Project duration: 1 year rotation. Potential to grow into thesis project.
PI: Néstor Espinoza (nespinoza@stsci.edu, www.nestor-espinoza.com)
Project motivation
How exoplanetary systems form and evolve is one of the main yet-to-be answered questions of modern astronomy, and one which is central to understanding the place our own Solar System has in our galaxy. Transiting exoplanets orbiting bright stellar hosts are, in particular, fundamental to answer this question: they are ideal for mass determination via follow-up radial velocities, which combined with the transit information allows us to peek into their interior structures, orbital configurations and, with further characterization, even provide rich constraints on their atmospheric properties. All these techniques hold the promise to inform us about their formation and evolution in the protoplanetary disks in which they lived and formed.
Perhaps the systems that can uncover the most information about their formation and evolution are warm giant exoplanets — planets with radii over 4 times that of Earth, with typical periods larger than about 10 days. These systems are thought to be formed by a set of possible mechanisms that lead to directly observed present-day properties that we can study in impressive detail (see Figure 1). Unlike their hotter counterparts, warm giant exoplanets are not impacted by either additional irradiation from their host stars or tidal interactions that could modify their physical & orbital properties and, as such, their present-day properties are true relics of the formation and evolution scenarios that placed them where we observe them today. Despite their importance, only tens of these systems are known, as they are very challenging to detect due to their long periods from ground-based surveys. This is set to change with the Transiting Exoplanet Survey Satellite (TESS) mission, which is expected to increase this number by a factor of 6 with data from its 2-year primary mission only. The number is even larger if we consider the mission has been recently extended for an extra two years. The mission is expected to detect not only the most interesting ones for mass determination, but also the most exciting objects for future atmospheric characterization. This latter is a unique opportunity to start studying cool worlds like the ones in our Solar System, and start bridging the connection with the observed properties of our neighboring planets in it.
Motivated by this exciting era of TESS, we have started the Warm GIaNts with TESS (WINE) Collaboration (PIs: Brahm, Espinoza, Jordán & Henning) — a multi-institutional collaboration which aims at confirming and characterizing over a hundred of these systems with TESS. The work of the collaboration involves all the stages of planetary detection and characterization: from detection of candidates using TESS photometry to follow-up of these systems with ground-based photometry and radial-velocities, aiming at obtaining precise radii, masses and orbital characterization of the systems. These exoplanets will, in turn, form the basis for a sample to study which of the formation mechanisms outlined in Figure 1 are the most prominent in nature, as well as to provide optimal targets for atmospheric characterization with the upcoming James Webb Space Telescope (JWST).
Student projects
There are currently two, 1-year student projects which could either be done by the same or different students. In addition to these projects, the student will also be allowed to participate in the collaboration, helping with the follow-up efforts and publication of new systems to be discovered with the TESS mission within the WINE collaboration:
- Stage 1: machine learning and transiting exoplanents. The objective of this stage is to minimize (hopefully to zero) the human interaction in the planetary candidate selection from the TESS photometry currently being used by the WINE collaboration. To date, the process is semi-automated in the sense that transiting exoplanet candidates are searched for in the TESS photometry using state-of-the-art algorithms, but before promoting the targets as promising/interesting for ground-based follow-up, visual inspection of the lightcurves is made in order to remove false-positives which are evident to the human eye but not to current detection algorithms. This has provided us with a large (~thousands) sample of false positives which we can use to train machine learning algorithms to perform the target selection in an automated fashion. The student will work on adapting already existing code to performing machine learning classification for the needs of the project, with the objective of incorporating these techniques into our planet detection workflow. We expect the student to publish this implementation, with a list of the detected candidates from our project which can form the basis for a sample of exoplanets that can be used to study planet ocurrence rates with the TESS mission.
- Stage 2: constraining warm giant planet formation scenarios with TESS. With the process of detection and posterior follow-up and characterization of the exoplanets within the project fully automated, the student will work with the PIs of the project in performing injection and recovery tests on the TESS data in order to generate occurrence rates of warm giant exoplanets. The objective of this stage of the project will be to study (1) how efficient is our pipeline for the detection of warm giant exoplanets with TESS given they come from the different formation scenarios depicted in Figure 1 and (2) given this discovery efficiency and our candidate list, what is the real underlying occurrence rate of warm giant exoplanets, and what of the given formation scenarios is the most likely to give rise to the observed population of warm giant exoplanets. We expect the student to again lead a publication on this study.
Figure 1: Warm giant exoplanets formation theories and example planet discoveries with our ongoing project. (a) The formation and posterior migration of warm giant exoplanets to the observed close-in orbits can happen through many mechanisms. Three of the most popular are depicted here: disk migration, in which planets migrate inwards through interactions with the disk which either damps their eccentricities if they are alone or forces resonances with nearby exoplanets; high-eccentricity migration in which planets interact with other exterior companions, which makes them undergo high-eccentricity variations which eventually puts them in close-in orbits and in-situ formation, in which planets are formed from collision of "seed" exoplanets that form the warm giant exoplanets at their currently observed close-in orbits. Note how each formation scenario predicts different outcomes. In particular, in-situ planet formation predicts the formation of Earth to super-Earth sized exoplanets, which we should be able to discover with our project. (b) Example discoveries that conform to some planet formation theories. The left panel shows transit timing variations observed in a Jupiter-sized exoplanet, which is evidence for dynamical interactions with a second non-transiting exoplanet. This conforms to planet formation via disk migration. The right panel shows the radial-velocity (RV) measurements of a star in which we detect both, the RV variation of a highly eccentric transiting exoplanet first detected in the TESS photometry and a long-term trend indicative of a long-period companion. This latter system conforms to planet formation via high-eccentricity migration.
