orbit every 3.74 days at a semi-major axis of approximately 0.051 AU, placing it in a close-in orbit that results in an equilibrium temperature of around 1,700 K.[1][2] With a mass of 0.49 Jupiter masses and a radius of 1.83 Jupiter radii, WASP-17b exhibits one of the lowest densities among known exoplanets at about 0.09 g/cm³, suggesting significant atmospheric inflation due to internal heating and stellar irradiation.[2][3] The host star WASP-17 is an F4-type dwarf located approximately 406 parsecs away in the constellation Scorpius, with an effective temperature of about 6,500 K and a radius 1.3 times that of the Sun.[2] WASP-17b's orbit is inclined at nearly 87° to our line of sight, enabling precise transit observations, but it features a rare retrograde spin-orbit misalignment of -149°, indicating a likely dynamical history involving gravitational interactions with companion bodies.[2] This misalignment was confirmed through spectroscopic measurements shortly after discovery. Atmospheric studies of WASP-17b have revealed a hydrogen- and helium-dominated envelope with evidence of water vapor and other molecules, observed via transmission spectroscopy with the Hubble Space Telescope.[4] More recently, the James Webb Space Telescope (JWST) detected tiny quartz (SiO₂) crystals in the planet's clouds during a 2023 observation, marking the first identification of silicates in an exoplanet atmosphere and providing insights into high-temperature cloud formation processes.[5][6] JWST observations in 2024 further measured a precise super-solar water abundance in the transmission spectrum and confirmed supersolar metallicity in the dayside atmosphere.[7][8] The planet's dayside emission shows non-uniform temperatures, with JWST Mid-Infrared Instrument (MIRI) data indicating a cooler nightside and potential heat redistribution inefficiencies. These observations highlight WASP-17b as a key target for understanding the formation, evolution, and atmospheric dynamics of inflated gas giants.[2]Discovery and Nomenclature
Discovery
WASP-17b was discovered through the transit method as part of the Wide Angle Search for Planets (WASP) consortium survey and announced on August 11, 2009, via submission to arXiv by the discovery team.[9] The planet was initially detected using photometric observations from the SuperWASP-South telescope array, which recorded 15,509 measurements of the host star between 2006 and 2008, identifying periodic dips in brightness indicative of a transiting exoplanet.[1] The discovery was led by David R. Anderson and a collaborative team from institutions including Keele University in the United Kingdom and the Geneva Observatory in Switzerland.[1] Confirmation followed through radial velocity measurements obtained with the CORALIE spectrograph on the Euler 1.2 m telescope at La Silla Observatory in Chile, supplemented by high-precision spectra from the HARPS instrument on the ESO 3.6 m telescope, which revealed the planet's orbital motion around the host star.[1] These observations ruled out false positives and provided initial constraints on the system's parameters. The initial findings, including the first light curve analysis using Markov Chain Monte Carlo methods to model the transit and determine an orbital period of approximately 3.74 days, were detailed in a paper published in the Astrophysical Journal in 2010.[1] Ground-based follow-up photometry was conducted at the South African Astronomical Observatory using the EulerCam on the Euler-Swiss telescope to confirm the transit depth and timing, ensuring the signal's consistency with a planetary transit.[1] This discovery marked WASP-17b as the first exoplanet suggested to possess a retrograde orbit, opposite to the direction of its host star's rotation.[1]Nomenclature
WASP-17b received its provisional designation upon discovery, following the standard convention for exoplanets detected by the Wide Angle Search for Planets (WASP) survey, where the host star is numbered sequentially (WASP-17) and planets are lettered alphabetically starting with 'b' for the innermost or first confirmed. In December 2019, as part of the International Astronomical Union (IAU)'s centennial NameExoWorlds contest, the system was assigned to Costa Rica for public naming, resulting in the official approval of Ditsö̀ for the planet and Dìwö for the host star. These names derive from the Bribri language of the indigenous Talamanca people in Costa Rica; Dìwö means "the Sun," while Ditsö̀ refers to the name bestowed by the creator god Sibö̀ upon the first Bribri people in Talamancan mythology, symbolizing reflection and origin. The IAU's exoplanet naming guidelines, established to promote global participation and cultural diversity, require that official names for exoplanets and their host stars form a thematic pair, draw from mythology, literature, or cultural heritage (preferring indigenous or lesser-known traditions), and avoid references to individuals, places, brands, or politically sensitive terms. This contest, held during the IAU's 100th anniversary, encouraged submissions from national organizing committees to foster international collaboration in astronomy and highlight underrepresented cultural narratives in celestial nomenclature.Host Star
Characteristics
WASP-17 is classified as an F6V main-sequence star with an effective temperature of 6550 ± 100 K.[1] Its mass is determined to be 1.20^{+0.10}{-0.11} M\sun, and its radius measures 1.38^{+0.20}{-0.18} R\sun.[1] The star exhibits sub-solar metallicity, with an iron abundance of [Fe/H] = -0.25 ± 0.09.[1] Isochrone fitting yields an estimated age for WASP-17 of 3.0^{+0.9}_{-2.6} Gyr.[1] The star displays low chromospheric activity. WASP-17 shows moderate rotational broadening, with a projected equatorial velocity of v \sin i = 9.0 ± 1.5 km s^{-1}, which corresponds to an expected rotation period of approximately 8.5–11 days given the stellar radius.[1]Location and Visibility
The WASP-17 system is located in the constellation Scorpius, at equatorial coordinates of right ascension 15ʰ 59ᵐ 51ˢ and declination −28° 03′ 42″ (J2000 epoch).[2] It lies approximately 1,310 light-years (403 parsecs) from Earth, a distance refined through parallax measurements from the Gaia mission's Data Release 3 (2022).[2][10] This places the system in a region of the southern celestial sky, accessible primarily to observers in the Southern Hemisphere. The host star WASP-17 has an apparent visual magnitude of V = 11.6, rendering it faint enough to require mid-sized telescopes (typically 8–12 inches in aperture) for detailed observation under dark skies.[1] Due to its southern declination, the system is best observed from latitudes south of 30° N, where it reaches higher altitudes and avoids horizon obstruction. Seasonal visibility peaks in the evening sky from April to July for Southern Hemisphere observers, when Scorpius transits near midnight.[11] In Galactic coordinates, WASP-17 resides at longitude 346° and latitude +19°, positioning it in the general direction of the Galactic plane and subject to moderate interstellar reddening along the line of sight.[2] Observations account for an estimated E(B–V) reddening of about 0.05 magnitudes, which minimally affects photometric studies but is corrected for in spectral analyses.[12]Orbital Characteristics
Parameters
The orbital parameters of WASP-17b were determined primarily through analysis of photometric transit light curves and radial velocity measurements, providing key geometric elements of its orbit around the host star WASP-17. These parameters describe the size, shape, and orientation of the orbit, essential for modeling the planet's transit events and dynamical evolution. The values reflect refinements from multiple observations, confirming a close-in, short-period orbit typical of hot Jupiters.| Parameter | Value | Source |
|---|---|---|
| Orbital period (P) | 3.735485 ± 0.000002 days | [13] |
| Semi-major axis (a) | 0.05151 ± 0.00035 AU | [14] |
| Eccentricity (e) | < 0.020 (nearly circular) | [14] |
| Inclination (i) | 87.22° ± 0.14° | [13] |