Beyond the architecture of planetary systems, the study of exoplanet structure and atmospheres holds key insights into their origins and history as well as the long-term prospect to remotely question the potential habitability of these other worlds. While this seems an extraordinary endeavour, in light of what we have learnt about Solar System planets, only a handful of key ingredients are required to start characterising an exoplanet. The fundamental quantities include the mass and the size of the planet, its temperature and some physical characteristics of major chemical ingredients present in its atmosphere. These fundamental parameters allow us to gather insights about the composition, formation and evolution history of planets. Our team is developing a comprehensive research program to expand our knowledge in these directions. A large set of observing facilities is used for that goal, including ground and space based equipments.
We present in the following some examples of ongoing work carried out by our research group.
Characterising clouds in exoplanet atmospheres
Our group leads several ground-based and space-based programmes aiming at detecting and characterising clouds in exoplanet atmospheres. To this end, we observe exoplanets at visible and infrared wavelengths to measure their geometric albedos and to explore how their reflectivity evolves with longitude. As for the Earth, clouds is of paramount importance for remote observations. They tend to damp the atomic and molecular features in exoplanet spectra, which represents a major challenge in the field. Understanding the formation, composition and evolution of clouds is a key question that our group investigates.
The figure on the left represents the optical phase-curve (as the planet revolves around its star) of the hot-Jupiter Kepler-7b obtained from 3.5 years quasi continuous data from the Kepler spacecraft. Along with Spitzer infrared data, these observations allowed us to constrain the first map of clouds in an exoplanet atmosphere.
Searching for terrestrial planets orbiting ultra-cool stars using K2
Our group is leading a series of programmes aiming at detecting planets that are similar to the Earth in terms of size, which orbit stars that are much cooler than our Sun (M dwarfs). One of this programme uses the Kepler spacecraft, now called “K2″. We observe a different 100 sq degrees field of view every three months to detect transits that are 0.1 to 1% deep. As K2 operates with two reaction wheels only (three are required to maintain fine-pointing capability), the spacecraft’s thrusters have to be operated regularly to maintain the telescope’s boresight pointing. Three 80-day long campaigns have been awarded for this programme and we are starting to analyse the corresponding data. M dwarfs often exhibit significant stellar variability (due to star spots and flares) that render the search for transiting planets challenging. As an example, a variable pattern is shown on the left for one M dwarf located in the campaign-0 field. The sine form of the lightcurve (phase-folded) appears clearly and allows a measurement of the rotation period of the star, found to be only 16.8 hours in this case. Constraining the properties of the planet host stars is very important, and this programme also represents an effort toward a better understanding of cool stars.
NGTS (2014 onwards)
The Next-Generation Transit Survey (NGTS) is a wide-field photometric survey designed to discover transiting Neptune-size and smaller exoplanets around bright stars (magnitude V<13). NGTS will be located at ESO/Paranal (Chile). The NGTS project is a partnership between the University of Cambridge, Queen’s University Belfast, University of Warwick, University of Leicester, Observatoire de Geneve, DLR (Berlin) and Universidad Catolica de Chile.
SPECULOOS (2016 onwards)
SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) is a photometric survey designed to discover Earth-size planets transiting the brightest (J ≤ 14) southern (DEC ≤ +10°) ultra-cool stars (dwarfs with spectral type later than M5). It consists of four 1m robotic telescopes equipped with CCD cameras operating in the very-near-IR (0.7 to 1 microns). The Facility will be installed at Paranal to benefit from its unique low water vapor and high transparency.
SPECULOOS is a collaboration between the University of Liège in Belgium (Lead), the University of Cambridge in UK, and the King Abdul Aziz University in Saudi Arabia.
CHEOPS (2017 onwards)
The CHaracterizing ExOPlanet Satellite (CHEOPS) will be the first mission dedicated to search for transits by means of ultrahigh precision photometry on bright stars already known to host planets. By being able to point at nearly any location on the sky, it will provide the unique capability of determining accurate radii for a subset of those planets for which the mass has already been estimated from ground-based spectroscopic surveys. It will also provide precision radii for new planets discovered by the next generation ground-based transits surveys (Neptune-size and smaller). The CHEOPS project is a partnership between european team members, including the University of Cambridge.
Terra Hunting Experiment (2018 onwards)
The Terra Hunting Experiment (THE) is a proposed radial velocity measurement survey for the Isaac Newton Telescope in La Palma. The 10 year experiment plans to target a set of our brightest G and K-type stars (V ≤ 8.5) to look for Earth-mass planets on orbital periods much greater than is currently possible (up to a few 100 days); this is approaching the regime of Earth-like planets. The experiment will build and use a close-copy of the HARPS spectrograph and over the 10 year survery will collect long series of daily RV measurements for each star. These intense series of regular measurements will help combat signal aliases (false detections) and closely monitor stellar activity to better enable low amplitude RV signals to be retrieved.
The Terra Hunting Experiment consortium is lead by University of Cambridge and partners with University of Exeter, University of Geneva, Leiden University, Instituto de Astrofísica de Canarias and Uppsala University.