WASP-17b
Encyclopedia
WASP-17b is an exoplanet in the constellation Scorpius
that is orbiting the star 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 changed traditional planetary theory. In terms of diameter, WASP-17b may be the largest exoplanet discovered yet, and at half Jupiter
's mass, this would make it the most diffuse known planet, and a puffy planet.
in Staffordshire
, England
, discovered the gas giant
, which is about 1,000 light years
(300 parsecs) from Earth, by observing it in transit
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
. Due to the involvement of the Wide Angle Search for Planets 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
were then able to use characteristic red shift
s and blue shift
s in the host star's spectrum
as its radial velocity
varied over the course of the planet's orbit
to measure the planet's mass and obtain an indication of its orbital eccentricity
. 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.
1.5–2 times that of Jupiter
and about half the mass
. Thus its mean density
is between 0.08 and 0.19 g/cm3, compared with Jupiter's 1.326 g/cm3 and the Earth
'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
), leading to tidal flexing and heating of its interior. The same mechanism is behind the intense volcanic activity of Jupiter's moon Io
.
WASP-17b is thought to have a retrograde orbit (with a sky-projected inclination 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 Rossiter-McLaughlin effect
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
.
Scorpius
Scorpius, sometimes known as Scorpio, is one of the constellations of the zodiac. Its name is Latin for scorpion, and its symbol is . It lies between Libra to the west and Sagittarius to the east...
that is orbiting the star WASP-17
WASP-17
WASP-17 is an F-type main sequence star approximately 1,000 light-years away in the constellation Scorpius. As of 2009, an extrasolar planet has been confirmed to orbit the star...
. 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 changed traditional planetary theory. In terms of diameter, WASP-17b may be the largest exoplanet discovered yet, and at half Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's mass, this would make it the most diffuse known planet, and a puffy planet.
Discovery
A team of researchers led by David Anderson of Keele UniversityKeele University
Keele University is a campus university near Newcastle-under-Lyme in Staffordshire, England. Founded in 1949 as an experimental college dedicated to a broad curriculum and interdisciplinary study, Keele is most notable for pioneering the dual honours degree in Britain...
in Staffordshire
Staffordshire
Staffordshire is a landlocked county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. Part of the National Forest lies within its borders...
, England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, discovered the gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
, which is about 1,000 light years
Light Years
Light Years is the seventh studio album by Australian recording artist Kylie Minogue. It was released on 25 September 2000 by Parlophone and Mushroom Records. The album's style was indicative of her return to "mainstream pop dance tunes"....
(300 parsecs) from Earth, by observing it in transit
Astronomical transit
The term transit or astronomical transit has three meanings in astronomy:* A transit is the astronomical event that occurs when one celestial body appears to move across the face of another celestial body, hiding a small part of it, as seen by an observer at some particular vantage point...
its host star WASP-17
WASP-17
WASP-17 is an F-type main sequence star approximately 1,000 light-years away in the constellation Scorpius. As of 2009, an extrasolar planet has been confirmed to orbit the star...
. Such photometric
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
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
South African Astronomical Observatory is the national center 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 function is to conduct research in astronomy and astrophysics...
. Due to the involvement of the Wide Angle Search for Planets SuperWASP
SuperWASP
SuperWASP is an international academic organisation performing an ultra-wide angle search for transiting extrasolar planets with the aim of covering the entire sky down to ~15th magnitude.-Equipment:...
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
Observatory of Geneva
Observatoire de Genève is an astronomical observatory at Sauverny in the municipality of Versoix, Canton of Geneva, Switzerland...
were then able to use characteristic red shift
Red shift
-Science:* Redshift, the increase of wavelength of detected electromagnetic radiation with respect to the original wavelength of the emission* Red shift, an informal term for a bathochromic shift...
s and blue shift
Blue shift
A blueshift is any decrease in wavelength ; the opposite effect is referred to as redshift. In visible light, this shifts the colour from the red end of the spectrum to the blue end...
s in the host star's spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
as its radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
varied over the course of the planet's orbit
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
to measure the planet's mass and obtain an indication of its orbital eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
. Careful examination of the Doppler shifts
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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.
Special properties
WASP-17b has a radiusRadius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
1.5–2 times that of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and about half the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
. Thus its mean density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
is between 0.08 and 0.19 g/cm3, compared with Jupiter's 1.326 g/cm3 and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
'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
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
and the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
), leading to tidal flexing and heating of its interior. The same mechanism is behind the intense volcanic activity of Jupiter's moon Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....
.
WASP-17b is thought to have a retrograde orbit (with a sky-projected inclination 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 Rossiter-McLaughlin effect
Rossiter-McLaughlin effect
The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's secondary star or an extrasolar planet is seen to transit across the face of the primary or parent star. As the main star rotates on its axis, one quadrant of its photosphere will be seen to be...
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
Kozai mechanism
In celestial mechanics, the Kozai mechanism, or the Lidov-Kozai mechanism, causes a periodic exchange between the inclination and eccentricity of an orbit...
.
See also
- HAT-P-7bHAT-P-7bHAT-P-7b is an extrasolar planet discovered in 2008. It orbits very close to its parent star GSC 03547-01402, and is both larger and more massive than Jupiter. Due to the extreme heat it receives from its star, the dayside temperature is predicted to be 2730 K...
, another exoplanet announced to have a retrograde orbit the day after the WASP-17b announcement - TrES-4TrES-4TrES-4b is an extrasolar planet discovered in 2006 and announced in 2007 by the Trans-Atlantic Exoplanet Survey using the transit method. It is away in the constellation Hercules....
, another large exoplanet with a low density - List of extrasolar planet extremes
External links
- Alexander, Amir. Scientists Detect "Wrong-Way" Planet. http://www.planetary.org/news/2009/0812_Scientists_Detect_WrongWay_Planet.html The Planetary Society, August 12, 2009. Accessed August 14, 2009.