Giovanna Tinetti

The Presence of Methane in the Atmosphere of an Extrasolar Planet

Molecules present in the atmospheres of extrasolar planets are expected to influence strongly the balance of atmospheric radiation, to trace dynamical and chemical processes, and to indicate the presence of disequilibrium effects. As molecules have the potential to reveal atmospheric conditions and chemistry, searching for them is a high priority. The rotational–vibrational transition bands of water, carbon monoxide and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jupiter planets. As these bands can overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy. Here we report a near-infrared transmission spectrum of the planet HD 189733b that shows the presence of methane. Additionally, a resolved water vapour band at 1.9 μm confirms the recent claim of water in this object. On thermochemical grounds, carbon monoxide is expected to be abundant in the upper atmosphere of hot-Jupiter planets, but is not identifiable here; therefore the detection of methane rather than carbon monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanism that leads to an enhancement of methane.

Biosignatures from Earth-like planets around M dwarfs.

Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible/near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs-AD Leo and GJ 643-and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earths was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planets atmosphere has been suggested as strong evidence for life. Planets circling M stars may be good locations to search for such evidence.

An afocal telescope configuration for the ESA Ariel mission

ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is one of the three candidates for the next ESA medium-class science mission (M4) expected to be launched in 2026. This mission will be devoted to observing spectroscopically in the infrared (IR) a large population of known transiting planets in the neighborhood of the Solar System, opening a new discovery space in the field of extrasolar planets and enabling the understanding of the physics and chemistry of these far away worlds. ARIEL is based on a 1-m class telescope ahead of two spectrometer channels covering the band 1.95 to 7.8 microns. In addition there are four photometric channels: two wide band, also used as fine guidance sensors, and two narrow band. During its 3.5 years of operations from L2 orbit, ARIEL will continuously observe exoplanets transiting their host star. The ARIEL optical design is conceived as a fore-module common afocal telescope that will feed the spectrometer and photometric channels. The telescope optical design is composed of an off-axis portion of a two-mirror classic Cassegrain coupled to a tertiary off-axis paraboloidal mirror. The telescope and optical bench operating temperatures, as well as those of some subsystems, will be monitored and fine tuned/stabilised mainly by means of a thermal control subsystem (TCU-Telescope Control Unit) working in closed-loop feedback and hosted by the main Payload electronics unit, the Instrument Control Unit (ICU). Another important function of the TCU will be to monitor the telescope and optical bench thermistors when the Payload decontamination heaters will be switched on (when operating the instrument in Decontamination Mode) during the Commissioning Phase and cyclically, if required. Then the thermistors data will be sent by the ICU to the On Board Computer by means of a proper formatted telemetry. The latter (OBC) will be in charge of switching on and off the decontamination heaters on the basis of the thermistors readout values.

Thermal architecture of the ESA ARIEL payload

The Atmospheric Remote-sensing Infrared Exoplanets Large-survey (ARIEL) is a space project selected by the European Space Agency for the Phase A study in the context of the M4 mission within the Cosmic Vision 2015-2025 programme. ARIEL will probe the chemical and physical properties of a large number of known exoplanets by observing spectroscopically their atmosphere, to extend our knowledge of how planetary systems form and evolve. To achieve its scientific objectives, the mission is designed as a dedicated 3.5-years survey for transit and eclipse spectroscopy, with an instrumental layout based on a 1-m class telescope feeding two spectrometer channels that cover the band 1.95 to 7.8 μm and four photometric channels in the visible to near-IR range. The high sensitivity requirements of the mission need an extremely stable thermo-mechanical platform. In this paper we describe the thermal architecture of the payload and discuss the main requirements that drive the design. The ARIEL thermal configuration is based on a passive and active cooling approach. Passive cooling is achieved by a V-Groove based design that exploits the L2 orbit favorable thermal conditions. The telescope and the optical bench are passively cooled to a temperature close to 50K to achieve the required sensitivity and stability. The photometric detectors are maintained at the operating temperature of 50K by a dedicated radiator coupled to cold space. The IR spectroscopic channel detectors require a lower temperature reference. This colder stage is provided by an active cooling system based on a Neon Joule-Thomson cold end, fed by a mechanical compressor, able to reach temperatures lower than 30K. Thermal stability of the telescope and detector units is one of the main drivers of the design. The periodical perturbations due to orbital changes, to the active cooling or to other internal instabilities make the temperature control one of the most critical issues of the whole architecture. The thermal control system design, based on a combination of passive and active solutions aimed at maintaining the required stability at the telescope and detector stages level, is described. We report here about the baseline thermal architecture at the end of the Phase A, together with the main trade-offs needed to enable the ARIEL exciting science in a technically feasible payload design. Thermal modeling results and preliminary performance predictions in terms of steady state and transient behavior are also discussed.

The SEE-COAST concept

The SEE COAST concept is designed with the objective to characterize extrasolar planets and possibly Super Earths via spectro-polarimetric imaging in reflected light. A space mission complementary to ground-based near IR planet finders is a first secure step towards the characterization of planets with mass and atmosphere comparable to that of the Earth. The accessibility to the Visible spectrum is unique and with important scientific returns. 1. Historic and context Radial Velocity technique has been one of the most prolific method to identify extrasolar planets in the past decade (374 objects listed in http://exoplanet.eu). The comparison with our own Solar System already suggests a large diversity (in mass, eccentricity, semimajor axis ...). One of the next step is the study of exoplanet atmospheres and we must be also prepared for diversity. In that context, direct detection is required as we need to collect the planetary photons to perform for instance spectroscopy. But also, a few self-luminous planets have been already detected either from the ground and space using direct imaging (Kalas et al. 2008, Lagrange et al. 2009) and temperature and mass have been estimated from evolutionary models. In addition, direct imaging covers a parameter space that is complementary to Radial Velocity as it is more sensitive to long period. The ever-growing interest for Super Earths (telluric planets that are more massive than the Earth) has led us to propose a space telescope operated in the visible for spectroscopic and polarimetric analysis of these objects. SEE COAST (Super Earth Explorer Coronagraphic Off-Axis Space Telescope) was first proposed to Cosmic Vision at ESA in 2007 but not selected. The science potential and requirements are briefly described 2. Astrophysical requirements and instrumental concept The Core Science program of SEE-COAST is to explore the diversity of planets especially focusing on Super Earths and mature Jovian planets as a goal (young/massive giants, brown dwarfs and debris disks are also part of the program). We anticipated that in the coming years Radial Velocity instruments will provided many targets accessible with SEE-COAST. Several tools are considered for this study : Spectroscopy: a spectral resolution of 40 to 80 in the visible and near IR (0.4-1.2μm) is required to measure the presence of several molecular species like H2O, O2, CH4,