WASP-4b is an exoplanet, specifically a hot Jupiter, approximately 891 light-years away in the constellation of Phoenix.

Discovery

The planet was the discovered by the Wide Angle Search for Planets team using images taken with the SuperWASP-South project's eight wide-angle cameras located at the South African Astronomical Observatory. Analysis of over 4000 images taken between May and November 2006 resulted in the detection of a transit occurring every 1.3 days. Follow-up radial velocity observations using the Swiss 1.2-metre Leonhard Euler Telescope confirmed that the transiting object was a planet.

Characteristics

The planetary equilibrium temperature would be 1650 K, but the measured dayside temperature is higher, with a 2015 study finding 1900 K and a 2020 study finding 1957 K. A study in 2012, utilizing the Rossiter–McLaughlin effect, determined the planetary orbit is probably aligned with the equatorial plane of the star, with misalignment equal to -1°. The planet's orbital period appears to be decreasing at a rate of 7.33 milliseconds per year, suggesting that its orbit is decaying, with a decay timescale of 15.77 million years. The anomalously high rate of orbital decay of WASP-4b is poorly understood as of 2021.