WASP-17b was the first exoplanet discovered with a retrograde orbit around its star.
A retrograde orbit travels opposite to the direction in which the host star rotates. Astronomers identified WASP-17b’s unusual motion by measuring the Rossiter–McLaughlin effect: during a transit, the planet temporarily blocks portions of the rotating star, leaving a Doppler signature that reveals the orbit’s direction.
The discovery was announced in 2009 and challenged the expectation that planets should orbit in the same general direction as their stars. WASP-17b is also an exceptionally inflated hot Jupiter, with roughly half Jupiter’s mass but a radius around 1.5–2 times Jupiter’s.
A common mix-up is HAT-P-7b, whose retrograde nature was announced soon afterward. HAT-P-7b was discovered first, but WASP-17b was the first whose retrograde orbit was reported, making the stored answer the standard answer to this wording.