The EChO science case
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Journal Article
Abstract
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.
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.
Date Issued
2015-12-01
Date Acceptance
2015-10-26
Citation
Experimental Astronomy, 2015, 40 (2-3), pp.329-391
ISSN
1572-9508
Publisher
Springer Verlag (Germany)
Start Page
329
End Page
391
Journal / Book Title
Experimental Astronomy
Volume
40
Issue
2-3
Copyright Statement
© The Author(s) 2015. This article is published with open access at Springerlink.com
License URL
Sponsor
Science and Technology Facilities Council (STFC)
Grant Number
ST/K001051/1
Subjects
Science & Technology
Physical Sciences
Astronomy & Astrophysics
Exoplanets
Spectroscopy
Atmospheric science
IR astronomy
Space missions
HUBBLE-SPACE-TELESCOPE
EXOPLANET HD 189733B
EXTRASOLAR PLANET ATMOSPHERE
INFRARED-EMISSION-SPECTRUM
HOT-JUPITER ATMOSPHERES
GIANT PLANETS
MU-M
TRANSMISSION SPECTROSCOPY
ENERGY-BALANCE
WATER-VAPOR
Publication Status
Published