HD 17156 b
A highly eccentric hot Jupiter with a three-week orbit.
HD 17156 b, also called Mulchatna, is a planet about 255 light-years away in the constellation Cassiopeia. It circles the yellow subgiant star HD 17156. Slightly smaller than Jupiter but larger than Saturn, it is considered a relatively cool hot Jupiter. Its orbit is highly eccentric, taking about three weeks to complete, and swings from roughly 0.0523 AU at periastron to about 0.2665 AU at apastron. The eccentricity is similar to that of 16 Cygni Bb, another so-called "eccentric Jupiter." Until 2009, it held the record as the transiting planet with the longest orbital period.
The planet was discovered on April 14, 2007, using the radial velocity method with the Keck and Subaru telescopes. The team conducted an initial search for transits but covered only 25% of the possible window, leaving the possibility open. After discussion on oklo.org, several groups followed up. A three-hour transit was confirmed on October 2, 2007, and a paper was published two days later.
Originally designated HD 17156 b, the planet was later named Mulchatna by the IAU as part of the NameExoWorlds contest, chosen by United States representatives. The name refers to the Mulchatna River, a tributary of the Nushagak River in southwestern Alaska. Its host star was simultaneously named Nushagak.
Careful radial velocity measurements detected the Rossiter–McLaughlin effect, which measures the angle between the planet's orbital plane and the star's equatorial plane. The spin-orbit angle was first measured in 2007 as +62 ± 25 degrees, later remeasured as +9.4 ± 9.3 degrees, and refined in 2012 to about 10 degrees.
Because of its high eccentricity and large distance from its star, the planet has a low probability of entering a secondary eclipse. The star's true temperature cannot be measured accurately. During each orbit, the planet's atmosphere experiences a 27-fold variation in stellar flux.
Quick Facts
- Discoverer
- Fischer et al.
- Discovery Site
- W. M. Keck Observatory / Subaru Telescope
- Discovered
- 14 April 2007
- Discovery Method
- Radial velocity and Transit
- Apsis
- astron
- Semimajor
- 0.1589 AU
- Eccentricity
- 0.6768 ± 0.0034
- Period
- 21.2163979 ± 0.0000159 d
- Inclination
- 86.573 ± 0.060
- Time Periastron
- 2454438.4835±0.0025
- Arg Peri
- 121.71 ± 0.43
- Semi-Amplitude
- 274.2 ± 2.0
Facts from the source article.
Lore & Background
The planet was discovered on April 14, 2007, by a team using the radial velocity method on the Keck and Subaru telescopes. The initial transit search was negative, covering only 25% of the search space, leaving the possibility of a transit open. After the possibility was discussed on oklo.org, various groups performed follow-on searches, confirming a three-hour transit on October 2, 2007, with a paper published two days later.
The planet was originally named HD 17156 b, being the second object in the system. It was later given the name Mulchatna by the IAU, chosen by United States representatives for the NameExoWorlds contest, with the comment that the Mulchatna River is a tributary of the Nushagak River in southwestern Alaska. Its parent star was simultaneously named Nushagak.
Careful radial velocity measurements detected the Rossiter–McLaughlin effect, allowing measurement of the spin-orbit angle. This angle was initially measured as +62 ± 25 degrees, later remeasured as +9.4 ± 9.3 degrees, and a 2012 study refined the misalignment angle to 10 degrees. Due to its high eccentricity and large distance from its star, the planet has a low probability of ever entering a secondary eclipse. The atmosphere undergoes a 27-fold variation in stellar flux during each orbit.
Reader's Guide
HD 17156 b holds significance as a benchmark for studying highly eccentric transiting exoplanets. Its discovery and transit confirmation demonstrated the value of collaborative follow-up efforts after initial radial velocity detection, with the transit window being identified through community discussion on oklo.org. The planet's orbit, with eccentricity comparable to 16 Cygni Bb, provides a natural laboratory for atmospheric dynamics under extreme flux variations—the 27-fold change in stellar flux over its orbit is among the largest known for a transiting planet. The Rossiter–McLaughlin measurements, which evolved from an initial misalignment of +62 degrees to a refined value near 10 degrees, illustrate the iterative nature of spin-orbit angle determinations and the importance of multiple observations. Until 2009, it held the record for the longest orbital period among transiting planets, a milestone that highlighted the expanding parameter space of transiting exoplanets beyond short-period hot Jupiters. Its naming through the NameExoWorlds contest, linking it to Alaskan geography, also reflects public engagement in exoplanet nomenclature.
Did You Know?
- HD 17156 b was the transiting planet with the longest orbital period until 2009.
- Its orbit has an eccentricity about the same as 16 Cygni Bb.
- The spin-orbit angle was initially measured as +62 ± 25 degrees and later refined to about 10 degrees.
More in Transiting Exoplanets, Part 3 1-24
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