Circumbinary planets

You've seen Star Wars, yes? If not, it may be helpful to visit this link and then return. Luke's home planet Tatooine is a circumbinary planet, meaning that it orbits around two stars and hence has a double sunset. We now know that such planets exist not just in science fiction but in nature too! The first 100% bonafide discovery was Kepler-16b, a Saturn-mass planet orbiting two stars that are ~65% and ~20% as big as our Sun. The Kepler space telescope has now made 11 such discoveries.

Orbital dynamics

For centuries people have worked to understand how gravitationally interacting bodies behave. It turns out that the general three body problem is non-integrable, which is to say that one cannot write a simple equation that will predict the positions of all three body at a future time based on their interactions. This means that there is an entire field dedicated to solving these equations using approximations and numerical methods, and also trying to understand the sometimes surprising behaviour of these gravitational interactions. Circumbinary planets mentioned above, which constitute a three-body system (2 stars, 1 planet), are a perfect example of the intersection between observational astronomy and orbital dynamics.

Hot-Jupiters

Hot Jupiters are a peculiar type of planet that is a gas giant, so comparable in size and mass to Jupiter, yet orbiting in a close, "hot" orbit around its star. It may be so close that its orbital period is less than one Earth day! The first exoplanet found around a main sequence star, 51 Peg, was a hot Jupiter. Ever since then their existence and formation has puzzled astronomers. I have approached this problem as a comparison to close binary formation. By seeing the similarities and differences we may better understand both types of astrophysical system

Planetary transits

Many surveys, e.g. Kepler from space and WASP from the ground monitor the brightness of thousands of stars. When a planet passes in front of its host star(s), known as a transit, the brightness drops and we obtain information about the planet. The separation of transits tells us the planet's period. The width and depth of the transit indicate the radius of the planet. If we also know the planet's mass, e.g. from radial velocity spectroscopy or transit timing variations, then we know the planet's bulk density and can start to probe its interior composition. Furthermore, when a planet transits its atmosphere (if it has one) will only block out part of the starlight. Based on the different wavelengths absorbed we can identify different elements and molecules within the atmosphere. In the future we may ultimately find biological markers indicative of life.

White dwarf planets

Searches for exoplanets have typically focused on single, main sequence stars. There are two main reasons for this. First, these are the easiest stars to target since binary stars can make detections harder (a large chunk of my research) and both young and evolved stars also have observational challenges. A second reason is that our own Sun is a single, main sequence star, and so it's natural to seek comparisons. I've tried to break this mould by looking at planets in binaries that contain a white dwarf, which is a degenerate remnant of a roughly Sun-like star after its full lifetime of stellar evolution. To have tight planets around white dwarfs, like those you might be able to see transit, it's argued that a binary is needed to get it there via Kozai-Lidov interactions. A cool thing is that a white dwarf is only about the radius of Earth, and so planets we find transiting it would often be larger (but less massive) than the star!

Close binary formation

Stars are believed to form out of condensing clouds of gas and dust. Sometimes these clouds fragment into two, three or even more stars. This picture helps us understand the wide diversity of multi-star systems in the Galaxy. However, it is believed that two stars will not naturally form very close together. They alternatively are thought to initially form far apart and then migrate closer. I have been tackling this problem from the perspective of circumbinary planets, and how this new observable helps constrain our theories. I have also helped coordinate observational surveys of low mass binaries, obtaining observational markers of their formation history (eccentricities, orbital obliquities, rotation rates etc).