WASP-4b
A hot Jupiter with debated orbital decay and a wide-orbit companion.
WASP-4b is a hot Jupiter exoplanet located approximately 891 light-years away in the constellation of Phoenix. It was discovered by the Wide Angle Search for Planets team using data from the SuperWASP-South project's eight wide-angle cameras at the South African Astronomical Observatory, with follow-up radial velocity observations confirming its planetary nature.
Quick Facts
- Discoverer
- Wide Angle Search for Planets
- Discovery Site
- South African Astronomical Observatory
- Discovered
- October 31, 2007
- Discovery Method
- Transit photometry
- Apsis
- astron
- Eccentricity
- <0.0033
- Star
- WASP-4
Facts from the source article.
Lore & Background
WASP-4b was discovered through analysis of over 4,000 images taken between May and November 2006, which revealed a transit recurring every 1.3 days. The Swiss 1.2-metre Leonhard Euler Telescope provided radial velocity measurements that confirmed the transiting object was a planet. Its planetary equilibrium temperature is 1,650 K, though measured dayside temperatures are higher: a 2015 study found 1,900 K, and a 2020 study found 1,957 K. A 2012 study using the Rossiter–McLaughlin effect determined the planet's orbit is probably aligned with the star's equatorial plane, with a misalignment of -1°.
Reader's Guide
WASP-4b gained attention for an apparent orbital decay, with its orbital period decreasing at 7.33 milliseconds per year, suggesting a decay timescale of 15.77 million years. This anomalously high rate was poorly understood as of 2021, thought to be driven by tidal interactions with its host star. However, in late 2025 it was determined that WASP-4b is not undergoing any orbital decay; the data previously interpreted as decay is instead explained by the light travel time effect of a wider-orbit planet, WASP-4c. This resolution highlights the importance of considering multiple-body effects in exoplanet timing studies and underscores how apparent orbital decay can be misattributed without accounting for additional companions. The planet remains a key example of a hot Jupiter with a well-characterized atmosphere and orbital geometry.
Did You Know?
- Its measured dayside temperature ranges from 1,900 K to 1,957 K depending on the study.
- The planet's orbit is probably aligned with its star's equator, with a misalignment of -1°.
- A 2025 study determined that WASP-4b is not undergoing orbital decay; the apparent decay was due to the light travel time effect of WASP-4c.
Frequently Asked Questions
What is WASP-4b?
WASP-4b is a hot Jupiter exoplanet situated roughly 891 light-years from Earth in the constellation Phoenix. It orbits its host star so closely that it completes one full lap in just 1.3 days.
How was WASP-4b discovered?
The Wide Angle Search for Planets team identified it by detecting its transit signal using the eight wide-field cameras of the SuperWASP-South array at the South African Astronomical Observatory. Radial-velocity measurements were then taken to confirm that the dimming was caused by a genuine planet rather than a stellar artifact.
How hot is WASP-4b's surface?
Its equilibrium temperature works out to about 1,650 K, but direct measurements of the dayside have pushed the figure higher—roughly 1,900 K in a 2015 study and 1,957 K in a 2020 analysis. That makes it one of the most intensely heated worlds known.
What is WASP-4b's orbital period?
It circles its star in approximately 1.3 days, classifying it as an ultra-short-period hot Jupiter. That extremely tight orbit is what drives its scorching temperatures and keeps it locked in a permanently scorching state.
Why do astronomers find WASP-4b particularly interesting?
There is an ongoing debate about whether its orbit is slowly shrinking over time, which would eventually doom the planet to spiral into its star. Additionally, a wide-orbit companion has been identified in the system, adding a layer of dynamical complexity that makes WASP-4b a favorite case study for planetary-evolution models.
More in Transiting Exoplanets, Part 5 1-24
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