WASP-17b

Lore & Background

A team led by David Anderson of Keele University discovered WASP-17b by observing it transit its host star using a telescope array at the South African Astronomical Observatory. The planet was the 17th found by the SuperWASP consortium, giving it its name. Astronomers at the Observatory of Geneva then measured its mass and orbital eccentricity via radial velocity variations, and determined the retrograde orbit through the Rossiter–McLaughlin effect. The spin-orbit angle was updated in 2012 to −148.7°.

WASP-17b's retrograde orbit is thought to result from a gravitational slingshot from a near-collision with another planet or the Kozai mechanism involving a smaller body. Its unusually low density is attributed to tidal flexing and heating from its orbital eccentricity and proximity to its star (less than one seventh the distance of Mercury from the Sun), similar to the mechanism driving Io's volcanism. In 2013, water was detected in its atmosphere; later observations found spectral signatures of water, aluminium oxide, titanium hydride, and carbon dioxide. In 2023, the James Webb Space Telescope detected evidence of quartz clouds.

Reader's Guide

WASP-17b's discovery on 11 August 2009 marked a milestone as the first exoplanet known to orbit its star in a retrograde direction, directly challenging conventional models of planetary system formation. Its extremely low density—among the lowest known at the time—made it a benchmark for studying inflated hot Jupiters and the role of tidal heating. The detection of water in its atmosphere in 2013, followed by signatures of aluminium oxide, titanium hydride, carbon dioxide, and quartz clouds in subsequent years, has made it a key target for atmospheric characterization. The planet's name, Ditsö̀, was chosen by Costa Rica in the NameExoWorlds campaign during the 100th anniversary of the International Astronomical Union, linking it to the mythology of the Bribri people. Its retrograde orbit and puffy nature continue to inform theories of planetary migration and atmospheric escape.

Did You Know?

• WASP-17b was the first exoplanet discovered to have a retrograde orbit.
• Its mean density is between 0.08 and 0.19 g/cm³, lower than water's density of 1 g/cm³.
• Its official name, Ditsö̀, comes from Talamancan mythology and was selected by Costa Rica.