The Exoplanet Characterisation Observatory (EChO) will be the first dedicated mission to investigate exoplanetary atmospheres, addressing the suitability of those planets for life and placing our Solar System in context.
EChO will provide high resolution, multi-wavelength spectroscopic observations. It will measure the atmospheric composition, temperature and albedo of a representative sample of known exoplanets, constrain models of their internal structure and improve our understanding of how planets form and evolve. It will orbit around the L2 Lagrange point, 1.5 million km from Earth in the anti-sunward direction.
The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. It will place our Solar System in context and by addressing the suitability of planets for life will allow us to address some of the fundamental questions of the Cosmic Visions programme:
- What are the conditions for planet formation and the emergence of life?
- Are systems like our Solar System rare or very common? How does the Solar System work?
EChO will provide high resolution, simultaneous multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission.
During a primary transit, when a planet passes in front of its star, the star’s light passes through the limb of the planet’s atmosphere, effectively providing an atmospheric transmission spectrum. During a secondary eclipse the planet passes behind its star; the dip in the flux reveals the emission, and at optical wavelengths, the reflection, spectrum of the planet. An orbital light-curve can be used to obtain the horizontal gradients of the temperature and composition of exoplanets. These combined approaches provide complementary information, including the temperature and composition profiles over ~3 decades of pressure,the planet’s cloud opacity and composition, and disk-wide temperature and composition variations. In all cases, instead of spatially separating the light of the planet from that of the star, EChO exploits temporal variations to extract the planet’s signal. The mission will study a large range of processes that shape the structure of planets, a few of which we highlight here.
EChO will provide constraints on the formation of the exoplanets. Primary and secondary observations will readily indicate (through the CH4, CO, CO2 and H2 features) the carbon and oxygen elemental abundance of the atmospheres which point to the formation mechanism of the planet: whether by the accretion of solids (as did our planets) or by gas collapse (as do stars). The planet’s C and O abundance can be compared to measurements of C and O in the primary star to determine whether the atmosphere’s abundance matches that of the star. A significant overabundance of heavy elements in the planet indicates formation by core accretion. A mild enrichment of heavy elements indicates formation by gas collapse.
EChO will characterise an exoplanet’s climate. The composition and thermal structure of the planet’s atmosphere will be compared to the primary star’s incident light and measured reflection to determine the partitioning of stellar radiation. Such measurements probe the greenhouse effect, which shapes planetary atmospheres and renders them habitable (as on Earth) or uninhabitable (as on Venus). Planetary climates are also affected by atmospheric dynamics: Earth’s circulation redistributes heat from equator to pole. EChO will study the dynamics of exoplanetary atmospheres through the derived vertical and horizontal temperature and compositional gradients.
EChO will study exoplanetary chemistry. Thermal equilibrium models will indicate if an exoplanet’s atmosphere is in equilibrium. With the exception of the deep atmospheres of the Giant Planets, atmospheric equilibrium is rare in the Solar System, so we expect to find atmospheres that are out of equilibrium. We plan to study photochemistry, the most important non-equilibrium chemical process which on Earth is responsible for O3 production, through the identification and measured abundances of principal photochemical products.
Monitoring stellar variability simultaneously with exoplanet atmospheric data is a key aspect of the mission. The best available indicator of chromospheric flux in the wavelength ranges accessible to EChO is the H Balmer alpha line at 0.66\,µm. Emission in the core of the line can be used to determine how variations in the stellar chromosphere affect planetary atmospheres, and to distinguish stellar variability from planetary variability. In addition, EChO will search for H3+, which indicates if a Jovian-type planet has a magnetosphere and is an indicator of the effects that stars of different types have on planetary atmospheres.
The investigation of exoplanetary atmospheres requires a dedicated space mission that is fine-tuned to this purpose. Such a mission must be capable of capturing a snapshot of the planet’s atmosphere and separating time variable characteristics from steady state conditions. It must be able to observe many systems, including the dimmer planets that approach the size of Earth and be optimised to eliminate systematic errors. Lastly, it must have a spectrometer with sufficient resolution to capture the spectral characteristics of the constituents that reveal the chemical and dynamical processes of the atmosphere. EChO fulfils all of these requirements.
EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However EChO’s configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral region to constrain from one spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen molecules, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution of the 4 channels are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres.
The target list for EChO includes planets ranging from Jupiter-sized, with an orbital semi-major axis one tenth that of Mercury and equilibrium temperatures, Teq up to 2000 K, to those of a few Earth masses, with Teq ~300 K. The list will include planets with no Solar System analog, such a as the recently discovered planet GJ1214b, whose density lies between that of terrestrial and gaseous planets.
As the number of detected exoplanets grows exponentially each year, and the mass of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems.
We have baselined a dispersive spectrograph design covering continuously the 0.4-16 µm spectral range in 6 channels (1 VIS, 5 IR) which allows the spectral resolution to be adapted to the target brightness from several tens (Lambda ≥ 11 µm) to several hundreds (Lambda ≤ 11 µ m). Thus optimising for the scientific objectives over the observation spectral range. The instrument is mounted behind a 1.2/1.5 m class telescope passively cooled and detectors with performance optimised precisely to each wavelength range, actively cooled in order to reduce the dark current to the required level. Stability and accuracy of the photometry is critical to the success of EChO and the design of the whole detection chain and satellite will be dedicated to achieving a high degree of photometric stability and repeatability. Calibration is also critical and requires detailed monitoring of the detector performance using both internal and external calibration sources.
EChO will be placed in a grand halo orbit around L2. This orbit, in combination with a nested thermal shield design, provides a highly stable thermal environment for the passive cooling of the instrument and telescope. The orbit and thermal shield design will also provide a high degree of visibility of the sky over the year and an ability to repeatedly observe several tens of targets whatever the epoch in the year.
The EChO CDF has now been completed, see sci.esa.int/echo
Measure the atmospheric composition, temperature and albedo of a highly representative sample of known extrasolar planets, orbiting different stellar types (F, G, K and M). The sample will include hot, warm, and habitable-zone exoplanets, down to the Super-Earth size (~1.5 Earth radii). Some of the key molecular, atomic and ionic species detectable by EChO between 0.4 and 16 µm at the baseline spectral resolving power are: H2O, CO, CO2, CH4, NH3, H3+, He, K, O3, H-alpha and hydrocarbons. The climate of a planet depends on the amount of solar radiation that is reflected out to space and the amount that is absorbed. Measuring the reflected light from the planet will provide information about the albedo of the planet and the possible presence of condensates and hazes. The combination of visible albedo and infrared temperature will be key to understanding how the energy is redistributed. In the Solar System albedo can range from ~0.05 for the asteroids belt to a maximum of 0.99 for Enceladus, with an average of ~0.3, like the case of the Earth. Photometric observations with MOST seem to indicate that the albedo of the Hot-Jupiters HD 209458b and HD 189733b is very low 1 , suggesting the presence of highly absorbing hazes or clouds.
- Measure the spatial (vertical & horizontal) and temporal variability of the thermal/chemical atmospheric structure of giants, Neptunes and hot Super-Earths. The photometric accuracy of EChO at multiple wavelengths will be sufficient to observe the planet not merely as day/night hemispheres or terminator, but to divide the planet into longitudinal slices, hence producing coarse maps of exoplanets (see proposal). Repeated ingress/ egress measurements and phase light-curves for bright eclipsing hot exoplanets will inform atmospheric modelling efforts. This spatial/temporal differentiation is necessary to:
- Understand the relative importance of thermochemical equilibrium, photochemistry, and transport-induced quenching in controlling the observed composition. These factors largely depend on the planet’s thermal structure, which in turn depends on the planet’s orbital distance and metallicity, and the host star’s luminosity and stellar type. The host star’s chromospheric activity level and the overall UV flux incident on the planet can also affect the photochemistry, but properties like planetary mass or radius play less of a role (see proposal).
Provide much needed constraints for atmospheric dynamics and circulation models. General arguments suggest that planets with orbital periods on the order of a few days are tidally locked 2, 3, 4, so that their permanent daysides are continuously subjected to intense stellar irradiation, while their night-sides may be heated only by a more modest internal energy flux. In the presence of such an uneven energetic forcing, leading to significant (horizontal) temperature gradients, the efficiency of the horizontal heat redistribution is an important open question, as it largely determines their observational properties. Several attempts have been made to address this general circulation problems. Longitudinal brightness maps from the light curve of phase variations, observed by EChO, promise to be powerful diagnostic tools for simulations of hot planets’ atmospheric dynamics. Vortices and waves are structures in exoplanet atmospheres that can produce observable temporal variability: these are usually long-lived and evolve with characteristic periodicities 5, 6 which can be captured by EChO’s observations (see proposal).
- Investigate the complex planet-star interaction. Proper characterisation of a planet’s host star is key to the interpretation and to the understanding of planetary data. The star has an impact on two major aspects, i) our ability to measure and interpret the data because of potential sources of systematic errors, and also ii) the modification of the structure and evolution of the planet atmosphere by being the overwhelmingly largest source of energy. Monitoring stellar variability simultaneously with the acquiring of data from which the exoplanet atmosphere will be measured is a key aspect of the EChO mission. Depending on the contrast temperature, our simulations indicate that the visible continuum variations can be a factor of 3-10 times larger than those of the near-IR and mid-IR, thus allowing for proper modelling and subsequent correction of the exoplanet data.
- Constrain the models of internal structure. Although EChO will by definition measure the characteristics of planetary atmospheres it will be also crucial in improving our knowledge of planetary interiors. EChO will of course be able to measure with exquisite accuracy the depth of the primary transit and thus the planetary size, but the major improvements for interior models will come from its ability to fully characterise the atmosphere in its composition, dynamics and structure. We give here a couple of examples (see proposal). – Giant planets are mostly made of hydrogen and helium and are expected to always be in gaseous form 7. Because they play an essential role in shaping planetary systems 8 determining precisely their internal structure and composition is essential to understanding how planets form. A large fraction of the known transiting planets are larger than expected, even when considering that they could be coreless hydrogen-helium planets 9, 10, 11, 12, 13, 14. There is thus missing physics that needs to be identified. – Neptune-like planets possess a thick atmosphere mostly composed of hydrogen and helium (several Earth masses). Mass and radius measurements obtained from radial velocities and transit observations, respectively, will not be sufficient to distinguish between this intermediate family and terrestrial planets with a significant amount of water 15. This degeneracy can be easily removed by sounding the atmosphere through primary transit spectroscopy. EChO’s observed spectra will allow the atmospheric scale height to be determined, hence the main atmospheric component (i.e. molecular hydrogen or water vapour in this case) and the gravity – the atmospheric temperature is estimated independently by eclipse measurements (see proposal).
- Improve our understanding of planetary formation/evolution mechanisms. High resolution spectroscopy will provide important information about the chemical constituents of planetary atmospheres, and this is expected to be related to both the formation location, and the chemical state of the protoplanetary disk. Atmospheric escape is an important factor in the evolution of exoplanet atmospheres, and may define the boundary between Neptune-like planets and Super Earths. H3+ is a key component for the thermal stability of exoplanets: there is a sharp inner limit to the distance a gas giant can be from its star and retain thermal stability. Inside this distance (which is about 0.16 AU for a Jovian sized body around a Sun-like star) the H2 from which the H3+ is made begins to dissociate, inhibiting the molecular ion’s formation, thus reducing the cooling effect. The planet responds by heating up enormously with its thermosphere expanding to many times its normal size 16. Our simulations show that EChO’s spectral resolving power of ~300 is sufficient to detect H3+ (see proposal).
- explore the thermal/chemical variability along the orbit of non transiting exoplanets, especially in high-eccentric orbit. This work was pioneered by Harrington et al. 17 phase curve measurements of non transiting Ups And b. Despite the fact that all planets in our Solar System have a circular orbit, more than 60% of the exoplanets discovered have a high eccentricity, e.g. HD 80606b 18. While non-transiting planets will not be a primary goal, EChO will give us the unique opportunity of studying the chemistry and thermal properties of very exotic objects. On top of that, EChO could:
- search for Exomoons. We estimate that moons down to 0.33REarth would be detectable with EChO for our target stars. Whilst Kepler may also be able to detect exomoons, EChO has numerous advantages in that: 1) we target brighter stars so higher SNRs 2) EChO works from 0.4-2.5 µm and can thus obtain NIR light curves which exhibit highly reduced distortion from limb darkening and stellar activity e.g. spots (Kepler is 0.4-0.9 µm) and 3) multi-colour light curves significantly attenuates degeneracy of fitted limb darkening parameters across all wavelengths (see proposal).
- identify potential biosignatures in the atmospheres Super-Earths in the habitable zones of late type stars. In addition to the basic parameters described above, which are common to all planets with atmospheres, a planet which harbours life may also exhibit astronomical biosignatures, i.e. the presence of chemically based life on a planet would change the composition of its atmosphere away from the a-biological steady state 19. The study of Super-Earths in the habitable zone of stars cooler than the sun, will challenge the paradigm of the Earth-twin orbiting a Sun-twin as the only possible cradle for life.
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