The Planet-Disk Connection: Accretion, Disk Structure, and Planet Formation

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"How do stars and planetary systems develop and is life exclusive to our planet?" is one of the major questions of humanity, and also one of the Science Challenges proposed by the STFC. In Dundee, we are a small but young and vibrant department with a strong focus in understanding the formation of stars and planets, and the group is growing within the larger context of the Scottish Universities Physics Alliance (SUPA) and the St Andrews Exoplanet Centre.Our main objective is to explore planet formation environments both observationally and computationally. Planets are born in protoplanetary disks formed around young stars, composed of gas and dust with sizes from very small particles to pebbles. We use multi-telescope, multi-wavelength data and a state-of-the-art computer simulations to track the formation of stars together with protoplanetary disks, to follow the evolution of disks as they disperse, and to study how planetary systems form and evolve out of a dissipating disk. One of the main problems of observationally studying the ongoing planet formation in disks is that the innermost part (which includes the habitable zone, the region where liquid water could enable planets to host life) is too small and too faint to be directly resolved even by the most powerful existing interferometers. Our project will develop special observational techniques to explore the conditions around young stars and in the innermost planet formation regions near the central stars, where many planets are now being discovered. Since observations of disks provide snapshots of disk evolution while observed exoplanets show the end state of planet formation, we also aim to connect the beginning and the end of planet formation (i.e. disks and planets) by using computer simulations, and will compare simulations with both observed disks and exoplanets. The first key is to explore the velocity and time dimension to track tiny spatial structures: using the Doppler effect and repeated observations over time, we can track the disk material as it orbits, spirals and accretes onto the central star, mapping the innermost regions at few stellar radii and determining the flow of matter throughout the disk. Both quantities are very important to explore whether the star will be quiescent enough in the future to allow for habitable planets, and the way matter moves through the disk in the planet-forming zone. It will also enable us to distinguish observational planetary signatures from those of accretion and activity and thus facilitate the detection of very young planets in the future. The second key is the "Rosseta Stone" technique, using observations at many different wavelengths (colours), taken with various telescopes. Similar to the different languages of the original Rosetta Stone, each wavelength tells us about aspects of the disk in a slightly different way. By combining them all for large numbers of objects, we can obtain an accurate picture of the global properties of protoplanetary disks and explore the signatures of planet formation. The third key is to use computer simulations to bridge the gap between protoplanetary disks and exoplanetary systems by exploring the formation of planets over a few million year timescale. We will construct a realistic disk model from our multi-wavelength observations for a large number of disks and use that as the starting point of planet formation simulations. We will then compare the properties of the simulated planetary systems with those of observed ones, which will also help us to rule out or confirm disk properties and physical processes that are not directly observable. Our goals are to put into context the known variety of planets by revealing from which kinds of disks they have originated, and to understand the observed variety of protoplanetary disks by exploring the effect of planet formation on the evolution of the disk.