The EChO science case
Journal article, 2015
The discovery of almost two thousand exoplanets has revealed an unexpectedly diverse planet population. We see gas giants in few-day orbits, whole multi-planet systems within the orbit of Mercury, and new populations of planets with masses between that of the Earth and Neptune—all unknown in the Solar System. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? How do planetary systems work and what causes the exceptional diversity observed as compared to the Solar System? The EChO (Exoplanet Characterisation Observatory) space mission was conceived to take up the challenge to explain this diversity in terms of formation, evolution, internal structure and planet and atmospheric composition. This requires in-depth spectroscopic knowledge of the atmospheres of a large and well-defined planet sample for which precise physical, chemical and dynamical information can be obtained. In order to fulfil this ambitious scientific program, EChO was designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large, diverse and well-defined planet sample within its 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allows us to measure atmospheric signals from the planet at levels of at least 10−4 relative to the star. This can only be achieved in conjunction with a carefully designed stable payload and satellite platform. It is also necessary to provide broad instantaneous wavelength coverage to detect as many molecular species as possible, to probe the thermal structure of the planetary atmospheres and to correct for the contaminating effects of the stellar photosphere. This requires wavelength coverage of at least 0.55 to 11 μm with a goal of covering from 0.4 to 16 μm. Only modest spectral resolving power is needed, with R ~ 300 for wavelengths less than 5 μm and R ~ 30 for wavelengths greater than this. The transit spectroscopy technique means that no spatial resolution is required. A telescope collecting area of about 1 m2 is sufficiently large to achieve the necessary spectro-photometric precision: for the Phase A study a 1.13 m2 telescope, diffraction limited at 3 μm has been adopted. Placing the satellite at L2 provides a cold and stable thermal environment as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. EChO has been conceived to achieve a single goal: exoplanet spectroscopy. The spectral coverage and signal-to-noise to be achieved by EChO, thanks to its high stability and dedicated design, would be a game changer by allowing atmospheric composition to be measured with unparalleled exactness: at least a factor 10 more precise and a factor 10 to 1000 more accurate than current observations. This would enable the detection of molecular abundances three orders of magnitude lower than currently possible and a fourfold increase from the handful of molecules detected to date. Combining these data with estimates of planetary bulk compositions from accurate measurements of their radii and masses would allow degeneracies associated with planetary interior modelling to be broken, giving unique insight into the interior structure and elemental abundances of these alien worlds. EChO would allow scientists to study exoplanets both as a population and as individuals. The mission can target super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300–3000 K) of F to M-type host stars. The EChO core science would be delivered by a three-tier survey. The EChO Chemical Census: This is a broad survey of a few-hundred exoplanets, which allows us to explore the spectroscopic and chemical diversity of the exoplanet population as a whole. The EChO Origin: This is a deep survey of a subsample of tens of exoplanets for which significantly higher signal to noise and spectral resolution spectra can be obtained to explain the origin of the exoplanet diversity (such as formation mechanisms, chemical processes, atmospheric escape). The EChO Rosetta Stones: This is an ultra-high accuracy survey targeting a subsample of select exoplanets. These will be the bright “benchmark” cases for which a large number of measurements would be taken to explore temporal variations, and to obtain two and three dimensional spatial information on the atmospheric conditions through eclipse-mapping techniques. [...]
IR astronomy
Spectroscopy
Exoplanets
Atmospheric science
Space missions
Author
G. Tinetti
University College London (UCL)
P. Drossart
Observatoire de Paris-Meudon
P. Eccleston
STFC Rutherford Appleton Laboratory
P. Hartogh
Max Planck Society
K. Isaak
European Space Research and Technology Centre (ESA ESTEC)
M. Linder
European Space Research and Technology Centre (ESA ESTEC)
C. Lovis
University of Geneva
G. Micela
Istituto nazionale di astrofisica (INAF)
M. Ollivier
Institut d'Astrophysique Spatiale
Observatoire de Paris-Meudon
L. Puig
European Space Research and Technology Centre (ESA ESTEC)
I. Ribas
Institute of Space Studies of Catalonia (IEEC)
I. Snellen
Leiden University
B. Swinyard
University College London (UCL)
STFC Rutherford Appleton Laboratory
F. Allard
École Normale Supérieure de Lyon
J. Barstow
University of Oxford
J. Cho
Queen Mary University of London
A. Coustenis
Observatoire de Paris-Meudon
C. Cockell
Royal Observatory
A. Correia
University of Aveiro
L. Decin
KU Leuven
R. J. De Kok
Netherlands Institute for Space Research (SRON)
P. Deroo
Jet Propulsion Laboratory, California Institute of Technology
T. Encrenaz
Observatoire de Paris-Meudon
F. Forget
LMD
A. Glasse
STFC UK-ATC
C. Griffith
University of Arizona
T. Guillot
Observatoire de la Cote d'Azur
T. Koskinen
University of Arizona
H. Lammer
IWF
J. Leconte
LMD
University of Toronto
P. Maxted
Keele University
I. Mueller-Wodarg
Imperial College London
R. Nelson
Queen Mary University of London
C. North
Cardiff University
E. Palle
Instituto de Astrofísica de Canarias
I. Pagano
Istituto nazionale di astrofisica (INAF)
G. Piccioni
Istituto nazionale di astrofisica (INAF)
D. Pinfield
University of Hertfordshire
F. Selsis
University of Bordeaux
A. Sozzetti
Istituto nazionale di astrofisica (INAF)
L. Stixrude
University College London (UCL)
J. Tennyson
University College London (UCL)
D. Turrini
Istituto nazionale di astrofisica (INAF)
M. Zapatero-Osorio
CAB
J.P. Beaulieu
Institut d 'Astrophysique de Paris
D. Grodent
University of Liège
M. Guedel
University of Vienna
D. Luz
University of Lisbon
H. U. Norgaard-Nielsen
DSRI
T. P. Ray
Dublin Institute for Advanced Studies
H. Rickman
Polish Academy of Sciences
Uppsala University
A. Selig
Netherlands Institute for Space Research (SRON)
M. Swain
Jet Propulsion Laboratory, California Institute of Technology
M. Banaszkiewicz
Polish Academy of Sciences
M. Barlow
University College London (UCL)
G. Branduardi-Raymont
University of Oxford
V. C. du Foresto
Observatoire de Paris-Meudon
L. Gizon
University of Liège
A. Hornstrup
Max Planck Society
C. Jarchow
Hungarian Academy of Sciences
F. Kerschbaum
Max Planck Society
G. Kovacs
University of Vienna
P.O. Lagage
Hungarian Academy of Sciences
T. Lim
The French Alternative Energies and Atomic Energy Commission (CEA)
T. L. Lim
STFC Rutherford Appleton Laboratory
G. Malaguti
Institute of Space Studies of Catalonia (IEEC)
E. Pace
Istituto nazionale di astrofisica (INAF)
E. Pascale
University of Florence
B. Vandenbussche
Cardiff University
G. Wright
KU Leuven
G.R. Zapata
STFC UK-ATC
R. Azzollini
Istituto nazionale di astrofisica (INAF)
R. Burston
STFC UK-ATC
J. Colomé
Max Planck Society
M. Crook
Institute of Space Studies of Catalonia (IEEC)
A. di Giorgio
STFC Rutherford Appleton Laboratory
A. M. di Giorgio
Istituto nazionale di astrofisica (INAF)
R. Hoogeveen
Cardiff University
K. Middleton
STFC Rutherford Appleton Laboratory
G. Morgante
STFC Rutherford Appleton Laboratory
F. Pinsard
Istituto nazionale di astrofisica (INAF)
R. Ottensamer
University of Vienna
M. Rataj
The French Alternative Energies and Atomic Energy Commission (CEA)
J.M. Reess
Polish Academy of Sciences
G. Savini
Observatoire de Paris-Meudon
Kay Justtanont
Chalmers, Earth and Space Sciences, Onsala Space Observatory
J.R. Schrader
Netherlands Institute for Space Research (SRON)
R. Stamper
STFC Rutherford Appleton Laboratory
B. Winter
University College London (UCL)
L. Abe
Observatoire de la Cote d'Azur
M. Abreu
University of Lisbon
Experimental Astronomy
0922-6435 (ISSN) 1572-9508 (eISSN)
Vol. 40 2-3 329-391Subject Categories
Physical Sciences
DOI
10.1007/s10686-015-9484-8