WASP-15b

Lore & Background

WASP-15b was first observed by the WASP-South branch of the SuperWASP project, operating from the South African Astronomical Observatory, between May 4, 2006, and July 17, 2006. Further observations occurred from January 31, 2007, to July 17, 2007, and from January 31, 2008, to May 29, 2008. Analysis of 24,943 collected data points revealed eleven full or partial transits. Follow-up observations at the 1.2 m Leonhard Euler Telescope at La Silla Observatory in Chile, using the CORALIE spectrograph between March 6, 2008, and July 17, 2008, ruled out an eclipsing binary star system as the cause of radial velocity variations. The spectra from CORALIE and the High Resolution Echelle Spectrometer (HIRES) were used to derive the star's characteristics, and best-fit models indicated the presence of a planet.

The planet orbits its host star, WASP-15, at a distance of 0.05 AU every 3.7520656 days. Its mass is 0.542 times that of Jupiter, but its radius is 1.428 times Jupiter's, giving it a density of 0.247 g/cm³. This extreme inflation is thought to require some other form of heating beyond stellar irradiation. A 2012 study using the Rossiter–McLaughlin effect determined the planetary orbit is strongly misaligned with the star's equatorial plane, with a spin-orbit angle of -139.6°, making the orbit retrograde.

WASP-15 is an F-type star in the Centaurus constellation, located approximately 290 parsecs (900 light years) from Earth with an apparent magnitude of 10.9. It is 1.18 times more massive than the Sun, with a radius 1.477 times larger, an effective temperature of 6300 K, and an age of 3.9 billion years. Its metallicity is [Fe/H] = -0.17, meaning it has 68% of the iron found in the Sun. In 2019, the IAU announced that WASP-15 and WASP-15b would receive official names chosen by school children from the Ivory Coast.

Reader's Guide

WASP-15b's significance lies in its extremely low density, which was among the lowest known for extrasolar planets at the time of its discovery. With a density only one quarter that of water, the planet is greatly inflated despite its close orbit of 0.05 AU from its host star. This inflation is not fully explained by proximity alone, and an internal heat source is suspected to contribute. The planet's retrograde orbit, with a spin-orbit misalignment of -139.6°, adds to its interest, as such misaligned orbits provide clues about planetary migration and dynamical interactions. The discovery was published on April 29, 2009, in the Astronomical Journal by the American Astronomical Society. The planet was detected by the SuperWASP project, which specializes in finding transiting exoplanets, and its existence was confirmed through radial velocity measurements from the CORALIE spectrograph and HIRES. The naming of the planet and its star by Ivory Coast school children in 2019 under the NameExoWorlds program highlights its cultural as well as scientific legacy.

Did You Know?

• WASP-15b has a density of only 0.247 g/cm³, one quarter that of water.
• Its orbit is retrograde, with a spin-orbit misalignment of -139.6° relative to its host star.
• The planet was discovered using data from the WASP-South branch of the SuperWASP project.
• WASP-15b was formally named Asye in 2019 by school children from the Ivory Coast.