Although astronomers have discovered large numbers of exoplanets in recent years, it is still far from clear what the nature of these planets are, how they formed and how they evolve. To make progress, the study of exoplanets must shift from 'discovery' towards 'studying and understanding' — as a starting point astronomers and planetary scientists need a simple taxonomy of planets and planetary systems. For this, a dedicated survey of a statistically well-defined, large and diverse sample of exoplanets is needed, with simultaneous observations gathered across a consistent wavelength range, in order to understand exoplanets both as individuals and as populations.
Towards a taxonomy for planets
ARIEL is a dedicated survey mission capable of observing a large, diverse and well-defined sample of exoplanets around a range of stellar types. It is designed to perform high-accuracy transit, eclipse, and phase-curve observations employing simultaneous multiband photometry in visible wavelengths and spectroscopy in near infrared wavelengths. Its payload comprises a 1-metre class, three-mirror telescope, an infrared spectrometer, and a Fine Guidance System module providing three narrow-band photometry channels (two used as guidance sensors as well as for science) and a low-resolution near-infrared spectrometer.
Key science questions:
- What are exoplanets made of?
- How do planets and planetary systems form?
- How do planets and their atmospheres evolve over time?
Key science objectives:
- Detect planetary atmospheres, and identify their composition and structure
- Determine vertical and horizontal temperature structure, and diurnal and seasonal variations
- Identify chemical processes at work (thermochemistry, photochemistry, transport quenching)
- Constrain planetary interiors (breaking the radius-mass degeneracy)
- Quantify the energy budget (albedo, temperature)
- Constrain formation and evolution models (evidence for migration)
- Detect secondary atmospheres around terrestrial planets (evolution)
- Investigate the impact of stellar and planetary environment on exoplanet properties
ARIEL will observe and study approximately 1000 preferentially warm and hot transiting gas giants, Neptunes, and super-Earths around a range of star types.
The planets targeted will preferentially be warm and hot (> 600 K) to ensure that their atmospheres are well-mixed and subject to minimal condensation and sequestration, allowing an accurate study of their bulk and elemental composition (from e.g. water (H2O), carbon dioxide (CO2), methane (CH4), ammonia (NH3), hydrogen cyanide (HCN), and hydrogen sulphide (H2S) through to more exotic metallic compounds, such as titanium oxide (TiO) and vanadium oxide (VO), and condensed species). Observations of these hot exoplanets will provide insight into the early stages of planetary and atmospheric formation during the nebular phase and the following few million years.
Transit and eclipse spectroscopy methods, whereby the signals from the star and planet are differentiated using knowledge of the planetary ephemerides, will allow ARIEL to measure atmospheric signals from the planets at levels of at least 10-4 relative to the star and, given the bright nature of targets, allow more sophisticated techniques — such as phase-curve analysis and eclipse mapping — that give a deeper insight into the nature of the atmospheres.
ARIEL will deliver an in-depth catalogue of planetary spectra, characterising molecular abundances, chemical gradients, atmospheric structure, diurnal and seasonal variations, clouds, and albedo measurements. The mission will thus provide a truly representative picture of the chemical nature of the exoplanets studied, and relate this directly to the type and chemical environment of their host stars.
ARIEL is complementary to several other exoplanet missions; potential targets will be identified and studied by a number of important missions in the next decade, including ESA's CHEOPS, Gaia, and PLATO missions, NASA's TESS survey mission, and the NASA/ESA/CSA James Webb Space Telescope. However, ARIEL is the only mission designed for and dedicated to performing a spectroscopic survey of a large, well defined sample of exoplanets.