WASP-17b is an

exoplanet 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 ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellation ...

Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gr ...

that is orbiting the

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

WASP-17. Its discovery was announced on 11 August 2009. It is the first planet discovered to have a retrograde orbit, meaning it orbits in a direction counter to the rotation of its host star. This discovery challenged traditional planetary formation theory. In terms of diameter, WASP-17b is one of the largest exoplanets discovered and at half

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-thousand ...

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 ele ...

, this made it the most

puffy planet Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter but that have very short orbital periods (). The close proximity to their stars and high surface-atmosphere temp ...

known in 2010. On 3 December 2013, scientists working with the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

reported detecting

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

in the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

. WASP-17b is named Ditsö̀. The name was selected in the

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 ...

campaign by

Costa Rica Costa Rica (, ; ; literally "Rich Coast"), officially the Republic of Costa Rica ( es, República de Costa Rica), is a country in the Central American region of North America, bordered by Nicaragua to the north, the Caribbean Sea to the no ...

, during the 100th anniversary of the

International Astronomical Union 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, outreac ...

. Ditsö̀ is the name that the god Sibö̀ gave to the first

Bribri people The Bribri are an Indigenous people in eastern Costa Rica and northern Panama. Today, most Bribri people speak the Bribri language or Spanish. There are varying estimates from government officials of the group's population. Estimates of the to ...

Talamancan mythology Talamancan mythology includes the traditional beliefs of the Bribri and Cabécar peoples, two groups of indigenous peoples in Costa Rica living in the Talamanca region. These peoples speak two different but closely related languages, and from a ...

Discovery

A team of researchers led by David Anderson of

Keele University Keele University, officially known as the University of Keele, is a public research university in Keele, approximately from Newcastle-under-Lyme, Staffordshire, England. Founded in 1949 as the University College of North Staffordshire, Keele ...

Staffordshire Staffordshire (; postal abbreviation Staffs.) is a landlocked county in the West Midlands region of England. It borders Cheshire to the northwest, Derbyshire and Leicestershire to the east, Warwickshire to the southeast, the West Midlands C ...

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...

, discovered the

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" ...

, which is about from Earth, by observing it transiting its host star WASP-17. Such photometric observations also reveal the planet's size. The discovery was made with a telescope array at 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 funct ...

. Due to the involvement of the

Wide Angle Search for Planets 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, ...

SuperWASP consortium of universities, the exoplanet, as the 17th found to date by this group, was given its present name. Astronomers at the

Observatory of Geneva The Geneva Observatory (french: Observatoire de Genève, german: Observatorium von Genf) is an astronomical observatory at Sauverny (CH) in the municipality of Versoix, Canton of Geneva, in Switzerland. It shares its buildings with the astronomy ...

were then able to use characteristic

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in fr ...

s and

blueshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase i ...

s in the host star's

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors ...

as its

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection ...

varied over the course of the planet's orbit to measure the planet's mass and obtain an indication of its

orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values bet ...

. Careful examination of the Doppler shifts during transits also allowed them to determine the direction of the planet's orbital motion relative to its parent star's rotation via the

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 secon ...

Orbit

WASP-17b is thought to have a retrograde orbit (with a sky-projected

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 Ea ...

of the orbit normal against the stellar spin axis of about 149°, not to be confused with the line-of-sight inclination of the orbit, given in the table, which is near 90° for all transiting planets), which would make it the first planet discovered to have such an orbital motion. It was found by measuring the

of the planet on the star's Doppler signal as it transited, in which whichever of the star's hemispheres is turning toward or away from Earth will show a slight blueshift or redshift which is dampened by the transiting planet. Scientists are not yet sure why the planet orbits opposite to the star's rotation. Theories include a gravitational slingshot resulting from a near-collision with another planet, or the intervention of a smaller planet-like body working to gradually change WASP-17b's orbit by tilting it via the Kozai mechanism. Spin-orbit angle measurement was updated in 2012 to −148.7°.

Physical properties

WASP-17b has a

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

between 1.5 and 2 times that of

and about half the

. Thus its mean

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

is between 0.08 and 0.19 g/cm³, compared with Jupiter's 1.326 g/cm³ and

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

's 5.515 g/cm³ (the density of water is 1 g/cm³). The unusually low density is thought to be a consequence of a combination of the planet's orbital eccentricity and its proximity to its parent star (less than one seventh of the distance between Mercury and the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

), leading to tidal flexing and heating of its interior. The same mechanism is behind the intense volcanic activity of Jupiter's moon Io.

WASP-39b WASP-39b, officially named Bocaprins, is a "hot Jupiter" extrasolar planet discovered in February 2011 by the Wide Angle Search for Planets, WASP project, notable for containing a substantial amount of water in its atmosphere. In addition WASP-3 ...

has a similarly low estimated density. Exoplanetary

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

in the atmosphere of the WASP-17 has been detected in 2018, but was not confirmed by 2021. Instead, the spectral signatures of water, aluminium oxide (AlO) and titanium hydride (TiH) were detected. The water signature was confirmed in 2022, together with

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

absorption. Clear to cloudy hot Jupiters

Notes

References

External links

*Alexander, Amir. Scientists Detect "Wrong-Way" Planet

The Planetary Society The Planetary Society is an American internationally-active non-governmental nonprofit organization. It is involved in research, public outreach, and political space advocacy for engineering projects related to astronomy, planetary science, a ...