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WASP-47 and Forming Hot Jupiters

Art by J. Becker
Schematic of the WASP-47 system, known so far to host three transiting planets including one hot Jupiter, and a distant, massive companion.

Named to JPL's list of the top 20 Most Intriguing Exoplanets, the WASP-47 system was the first system containing a hot Jupiter to be found to also contain additional, close-in planetary companions. The next two systems found to have a similar geometry were Kepler-730 and TOI-1130. In our 2015 discovery paper on the two additional nearby planets in the WASP-47 system, we also confirmed the planetary nature of the ~9 day period candidate and predicted the planetary masses in the WASP-47 system by fitting the transit timing variations. The masses derived with this method are consistent with the RV masses later measured by other methods. Ideally, the discovery of more systems of this nature will enable an eventual statistical study of the differences between hot Jupiters with and without nearby companions. The story of hot Jupiters doesn't end here, though - one intriguing property of WASP-47 led us to look at systems that share the same geometry more generally.

Art by J. Becker

The obliquities of cool stars hosting hot Jupiters tend to be aligned: this was true in the WASP-47 system, but also holds true more generally. Some of these systems have extra companions, with orbits well exterior to the hot Jupiter's orbit. Because these companions were discovered with RVs, we do not know their inclinations relative to the hot Jupiter's orbit (see schematic above). Using WASP-47 as our inspiration, we examined the dynamical states of other systems with this same geometry: a hot Jupiter orbiting a cool star, an exterior companion residing very far from the hot Jupiter, and a measure stellar obliquity with respect to the hot Jupiter. The results showed that for these systems around cool stars, the discovered exterior companions must reside in close to the same plane as the hot Jupiter (within 30 degrees or so). This result indicates a disk-driven rather than violent, tidally induced migration mechanism for the hot Jupiter: consistent with our expectations from the existence of WASP-47 and its companions!

Art by J. Becker
The three main mechanisms by which hot Jupiters can form and migrate to their final orbital locations.
    See the papers and links related to this project:
  1. The discovery paper of the extra planets in the WASP-47 discovered by K2: Becker, Vanderburg, et al. (2015)
  2. The exact masses of the WASP-47 planets, using HARPS-N RVs: Vanderburg, Becker, et al. 2017
  3. A simultaneous fit of the RVs and TTVs: Weiss et al. 2017
  4. Our paper studying the inclination of companions to hot Jupiters orbiting cool stars: Becker, Vanderburg, et al. (2017)
  5. I discussed my work on the population of hot Jupiters orbiting cool stars at the 2019 KITP Conference, Planet-Star Connections in the Era of TESS and Gaia. The talk I gave is online.