WASP-4b

WASP-4b is an exoplanet, specifically a hot Jupiter, approximately 891 light-years away^[5] in the constellation of Phoenix.^[6]

WASP-4b

Size comparison of WASP-4b with Jupiter.
Discovery[1]
Discovered by	Wide Angle Search for Planets
Discovery site	South African Astronomical Observatory
Discovery date	October 31, 2007
Detection method	Transit photometry
Orbital characteristics[2]
Semi-major axis	0.02239±0.00084 AU
Eccentricity	<0.0033[3]
Orbital period (sidereal)	1.338231587(22) d
Inclination	88.02°±0.69°
Semi-amplitude	232.7+2.5 −2.2 m/s[3]
Star	WASP-4
Physical characteristics[2]
Mean radius	1.312±0.045 RJ
Mass	1.164±0.082 MJ
Mean density	0.639±0.079 g/cm3
Temperature	1957±68 K (1,684 °C; 3,063 °F)[4]

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.^[7][8][1] 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.^[1]

The radial velocity trend of WASP-4, caused by the presence of WASP-4 b.

Characteristics

The planetary equilibrium temperature would be 1650±30 K,^[9] but the measured dayside temperature is higher, with a 2015 study finding 1900±100 K^[10] and a 2020 study finding 1957±68 K.^[4]

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⁺¹⁴
₋₁₂°.^[11]

Orbital decay and tidal interactions

The planet's orbital period appears to be decreasing at a rate of 7.33±0.71 milliseconds per year, suggesting that its orbit is decaying, with a decay timescale of 15.77±1.57 million years. The anomalously high rate of orbital decay of WASP-4b was poorly understood as of 2021.^[2] In 2025, the planet has been confirmed to have its orbit in decay. This decay is primarily driven by tidal interactions between the planet and its star. Studies based on Transit timing variations (TTVs) indicate that the planet’s orbit is shrinking due to energy dissipation caused by stellar tides. A tidal quality factor of approximately 80,000 was derived from these observations, suggesting that the star is more evolved than previously thought. This phenomenon is consistent with a model of tidal dissipation involving internal gravity waves within the star, although further modeling of the star's properties is needed for a complete understanding.^[12]