 Artist impression of Ditsö̀ | |
| Discovery[1] | |
|---|---|
| Discovered by | David R. Andersonet al |
| Discovery date | 11 August 2009 |
| Transit (including secondary eclipse) | |
| Orbital characteristics[2] | |
| 0.05151±0.00035 AU | |
| Eccentricity | <0.020 |
| 3.7354845±0.0000019 d | |
| Inclination | 86.83°+0.68° −0.53° |
| −70 or 210[citation needed] | |
| Semi-amplitude | 56.0+4.1 −4.0 m/s |
| Star | WASP-17 |
| Physical characteristics[2] | |
| 1.991±0.081 RJ | |
| Mass | 0.512±0.037 MJ |
Meandensity | 0.080+0.013 −0.011 g/cm3 |
| Temperature | 1,550+170 −200 K[3] |
WASP-17b, officially namedDitsö̀[pronunciation?], is anexoplanet in theconstellationScorpius that is orbiting thestarWASP-17. Its discovery was announced on 11 August 2009.[1] It is the first planet discovered to have aretrograde orbit, meaning it orbits in a direction counter to the rotation of its host star.[1] This discovery challenged traditional planetary formation theory.[4] In terms of diameter, WASP-17b is one of thelargest exoplanets discovered and at halfJupiter'smass, this made it the mostpuffy planet known in 2010.[5] On 3 December 2013, scientists working with theHubble Space Telescope reported detectingwater in theexoplanet'satmosphere.[6][7]
WASP-17b's name was selected in theNameExoWorlds campaign byCosta Rica, during the 100th anniversary of theInternational Astronomical Union. Ditsö̀ is the name that the godSibö̀ gave to the firstBribri people inTalamancan mythology.[8][9]
A team of researchers led by David Anderson ofKeele University inStaffordshire,England,discovered thegas giant, which is about 1,000light-years (310parsecs) from Earth, by observing ittransiting its host starWASP-17. Suchphotometric observations also reveal the planet's size. The discovery was made with a telescope array at theSouth African Astronomical Observatory. Due to the involvement of theWide Angle Search for Planets (SuperWASP) consortium of universities, the exoplanet, as the 17th found to date by this group, was given its present name.[10]
Astronomers at theObservatory of Geneva were then able to use characteristicredshifts andblueshifts in the host star'sspectrum as itsradial velocityvaried over the course of the planet's orbit to measure the planet's mass and obtain an indication of itsorbital eccentricity.[1] Careful examination of theDoppler shifts during transits also allowed them to determine the direction of the planet's orbital motion relative to its parent star's rotation via theRossiter–McLaughlin effect.[1]
WASP-17b is thought to have aretrograde orbit (with a sky-projectedinclination of the orbit normal against the stellar spin axis of about 149°,[11] not to be confused with theline-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 theRossiter–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 agravitational 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 theKozai mechanism.[12]Spin-orbit angle measurement was updated in 2012 to −148.7+7.7
−6.7°.[13]
WASP-17b has aradius between 1.5 and 2 times that ofJupiter and about half themass.[1] Thus its meandensity is between 0.08 and 0.19 g/cm3,[1] compared with Jupiter's 1.326 g/cm3[14] andEarth'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 betweenMercury and theSun), leading totidal flexing and heating of its interior.[1] The same mechanism is behind the intense volcanic activity of Jupiter's moonIo.WASP-39b has a similarly low estimated density.
Exoplanetarysodium in theatmosphere of theWASP-17 has been detected in 2018,[3] but was not confirmed by 2021. Instead, the spectral signatures ofwater,aluminium oxide (AlO) andtitanium hydride (TiH) were detected.[15] Thewater signature was confirmed in 2022, together withcarbon dioxide absorption.[16] In 2023, evidence ofclouds made ofquartz was detected on theplanet by theJames Webb Space Telescope.[17][18]
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