Project Description

Hot Jupiters are gas giant planets with orbital period less than 10 days. The short period means that hot Jupiters are very close to their host stars, usually less than 0.1 AU, one tenth of the distance between the Earth and the Sun. Hot Jupiters dominated planet discoveries for at least one decade because they are the easiest to find with the radial velocity (Doppler) technique and the transit method. However, the occurrence rate of hot Jupiters differs by a factor of 2-3 between Doppler planet surveys and transit planet surveys.

In addition to the discrepancy of occurrence rate, there are other puzzling questions with regard to hot Jupiters. Hot Jupiters are too massive to form in situ because a lack of building materials close to a star. One possible solution is that hot Jupiters form further out, where building materials are sufficient, then migrate to their current positions. Migration of hot Jupiters can be caused by different mechanisms. Some think that the imbalance toque in a protoplanetary disk is the cause. Some think that the orbits of hot Jupiter are excited to a very high eccentricity. The high eccentricity causes hot Jupiters to approach the central stars, so close that the orbital energy of hot Jupiters is tidally dissipated. The tidal energy dissipation shrinks and circularizes the orbits of hot Jupiters. What excites the high eccentricity is another issue under debate. Some think that planet-planet scattering is the cause. some think that the perturbation of a companion star is the cause.

My research focuses on answer two of the above questions: (1), why there is a discrepancy of hot Jupiter occurrence rate between Doppler and transit planet surveys; (2), is it the case that a companion star excites the orbital eccentricity and causes hot Jupiter migration? For the first question, I find that, after all, there is no discrepancy between the Doppler and transit planet surveys. It is the difference of stellar properties between two populations of stars. The Doppler planet surveys focus on a population of stars that are in general more metal-rich, with a higher fraction of main sequence stars and a lower fraction of subgiant stars. These stellar properties determine that the occurrence rate of hot Jupiters is higher compared the population of stars for the transit surveys.

For the first question, I find that, interestingly, there is no discrepancy between the Doppler and transit planet surveys. It is the difference of stellar properties between two populations of stars. The Doppler planet surveys focus on a population of stars that are in general more metal-rich, with a higher fraction of main sequence stars and a lower fraction of multiple star systems. These stellar properties determine that the occurrence rate of hot Jupiters is higher compared the population of stars for the transit surveys.

For the second question, whether hot Jupiters migrate inward with a high eccentricity and whether the high eccentricity is caused by a companions star, we can design an experiment to test this hypothesis. In the experiment, we have a sample of stars with hot Jupiters. We choose a control sample of stars with gas giant planets that are further away from their host stars. If the occurrence rate of companion stars is higher for the hot Jupiter sample than it is for the control sample, then the hypothesis if confirmed. This is still a work in progress. We have obtained data from the Palomar and the Keck observatory. Please stay tuned!

Publications

Ji Wang, Debra A. Fischer, Elliott P. Horch, Xu Huang. On the Occurrence Rate of Hot Jupiters in Different Stellar Environments. 2015, ApJ, 799, 229