Exoplanet science is one of the fastest growing fields in astronomy, with more than 3500 confirmed planets outside our solar system known to date. This wealth of discoveries has uncovered several important questions: How did gas-giant planets in close-in orbits (hot Jupiters) form? What are the origin and compositions sub-Neptune sized planets, for which we have no equivalent in the solar system? What are the occurrence rates of exoplanets as a function of their size, mass, orbital architecture, as well as their host star spectral type and evolutionary state? Do habitable planets exist outside our solar system?
Our ability to answer these questions depends strongly our understanding of the host stars since most exoplanets are discovered indirectly. In addition to deriving planet properties, host star characteristics are also crucial to understand planetary environments such as the extent of the habitable zone. The research projects of my group focus on understanding exoplanets through the characterization of their host stars, in particular using asteroseismology. A recent review article on this topic can be found here, and a few recent results are described below.
Precise Radii of Exoplanets
Measuring precise radii of exoplanets is crucial to understand their composition. Using asteroseismology to characterize host stars (and hence their planets) is particularly elegant since stellar oscillations manifest themselves in brightness variations, and hence can be measured using the same data that are used to detect transiting exoplanets. The image on the right shows a light curve with a single transit of Kepler-36c. The red solid line is the transit model, and the inset shows the oscillations of the host star. The transit depth yields the size of the planet relative to the star, and the oscillation periods can be used to independently measure the size of the star. Combining these two measurements allows us to precisely measure the radius of the transiting planet.
I have led a paper announcing the discovery of oscillations in 66 Kepler planet-candidate host stars, which increased the total number of host stars with asteroseismic detections by a factor of 7 and allowed precise radius measurements of over 100 planet-candidates detected by Kepler. I have also led the asteroseismic characterization of Kepler-37, which hosts the smallest exoplanet known to date, and Kepler-452, which hosts a super Earth-sized planet in the habitable zone of its G-type host star. One of the most exciting discoveries so far has been Kepler-444, a 11 billion year old star hosting five sub-Earth sized planets with orbital periods of less than 10 days. Our most recent projects is to hunt to for giant planets orbiting oscillating giant stars with the K2 Mission, which allows us to study the influence of stellar evolution on the properties of planets. The first results of the project, led by IfA graduate student Sam Grunblatt, can be found here.
Spin-Axis Inclinations of Exoplanet Host Stars
The angle between the equator of a star and the orbital plane of its planets reveals important information about planet formation. In particular, hot Jupiters are often found to be misaligned with their host stars, which has been interpreted as evidence that they must have experienced dynamical disruption during their formation rather than forming through a quiescent migration through the protoplanetary disk. Multiplanet systems like our solar system, on the other hand, have so far been found to be well aligned.
Asteroseismology allows us to measure the inclination of the spin axis of a star by measuring the relative height of dipole pulsation modes split by rotation. Here is very neat animation (courtesy of Andrea Miglio) showing the dependence of pulsation amplitudes on the inclination of the star to the line of sight:
In a paper published in Science Magazine, we used this technique to discover the first multiplanet system in which host star is misaligned with the orbital plane of its planets. Follow-up observations using the Keck telescope on Mauna Kea also revealed that the system has a companion on a wide orbit. Our favored explanation for the misalignment is that the outer companion is inclined to the orbit of the inner planet, hence exerting a torque which periodically misaligns the orbital plane of the inner planets with the equatorial plane of the host star (see graphical sketch on the right). This theory has since been supported by follow-up studies which measured the orbit and mass or the outer planet, and theoretical studies showing that torquing by a third companion is indeed efficient. Our current projects focus on finding more examples of systems like Kepler-56 to demonstrate whether they are common.