HD 17156 b

HD 17156 b is an extrasolar planet approximately 240 light-years away in the constellation of Cassiopeia. The planet was discovered orbiting the yellow subgiant star HD 17156 in April 2007. The planet is classified as a relatively cool hot Jupiter planet slightly smaller than Jupiter but slightly larger than Saturn in a so-called "torched orbit". This highly-eccentric three-week orbit takes it approximately 0.0523 AU of the star at periastron before swinging out to approximately 0.2665 AU at apastron. Its eccentricity is about the same as 16 Cygni Bb, a so-called "eccentric Jupiter". Until 2009, HD 17156 b was the transiting planet with the longest orbital period.TransitSearch (See: HD 80606 b)
The planet was discovered on April 14, 2007 by a team using the radial velocity method on the Keck and Subaru telescopes. The team made an initial, negative, transit search, but they were only able to cover 25% of the search space. This left the possibility of a transit open.
After the possibility of a transit was discussed on oklo.org, various groups performed a follow-on search. These searches confirmed a three-hour transit on October 2, 2007 and a paper was published two days later.
Careful radial velocity measurements have made it possible to detect the Rossiter-McLaughlin effect, the shifting in photospheric spectral lines caused by the planet occulting a part of the rotating stellar surface. This effect allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. This planet's spin-orbit angle was initially measured by Narita in 2007 as +62 ± 25 but has been remeasured by Cochran +9.4 ± 9.3 degrees. By analogy with HD 149026 b, a low angle would imply the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle suggests a violent interplay with other protoplanets. Given its eccentricity the latter is equally probable until the discrepancy of the Rossiter results is resolved.
It is so distant from its star and eccentric, that it will never enter a secondary eclipse relative to Earth, of the planet behind the star. The star's true temperature, and so the planet's surface temperature, cannot be measured with accuracy. Due to the high eccentricity the atmosphere of the planet undergoes a factor of 27 variation in stellar flux during each orbit.
== See also ==
* 16 Cygni Bb
* HD 17156 c
== References ==
== External links ==
Category:Exoplanets
Category:Cassiopeia (constellation)
Category:Transiting exoplanets
Category:Gas giants
Category:Exoplanets discovered in 2007
Category:Exoplanets detected by radial velocity