Chair of Session 1
Professor Bill Borucki
Over 2700 planetary candidates have been found with an enormous range of sizes, temperatures, and types of stellar hosts. In particular, exoplanets near the size of Earth’s moon to those larger than Jupiter have been found orbiting stars much cooler and smaller than the Sun as well to stars hotter and often larger than the Sun. Orbital periods range from 0.84 days to over 1000 days and orbital distances range from 0.01 AU to many AU. Several planets have been discovered orbiting binary stars.
Calculated radiative equilibrium temperatures (Teq) range from higher than molten lava (~1830K for Kepler-10b) for planets near the surface of their host star to temperatures to those as cold as -70C. The Kepler Mission has even found a planetary candidate in the HZ of a binary star.
Masses of those planets with large masses and/or short orbital-periods can be are being determined by radial velocity and transit timing methods. By combining these results with the sizes obtained from transit photometry, densities of these planets are being calculated. These range from 0.2 gr/cc for Kepler-7b to 8.8 gr/cc for Kepler-10b. The results are indicative of planets that range from mostly gas, to water planets, and to iron-rich rocky planets. Surprisingly, a very wide range of densities has been found for closely-packed planets orbiting the same star (Kepler-11). This result implies that contrary to what is observed in our Solar System, the composition (whether rocky, water-rich, or gas) cannot be surmised from its semi-major axis or insolation.
Because it is much more difficult to find small planets in the HZ compared to finding large planets in short period orbits, only a few dozen planetary candidates and confirmed planets have been found in the HZ. Unfortunately, no densities of planets in the HZ are available because the amplitudes of the RV and transit-timing signals they produce are too small for detection.
A summary of the known characteristics of exoplanets, especially small planets and all those in the HZ will be presented.
Direct detection of exoplanets: results, consequences on planet formation theories, and future
Dr Anne-Marie Lagrange
Most of the exoplanets known today have been discovered by indirect techniques, based on the study of the host star radial velocity or photometric temporal variations. These detections allowed the study of the planet content in the first 5-8 AU from the star. The numerous observations have provided precious information on the way planet form and evolve at such separations. One of the most interesting outcomes of these studies is that dynamical evolution (through e.g. disk-planet interaction, planet-planet interactions) plays an important role in the final architecture of the systems. In a few cases, spectroscopic observations of transiting planets allowed to make first exploration of the planet atmospheres.
Direct imaging allows to detect giant planets at larger separations (currently typ. > 5-10 AU), complementing then the indirect techniques. Given the separations considered, most of them are expected to be formed by gravitational instability within a disk rather than by accretion of gas on to a solid core, which is the preferred scenario to explain solar system giant planets as well as most of radial velocity and transiting giant planets. Hence, direct imaging provides an opportunity to study this alternative mode of planet formation. Coupled to spectroscopy, it allows the exploration of the planets atmospheres.
The surveys performed allowed to derive first statistics on the presence of giant planets at large separations. So far, only a few planets have been detected in direct imaging around young stars, but each of them provides an opportunity of very interesting individual studies of their orbital, physical and atmospheric properties and sometimes also on the interaction with "second generation", debris disks.
I will present the direct imaging approach,he detections made so far, and what they already tell us about giant planet formation and evolution. I will also point out the limitations of this approach, as well as the needs for further work in terms of planet formation modelling. I will finally present the tremendous progress that are expected in this field thanks to forthcoming planet imagers on 8-meter class telescopes, on space telescopes (JWST) and later, on Extremely Large telescopes.
Overview, limitations and prospects on exoplanet insights from HARPS and transiting planet perspective
Professor Didier Queloz, University of Cambridge, UK
The discovery of first exoplanets sparked a real revolution in astronomy. Today, about 1000 such objects have been found and confirmed. We have learned that planets are quite common, and that their properties are much more diverse than originally predicted. The improvements and intensive efforts made during the last decade by teams carrying out surveys and dedicated space missions have lead to the identification of numerous planetary systems hosting Neptune-mass planets and super-Earths. While numerously found, we know little on the nature and origines of these planets. They remain a matter of fierce debate in the community.
In my talk I will discuss current limitations and describe a path towards improving this situation. More particularly I'll present New Generation Transit Survey (NGTS) ground facility and CHEOPS transit finder satelite as the two next facilites to play a important role by gaining insights about the true nature of small planetary systems.
Radial velocity studies of cool stars
Professor Hugh Jones
Over the last two decades, the field of exoplanets has made extraordinary progress. Rather than wondering if we are a lone or typical planetary system a series of independent experiments suggest that the architecture of Solar System is not so common but that stars do normally seem to have planets. One significant hole in our exoplanet knowledge are the exoplanets around the dominant stellar population by number so called low-mass stars or M dwarfs. The two traditional drawbacks of such investigation faintness and activity are addressed by investigating radial velocity signals as a function of wavelength and by a mini-survey at red-optical wavelengths where M dwarfs exhibit dramatically more flux. These experiments allow us to glimpse that M dwarf planetary systems appear to be scaled down versions of those found around more massive stars. One particularly interesting aspect of these new M dwarfs systems is the relative abundance of habitable zone exoplanets.
The low-mass exoplanet population discovered by HARPS
Professor Christophe Lovis
I will discuss the recent progress made on the discovery and characterization of low-mass exoplanets in the solar neighbourhood. This involves in particular the results from the high-precision radial velocity surveys made with the HARPS spectrograph. The ultimate goals of these projects are twofold: an investigation into the statistical properties of planetary systems in general, and a search for nearby, bright transiting objects that will become the targets of choice for a detailed characterization of their internal structure and atmosphere. I will argue that radial velocities have significant advantages when it comes to improving our understanding of system architectures, in particular for studying the dichotomy between low-mass, compact systems and Solar-System-like architectures comprising giant planets orbiting at several AUs from the star. Finally, I will review the presently-known sample of transiting exoplanets that are sufficiently close to us to allow for atmospheric studies, and discuss how to expand this relatively small sample using new instrumentation, in particular HARPS-N and ESPRESSO, the high-precision velocimeter for ESO VLT.