Formation and Early Evolution of Planetary Systems

My work has two main components:

1) One is the study of the formation of planetesimals from small dust grains through the run-away convergence of radial drift known as the streaming instability. Planetesimals are the building blocks of terrestrial planets, and the cores of giant planets. My work has shown that small particles (Stokes number as low as 0.003) can participate in the streaming instability. In an upcoming paper I also show that planetesimals are a natural outcome of the evolution of protoplanetary disks.

2) The second part is the study the dynamical evolution of planetary systems where the giant planets are dynamically unstable. In a paper that is currently submitted to MNRAS I show that the present-day eccentricity of observed giant planets can be used to constrain the probability that a terrestrial planet in that system would have survived.

Planets are very common in the Galaxy. Over the past 20 years we have discovered that most Sun-like stars are surrounded by planetary systems. We also know that there is a lot of variety among exoplanets. About 50\% of all Sun-like stars are accompanied by close-in super-Earths (planets with a mass or radius between that of Earth and Neptune) meaning that the solar system is not typical. We have found evidence that many planetary systems experienced a period of dynamical chaos, in which the gravitational interactions between planets radically altered their orbits and probably led to the ejection of one or more planets. There is even evidence that something similar might have happened in the solar system. The goal of this thesis is to improve our understanding of how planets form, and how they evolve. My work can be divided into two parts:

1) Half of my work is focused on the formation of planetesimals. Planetesimals are one of the key stages in the development of planets. They are large bodies, between 1 and 100 km in diameter. Planetesimals are the building blocks of terrestrial planets like the Earth, and they are also required to form the rocky cores that are thought to be at the centre of gas giant planets like Jupiter and Saturn. In my first investigation, I used computer simulations to determine the conditions needed for planetesimals to form (it mainly depends on the solid and gas density, and the size of the solids). In my most recent paper I combined this result with a computer model of a planet-forming disk. We found that planetesimals begin to form in the outer-most regions of the disk. As time passes, planetesimals gradually start to form closer to the star.

2) The rest of my work is focused on the evolution of planetary systems after they form. In a planetary system, the gravitational interactions between planets are small, but their effect can accumulate over millions of years and can result in planets colliding or becoming ejected from the system. When this happens, the planets that survive are left in more eccentric (elliptical) orbits. I have developed methods to use this eccentricity to gain information about the history of planetary systems. In one published work I estimate the probability that a habitable planet would have survived and remained habitable. In an on-going work, I estimate the masses of planets that were ejected, and estimate how often planetary systems have their orbits altered by a second star in a binary system.