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Exoplanet List of exoplanets
Exoplanet Comparison WASP-17 b.png
Size comparison of WASP-17b (right) with Jupiter (left).
Parent star
Star WASP-17
Constellation Scorpius
Right ascension (α) 15h 59m 51s
Declination (δ) −28° 03′ 42″
Apparent magnitude (mV) 11.6
Distance 1000 ly
(300 pc)
Spectral type F6V
Orbital elements
Semi-major axis (a) 0.0515 (± 0.00034) AU
Eccentricity (e) 0.028 +0.018
Orbital period (P) 3.735438 (± 6.8e-06) d
Inclination (i) 86.83 +0.56
Argument of
(ω) −70°
Time of transit (Tt) 2454577.85806 (± 0.00027)[1] JD
Physical characteristics
Mass (m) 0.486 (± 0.032) MJ
Radius (r) 1.991 (± 0.081)[note 1][1][2] RJ
Discovery information
Discovery date 11 August 2009
Discoverer(s) David R. Anderson et al.[2]
Discovery method Transit (including secondary eclipse)
Other detection methods Radial velocity
Discovery status Published[2]
Database references
Extrasolar Planets
Exoplanet Archive data
Open Exoplanet Catalogue data

WASP-17b is an exoplanet in the constellation Scorpius that is orbiting the star WASP-17. Its discovery was announced on 11 August 2009.[2] 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.[2] This discovery challenged traditional planetary formation theory.[3] In terms of diameter, WASP-17b is the second-largest exoplanet discovered yet, and at half Jupiter's mass, this makes it the most puffy planet known.[4] On 3 December 2013, scientists working with the Hubble Space Telescope reported detecting water in the atmosphere of the exoplanet.[5][6]


A team of researchers led by David Anderson of Keele University 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.[7]

Astronomers at the Observatory of Geneva were then able to use characteristic red shifts and blue shifts 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.[2] 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.[2]

Special properties[edit]

WASP-17b has a radius 1.5–2 times that of Jupiter and about half the mass.[2] Thus its mean density is between 0.08 and 0.19 g/cm3,[2] compared with Jupiter's 1.326 g/cm3[8] and 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.[2] 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°,[9] 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.[10]

See also[edit]


  1. ^ for Case I in paper page 6, others range from 1.41 to 2.07.


  1. ^ a b c "Planet: WASP-17 b". The Extrasolar Planet Encyclopedia. Retrieved 14 August 2009. 
  2. ^ a b c d e f g h i j Anderson, D. R.; et al. (2010). "WASP-17b: An Ultra-Low Density Planet in a Probable Retrograde Orbit". The Astrophysical Journal 709 (1): 159–167. arXiv:0908.1553. Bibcode:2010ApJ...709..159A. doi:10.1088/0004-637X/709/1/159. 
  3. ^ "A planet going the wrong way", Phys Org. June 7, 2011. Accessed June 10, 2011
  4. ^ Rachel Kaufman (17 August 2009). ""Backward" Planet Has Density of Foam Coffee Cups". National Geographic. National Geographic Society. Retrieved 6 February 2011. 
  5. ^ Staff (3 December 2013). "Hubble Traces Subtle Signals of Water on Hazy Worlds". NASA. Retrieved 4 December 2013. 
  6. ^ Mandell, Avi M.; Haynes, Korey; Sinukoff, Evan; Madhusudhan, Nikku; Burrows, Adam; Deming, Drake (3 December 2013). "Exoplanet Transit Spectroscopy Using WFC3: WASP-12 b, WASP-17 b, and WASP-19 b". Astrophysical Journal 779: 128. arXiv:1310.2949. Bibcode:2013ApJ...779..128M. doi:10.1088/0004-637X/779/2/128. Retrieved 4 December 2013. 
  7. ^ Paul Rincon (August 13, 2009). "New planet displays exotic orbit". BBC News. Retrieved 2009-08-13. 
  8. ^ "Jupiter Fact Sheet". Retrieved 2009-08-13. 
  9. ^ Amaury H.M.J. Triaud et al. Spin-orbit angle measurements for six southern transiting planets. Accepted for publication in A&A 2010. arXiv preprint
  10. ^ Lisa Grossman (August 13, 2009). "Planet found orbiting its star backwards". New Scientist. Retrieved 2009-08-13. 

External links[edit]

Media related to WASP-17b at Wikimedia Commons

  • Alexander, Amir. Scientists Detect "Wrong-Way" Planet. [1] The Planetary Society, August 12, 2009. Accessed August 14, 2009.