WASP-15b, formally named Asye, is an

extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

discovered in 2008 by the

SuperWASP WASP or Wide Angle Search for Planets is an international consortium of several academic organisations performing an ultra-wide angle search for exoplanets using transit photometry. The array of robotic telescopes aims to survey the entire sky, ...

collaboration, which seeks to discover exoplanets that

transit Transit may refer to: Arts and entertainment Film * ''Transit'' (1979 film), a 1979 Israeli film * ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world * ''Transit'' (2006 film), a 2006 ...

their host stars. The planet orbits its host star at a distance of 0.05 AU every four days. The mass of this planet is about one half that of

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...

, but its radius is nearly 50% larger than Jupiter's, making the density of this planet only one quarter that of water; it is thought that some other form of heating must explain its extremely low density. WASP-15b's discovery was published on April 29, 2009.

Discovery

WASP-15 was first observed by the WASP-South branch of the

project, which operates from the

South African Astronomical Observatory South African Astronomical Observatory (SAAO) is the national centre for optical and infrared astronomy in South Africa. It was established in 1972. The observatory is run by the National Research Foundation of South Africa. The facility's func ...

, between May 4, 2006, and July 17, 2006. It was later observed by WASP-South from January 31, 2007, to July 17, 2007, and from January 31, 2008, to May 29, 2008. Further analysis taken from 24,943 collected data points revealed eleven full or partial transits. Follow-up observations were conducted by a European and American science team at the 1.2 m

Leonhard Euler Telescope Leonhard Euler Telescope, or the Swiss EULER Telescope, is a national, fully automatic reflecting telescope, built and operated by the Geneva Observatory. It is located at an altitude of at ESO's La Silla Observatory site in the Chilean Norte ...

La Silla Observatory La Silla Observatory is an astronomical observatory in Chile with three telescopes built and operated by the European Southern Observatory (ESO). Several other telescopes are located at the site and are partly maintained by ESO. The observatory is ...

in Chile, which further raised the possibility of the existence of a planet in WASP-15's orbit; use of the

CORALIE spectrograph Leonhard Euler Telescope, or the Swiss EULER Telescope, is a national, fully automatic reflecting telescope, built and operated by the Geneva Observatory. It is located at an altitude of at ESO's La Silla Observatory site in the Chilean Nor ...

on the Euler Telescope between March 6, 2008, and July 17, 2008, revealed that the variations in radial velocity measurements were not because of an

eclipsing binary A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...

star system. CORALIE and the

High Resolution Echelle Spectrometer The W. M. Keck Observatory is an astronomical observatory with two telescopes at an elevation of 4,145 meters (13,600 ft) near the summit of Mauna Kea in the U.S. state of Hawaii. Both telescopes have aperture primary mirrors, and when co ...

(HIRES) revealed the spectrum of WASP-15, which was used to derive the star's characteristics. The science team studying WASP-15 found that, after running best-fit models, WASP-15's radial velocity and transit shifts were most likely due to the existence of a planet. WASP-15's planet, WASP-15b, had one of the lowest densities known amongst extrasolar planets when it was discovered. Its discovery paper was published by the American Astronomical Society on April 29, 2009, in the ''

Astronomical Journal ''The Astronomical Journal'' (often abbreviated ''AJ'' in scientific papers and references) is a peer-reviewed monthly scientific journal owned by the American Astronomical Society (AAS) and currently published by IOP Publishing. It is one of the ...

''.

Host star

WASP-15 is an F-type star located in the

Centaurus Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. I ...

constellation. It is located approximately 290

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

s (900

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s) from Earth and has an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

of 10.9, making it invisible to the unaided eye. The star is 1.18 times more massive than the Sun, and has a

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

that is 1.477 times larger than that of the Sun, making it more diffuse. WASP-15 has an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 6300 K, and is thus hotter than the Sun, although at 3.9 billion years, it is also younger. WASP-15 has a

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

of e/Hof -0.17, which means that it has 68% of the iron found in the Sun.

Characteristics

WASP-15b has a

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...

of 0.542 times Jupiter's mass and a

that is 1.428 times Jupiter's radius. Due in part to its proximity to its host star, a distance of 0.0499 AU (7,500,000 km), WASP-15b is greatly inflated, with a density of 0.247 g/cm³. Another factor, such as an internal heat source, is suspected to add to this extremely high radius and extremely low density. WASP-15b orbits its host star every 3.7520656 days. It also has an

orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Ear ...

of 85.5º, making it almost edge-on as seen from the Earth's perspective. The study in 2012, utilizing a

Rossiter–McLaughlin effect The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when an object moves across the face of a star. Description The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's second ...

, have determined the planetary orbit is strongly misaligned with the equatorial plane of the star, misalignment equal to -139.6°. This spin-orbit angle is making orbit of WASP-15b retrograde.

Naming

In 2019 the

IAU The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

announced as part of

NameExoWorlds NameExoWorlds (also known as IAU NameExoWorlds) is the name of various projects managed by the International Astronomical Union (I.A.U.) to encourage names to be submitted for astronomical objects, which would later be considered for official ad ...

that WASP-15 and its planet WASP-15b would be given official names chosen by school children from

The Ivory Coast Ivory Coast, also known as Côte d'Ivoire, officially the Republic of Côte d'Ivoire, is a country on the southern coast of West Africa. Its capital is Yamoussoukro, in the centre of the country, while its largest city and economic centre is ...

References

External links