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Characterising exoplanets: detection, formation, interiors, atmospheres and habitability

Event

Starts:

March
112013

09:00

Ends:

March
122013

17:00

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Artist's impression of 'Waterworld' Kepler 22b

Scientific discussion meeting organised by Professor Athena Coustenis, Professor Steve Miller, Professor Peter Read and Professor Jonathan Tennyson FRS

Event details

This meeting aims to set the agenda for the next decade in this rapidly expanding field of extra-solar planet science. Some 800 planets orbiting stars outside of our solar system have now been detected, with varying masses and orbital parameters. The challenge now is to move from detection to understanding these planets as bodies in their own right.

Biographies of speakers and their talk abstracts can be found below and you can download the programme. Recorded audio of the presentations are available below. Papers of this meeting have been published in Philosophical Transactions A.

Enquiries: Contact the events team.

Event organisers

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Schedule of talks

Session 1: Exoplanet detection

5 talks Show detail Hide detail

Chair of Session 1

Professor Bill Borucki

Abstract

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.

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Direct detection of exoplanets: results, consequences on planet formation theories, and future

Dr Anne-Marie Lagrange

Abstract

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.

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Overview, limitations and prospects on exoplanet insights from HARPS and transiting planet perspective

Professor Didier Queloz, University of Cambridge, UK

Abstract

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.

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Radial velocity studies of cool stars

Professor Hugh Jones

Abstract

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.

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The low-mass exoplanet population discovered by HARPS

Professor Christophe Lovis

Abstract

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.

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Session 2: Formation and interiors of exoplanets

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Chair of Session 2

Dr James Cho

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Material behavior in exoplanet interiors

Professor Lars Stixrude

Abstract

What are exoplanet interiors made of?  The answer matters because interiors comprise nearly all the planetary mass and therefore shape our ideas about formation, as well as controlling long-term evolution, the generation of planetary magnetic fields, and in the case of super-Earths the generation of surface environments, and the prospects for habitability.  We examine possible connections between the interior and atmospheric composition, focusing on processes of differentiation, buoyant segregation, core erosion, and de-gassing.  These processes highlight the importance of learning more about material behavior in the largely unexplored region of pressure-temperature space typical of most exoplanetary interiors discovered so far.  We discuss two examples of progress in this area, focusing on ab initio quantum mechanical simulations of fluid helium and solid iron, and implications for the luminosity of giant planets and the generation of magnetic fields in super-Earths.

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M-R relationships for highly compressed rocky cores – a possible new family of planets

Professor Olivier Grasset

Abstract

Super-Earths are one of the most important target of space exploration for the next decade. With the discovery of potential candidates that suggests a high probability for having other Earths in our galaxy , it is important to ascertain how confident we can be that a planet with adequate mass and radius is indeed like our Earth’s planet.

It has already been shown in previous studies that similar mass and radius relationships can be found for Super-Earths and mini-Neptune. A third family can also be suggested. Since planetary migration is common process in stellar systems, it is highly possible that a few “planets“ discovered close to their stars are in fact the remnants of the inner core of gaseous exoplanets. In this paper, we investigate the M-R relationships of highly compressed rocky planets, similar to what could be the remants of Uranus-like planets. Characteristics of these bodies, as well as the probability of such occurrences, will be discussed.

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Scenarios of giant planet formation and evolution and their impact on the formation of habitable terrestrial planets

Dr Alessandro Morbidelli, Nice Observatory, France

Abstract

The giant planets in our Solar System remained on orbits beyond the habitable zone. In addition, they remained  on orbits with very small eccentricities and inclinations. These two dynamical features allowed the formation of terrestrial planets, one of which turned out to be habitable and inhabited. However, it is obvious from the orbital distribution of the extrasolar giant planets discovered to date that the evolution of the giant planets of our Solar System is not generic. Many giant planets migrated towards their parent star, crossing through or stopping near the habitable zone. In many cases, the giant planets also acquired eccentric orbits. I will discuss briefly the origin of the orbital diversity of giant planets and the effects that migration and eccentricity excitation can have on the formation and survival of terrestrial planets. I will also consider the case in which no giant planet forms, arguing that a number of icy super-Earths or Neptune-mass planets are likely to accrete  beyond the snowline and migrate into the inner system, possibly jeopardizing the formation of rocky terrestrial planets.

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The role of migration in planetary system formation

Professor Richard Nelson

Abstract

Radial velocity and transit observations of extrasolar planet systems provide strong evidence that orbital migration, driven by interaction with their gaseous protoplanetary disc, plays an important role during planet formation. There has been significant development in our understanding of migration processes in recent years, and I will review this progress and the current state-of-the-art in my talk. Migration, planetary mass growth, and planet-planet mutual interactions are tightly coupled processes, and an improved understanding of planet formation requires models that incorporate this coupling explicitly. I will present the results of recent simulations that attempt to include these processes self-consistently, and compare them with the known population of extrasolar planets.

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Session 3: Chemistry and spectroscopy of exoplanets

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Chair of Session 3

Professor Jonathan Tennyson FRS, University College London, UK

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Exoplanet kinetics: hot topic or out cold?

Dr Julianne Moses

Abstract

Chemical kinetics plays an important role in controlling the atmospheric composition of all planetary atmospheres, including those of extrasolar planets.  For the hottest exoplanets, the composition can closely follow thermochemical-equilibrium predictions, at least in the visible/infrared photosphere at dayside (eclipse) conditions.  However, for atmospheric temperatures < ~2000 K, and in the uppermost atmosphere at any temperature, chemical kinetics matters.  The two key mechanisms by which kinetic processes drive an exoplanet atmosphere out of equilibrium are photochemistry and transport-induced quenching.  I will review these disequilibrium processes in detail, discuss observational consequences, and examine some of the current evidence for kinetic processes on extrasolar planets.

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Infrared spectroscopy of exoplanets: observational constraints

Professor Therese Encrenaz

Abstract

The exploration of transiting extrasolar planets is an exploding research area in astronomy. With more than 200 exoplanets identified so far, these discoveries have made possible the development of a new research field, the spectroscopic characterization of exoplanets’atmospheres, using both primary and secondary transits. However, these observations have been so far limited to a very small number of targets. In this paper, we will first review the advantages and limitations of both primary and secondary transit methods. Then we will analyse what kind of infrared spectra can be expected for different types of planets, and we will discuss how to optimize the spectral range and the resolving power of the observations. We will also try to identify the most promising targets for present and future ground‐based observations.

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Molecular opacities for exoplanets

Professor Peter Bernath

Abstract

Spectroscopic observations of exoplanets are now possible by transit methods and direct emission. Spectroscopic requirements for exoplanets will be reviewed based on existing measurements and model predictions for hot Jupiters and super-Earths. Molecular opacities needed to simulate astronomical observations can be obtained from laboratory measurements, ab initio calculations or a combination of the two approaches. The talk will focus mainly on laboratory spectroscopy of hot molecules as needed for exoplanets.

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Session 4: Atmospheres of exoplanets

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Chair of Session 4

Professor Isabelle Baraffe

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Climates on terrestrial exoplanets

Dr Francois Forget

Abstract

The nature of the possible climates on extrasolar planets cannot yet be detected with telescopes. Nevertheless, key scientific questions motivates some investigations on the subject, for instance to prepare future observations, or to assess planetary habitability. For this purpose, we can use numerical climate models derived from the one used on the Earth to study climate changes, and which have been successfully tested on the terrestrial atmospheres of our solar system (Venus, Mars, Titan, Triton, Pluto). Such models can be considered as “planet simulators” that aims to simulate the complete environment on the basis of universal equations only. On the basis of our experience in the solar system, we have developed a new type of climate model flexible enough to simulate the wide range of conditions that may exist on terrestrial exoplanets, including any atmospheric cocktail of gases, clouds and aerosols for any planetary size, and around any star. Much can be learned from such models. Of course, to make further progress, we also have to learn about the possible composition of the atmospheres, which depends on their origin, escape, and long term chemical evolution.

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Exploring the diversity of Jupiter-class planets

Dr Leigh Fletcher

Abstract

Of the 860+ exoplanets discovered to date, 75% have masses larger than Saturn (0.3MJ), 55% are more massive than Jupiter, and 60% are within 1 AU of their host stars.  And yet the term 'hot Jupiter' betrays the incredible diversity of this class of astrophysical object.  This presentation will give an overview of our expectations for the temperatures, molecular composition and cloud properties of Jupiter-class objects under a variety of different conditions.  We will review the taxonomy and classification schemes for these Jupiter-class planets proposed to date, including the implications for our own Solar System giant planets.  We will attempt to show the potential categories of planetary types, accounting for: (i) thermochemical equilibrium expectations for cloud condensation and favoured chemical stability fields; (ii) the metallicity and formation mechanism for these giant planets; (iii) the importance of optical absorbers for energy partitioning and the generation of a temperature inversion; (iv) the favoured photochemical pathways and expectations for minor species (e.g., saturated hydrocarbons and nitriles); (v) molecular confusion arising from  vertical mixing of species above their quench levels; and (vi) methods for energy redistribution throughout the atmosphere, from the radiative to the convective zones.   Finally, we will discuss the benefits and pitfalls of retrieval techniques for establishing a family of atmospheric solutions that reproduce the available data without bias to a particular planetary model, and the requirements for future spectroscopic characterisation of a reference set of Jupiter-class objects to test our physical and chemical understanding of these planets.

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Interpretation of transiting exoplanets measurements

Professor Caitlin Griffith

Abstract

The atmospheres of transiting exoplanets are characterized by two distinct measurements.  The primary transit provides transmission spectra of the exoplanet's limb as the planet passes in front of the star. The secondary eclipse, which inducates the star's emission alone, when compared to the star plus planet emission, recorded outside of eclipse, yields the planet's emission spectrum.  Infrared transmission and emission spectroscopy have revealed over the past decade the primary carbon and oxygen molecular species (CH4, CO2, CO, and H2O) in a few exoplanets. However, efforts to constrain the molecular abundances to within several orders of magnitude are thwarted by the degenerate effectsthat the temperature and composition have on the emission spectra.  Similarly, transmission spectra, while less sensitive to the atmospheric temperatures, provide a degenerate set of solutions, because the composition derived is highly dependent to the assumed radius.  Problems in the interpretation of both spectra also arise from the potential variability of host stars. Apart from the challenges of deriving the basic properties of an exoplanet, the interpretation of the chemistry and possibly the evolution of an exoplanet requires knowledge of the starting composition of the planetary system.  Here we discuss general methods for addressing these uncertainties, which include ground-based high resolution spectroscopy of host stars, optical photometry of exoplanet transmission, and the joint analysis of transit and secondary eclipse data of exoplanets.   These methods will be explored with a discussion of recent and former measurements of the planet XO-2b, its host star (XO-2N) and stellar companion (XO-2S).

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Thermal escape from extrasolar giant planets

Dr Tommi Koskinen

Abstract

The detection of hot atomic hydrogen and heavy atoms and ions at high altitudes around some close-in extrasolar giant planets (EGPs) such as HD209458b imply that these planets have hot and rapidly escaping atmospheres that extend to several planetary radii.  However, these characteristics cannot be generalized to all close-in EGPs.  The thermal escape mechanism and mass loss rates from EGPs depend on a complex interplay between photochemistry and radiative transfer driven by the stellar UV radiation.  In this work we explore how these processes change under different levels of irradiation on giant planets with different characteristics.  We confirm that there are two distinct regimes of thermal escape from EGPs, and that the transition between these regimes is relatively sharp.  Our results have significant implications on thermal mass loss rates from different EGPs that we discuss in the context of currently known planets and the detectability of their upper atmospheres.

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Session 5: Exoplanet atmospheres and signatures of habitability

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Chair of Session 5

Dr Daphne Stam

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Finding extraterrestrial life using ground-based high-dispersion spectroscopy

Dr Ignas Snellen

Abstract

The cancellation of both the Terrestrial Planet Finder and Darwin missions means that it is unlikely that a dedicated space telescope to search for biomarker gases in exoplanet atmospheres will be launched within the next

25 years. In this talk I will advocate that ground-based telescopes provide a strong alternative for finding biomarkers in exoplanet atmospheres through high-resolution transit observations. I will review some of the exciting result we have recently obtained on hot-Jupiter atmospheres, and will show what can be done with the planned E-ELT.

Ultimately, large arrays of dedicated flux collector telescopes equipped with high-dispersion spectrographs can provide the large collecting area needed to perform a statistical study of life-bearing planets in the solar neighborhood.

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Habitable worlds with no signs of life

Professor Charles Cockell

Abstract

Habitable planets may turn out to be abundant in the Universe. However, the search of life on them could yield many negative results. Habitable worlds with no signs of life could include: 1) planets that are habitable, but have no biosphere (uninhabited habitable worlds); 2) planets with life, but where biologically-derived atmospheric gases out of equilibrium with abiotic processes are within the thermodynamic uncertainty in the atmospheric constituents. The uncertainties in the sources and sinks of the atmospheric trace gases, CO and H2 on Mars, illustrate the problem, even in our own Solar System; and 3) planets that have abundant life, but lack surface signatures of that life. This can be demonstrated in the laboratory. Accepting obvious limitations in instrument resolving power, these scenarios can be examined with the experimentally testable hypothesis that the universe contains many habitable worlds with no signs of life. If the hypothesis is rejected and most or all habitable planets have signatures of life, then we could conclude that: life is inevitable on any planet with habitable conditions, it always rapaciously colonizes a planet wherever it emerges, giving rise to detectable gaseous and surface signatures, and it always ‘discovers’ oxygenic photosynthesis that is then produced in high abundance. The acceptance of the hypothesis would suggest the possible rarity of the origin of life or the lack of inevitability of the ‘discovery’ of oxygenic photosynthesis by biological evolution.

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Spectroscopy of planetary atmospheres in our galaxy

Dr Giovanna Tinetti, University College London, UK

Abstract

About 20 years after the discovery of the first extrasolar planet, the number of planets known has grown by three orders of magnitude, and continues to increase at breakneck pace. For most of these planets we have little information, except for the fact that they exist and possess an address in our Galaxy. For about one third of them, we know how much they weigh, their size and their orbital parameters. For less than twenty, we start to have some clues about their atmospheric temperature and composition. How do we progress from here? We are still far from the completion of a hypothetical Hertzsprung–Russell diagram for planets comparable to what we have for stars, and today we do not even know whether such classification will ever be meaningful for planetary objects. But one thing is clear: planetary parameters such as mass, radius and temperature alone will not explain the diversity revealed by current observations. The chemical composition of these planets is needed to trace back their formation history and evolution, as it happened for the planets in our Solar System. As in situ measurements are and will remain off-limits for exoplanets to study their chemical composition, we will have to rely on remote sensing spectroscopic observations of their gaseous envelopes.


Little more than 10 years ago, the detection of a signal from an exoplanet atmosphere was still in the realm of science fiction. Pioneering results were then obtained through transit spectroscopy with Hubble, Spitzer and ground-based facilities, making possible the detection of ionic, atomic and molecular species and of the planet’s thermal structure. The few available transit spectra of hot exoplanets are comparable in quality with the Solar System planetary spectra known in the 1970s, before the Voyager era. With the arrival of improved or dedicated instruments in the coming decade, planetary science will expand beyond the narrow boundaries of our Solar System to encompass our whole Galaxy.

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Characterising exoplanets: detection, formation, interiors, atmospheres and habitability The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK