Formation and Migration Histories of Giant Exoplanets in Multi-stellar Systems

Citation

Ngo, Henry Hoang Khoi (2017) Formation and Migration Histories of Giant Exoplanets in Multi-stellar Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9KD1VZW. https://resolver.caltech.edu/CaltechTHESIS:05262017-234026021

Abstract

The first planets discovered outside of our solar system were very different from the solar system planets. These discoveries raised new challenges to planet formation models, which were designed to explain the origin of the solar system planets. One particularly intriguing population, the "hot Jupiters" were some of the first planets discovered. These gas giant planets have masses similar to Jupiter and Saturn, however, they were found on orbits 100 times closer to their star than Jupiter is to the sun. Proposed formation scenarios involve models that argue for formation at presently observed locations, but these are challenged by the lack of planet-building materials so close to the host star. Other models assume these planets form at more moderate locations, perhaps in a manner similar to Jupiter and Saturn, followed by inward migration via some other mechanism. These models are challenged by the lack of a known migration mechanism.

This dissertation compiles three studies conducted over the past five years to investigate the formation and migration histories of gas giant exoplanets. After the discovery of the first hot Jupiter, additional discoveries revealed some population characteristics that could provide evidence for certain formation or migration scenarios. A large fraction of hot Jupiters were found to have eccentric orbits and/or misaligned orbits relative to the star's spin axis. These properties suggest that a gravitational interaction with an additional massive object may have played a role in the dynamical history of these hot Jupiters. Studies of stellar multiplicity for nearby, sun-like stars have also revealed that multi-stellar systems are common.

The studies presented in this dissertation investigate whether stellar companions to giant planet systems influence the planets. In the first and second study, we conduct a survey for stellar companions around stars that host hot Jupiters detected by the transiting method. The first study examines whether stars hosting misaligned planets are more likely to host a companion star. We found no such correlation, suggesting that stellar companions do not play a dominant role in causing planetary misalignment. In the second study, we look at the population of stellar companions as a whole to quantify the fraction of hot Jupiters that might have migrated due to stellar interactions. We find that less than 20% of hot Jupiters might have experienced this migration scenario. However, we do find that hot Jupiters are three times more likely to be in a wide multi-stellar system compared to nearby stars that do not host hot Jupiters, suggesting some other connection between the companion star and the giant planet.

In the third study, we search for stellar companions around stars that host giant planets over a wide range of separations, from the close-in hot Jupiters to giant planets as far away as Jupiter is to our sun. These planets were found via the radial velocity method. We compare the giant planets' orbital properties for single- and multi-stellar systems to determine whether planets in multi-stellar systems show some evidence for star-planet interactions. With the current dataset, we find no evidence to support the hypothesis that planets in multi-stellar systems have a different set of orbital properties.

Finally, we present preliminary results of an ongoing survey to understand giant planet formation on the other extreme end. Instead of hot Jupiters on close-in orbits, this survey seeks to explain the origin of the very distant giant planetary mass objects found by direct imaging surveys. These objects are often found on separations that are ten to one hundred times farther away than Neptune is to our sun. Due to their distance and size, it's not certain if these objects are some of the biggest planets in existence or if they are the smallest stars. This new survey will search for planets to serve as the link between known giant exoplanets and these unknown directly imaged objects.

Thesis Files