Bibcode
Tinetti, Giovanna; Drossart, Pierre; Eccleston, Paul; Hartogh, Paul; Isaak, Kate; Linder, Martin; Lovis, Christophe; Micela, Giusi; Ollivier, Marc; Puig, Ludovic; Ribas, Ignasi; Snellen, Ignas; Swinyard, Bruce; Allard, France; Barstow, Joanna; Cho, James; Coustenis, Athena; Cockell, Charles; Correia, Alexandre; Decin, Leen; de Kok, Remco; Deroo, Pieter; Encrenaz, Therese; Forget, Francois; Glasse, Alistair; Griffith, Caitlin; Guillot, Tristan; Koskinen, Tommi; Lammer, Helmut; Leconte, Jeremy; Maxted, Pierre; Mueller-Wodarg, Ingo; Nelson, Richard; North, Chris; Pallé, Enric; Pagano, Isabella; Piccioni, Guseppe; Pinfield, David; Selsis, Franck; Sozzetti, Alessandro; Stixrude, Lars; Tennyson, Jonathan; Turrini, Diego; Zapatero-Osorio, Mariarosa; Beaulieu, Jean-Philippe; Grodent, Denis; Guedel, Manuel; Luz, David; Nørgaard-Nielsen, Hans Ulrik; Ray, Tom; Rickman, Hans; Selig, Avri; Swain, Mark; Banaszkiewicz, Marek; Barlow, Mike; Bowles, Neil; Branduardi-Raymont, Graziella; du Foresto, Vincent Coudé; Gerard, Jean-Claude; Gizon, Laurent; Hornstrup, Allan; Jarchow, Christopher; Kerschbaum, Franz; Kovacs, Géza; Lagage, Pierre-Olivier; Lim, Tanya; Lopez-Morales, Mercedes; Malaguti, Giuseppe; Pace, Emanuele; Pascale, Enzo; Vandenbussche, Bart; Wright, Gillian; Zapata, Gonzalo Ramos; Adriani, Alberto; Azzollini, Ruymán; Balado, Ana; Bryson, Ian; Burston, Raymond; Colomé, Josep; Crook, Martin; Di Giorgio, Anna; Griffin, Matt; Hoogeveen, Ruud; Ottensamer, Roland; Irshad, Ranah; Middleton, Kevin; Morgante, Gianluca; Pinsard, Frederic; Rataj, Mirek; Reess, Jean-Michel; Savini, Giorgio; Schrader, Jan-Rutger; Stamper, Richard; Winter, Berend; Abe, L.; Abreu, M.; Achilleos, N.; Ade, P.; Adybekian, V.; Affer, L. et al.
Bibliographical reference
Experimental Astronomy, Volume 40, Issue 2-3, pp. 329-391
Advertised on:
12
2015
Journal
Citations
9
Refereed citations
7
Description
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. If EChO were
launched today, the exoplanets currently observed are sufficient to
provide a large and diverse sample. The Chemical Census survey would
consist of > 160 exoplanets with a range of planetary sizes,
temperatures, orbital parameters and stellar host properties.
Additionally, over the next 10 years, several new ground- and
space-based transit photometric surveys and missions will come on-line
(e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on
finding bright, nearby systems. The current rapid rate of discovery
would allow the target list to be further optimised in the years prior
to EChO's launch and enable the atmospheric characterisation of hundreds
of planets.
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