Twinkle will provide high-quality infrared spectroscopic characterisation of the atmospheres of a population of exoplanets, covering a wide range of planetary types. Additionally, the mission will be able to provide phase curves for hot, short-period planets around bright stars and ultra-precise spectrophotometric light curves to accurately constrain orbital parameters, including ephemerides and TTVs/TDVs present in multi-planet systems.
Simulations of Twinkle’s performance on the currently known exoplanets and those predicted to be found by TESS have been used to create a catalogue of targets for atmospheric characterisation. Assuming an SNR > 7 is required on 3 scale heights of atmosphere, Twinkle’s catalogue will comprise a wide range of exoplanet types including Jupiters, Saturns, Neptunes and super-Earths.
Twinkle catalogue: Known Exoplanets
Twinkle catalogue: Known Exoplanets & TESS Predictions
Of the 400+ exoplanets in Twinkle’s field of regard, hot Jupiters orbiting bright stars, such as HD 209458 b, are ideal targets for observation. In many cases, such planets can be observed in a single transit with a high SNR at the highest resolution possible with Twinkle’s instrumentation. Modelling of HD 209458 b has shown that, with 3 transits, Twinkle will be able to identify the main molecular constituents, accurately recover their abundances and reveal the presence of clouds. Here the modelled atmosphere has been based on previous observations with the Hubble Space Telescope.
Simulated spectrum of HD 209458 b: 3 transits
Posteriors for spectral retrieval of HD 209458 b
Twinkle’s broad, simultaneous wavelength coverage will allow for a more thorough characterisation of this planet, and others like it, by providing sensitivity to the presence of additional spectral features. The posterior distributions from a retrieval of these simulated data show that Twinkle can effectively constrain abundances of the major species present, in this case H2O and NH3. For the trace molecules in the atmosphere, such as the CH4 modelled here, Twinkle will be able to place upper limits on their abundances. Such limits are useful in constraining and interpreting the chemical processes in the atmosphere. For hot and ultra-hot Jupiters, Twinkle’s broad wavelength coverage will be effective for confirming or refuting the presence of optical absorbers such as TiO and VO.
The atmospheres of smaller planets are often more challenging to study due to their reduced feature size. Additionally, many smaller planets that have been studied show evidence of clouds or hazes that obscure other spectral features, making it hard to uncover their full atmospheric composition. GJ 1214 b is perhaps the most famous of these, having been heavily observed with Hubble. However, following 12 observations with Hubble, no discernible atmospheric features over the wavelength region studied (1.1–1.7 microns) were detected.
Simulated Spectrum of GJ 1214 b: 20 Transits
Posteriors for spectral retrieval of GJ 1214 b
Further studies of planets like GJ 1214 b at longer wavelengths, including those covered by Twinkle, may reveal molecular absorption signatures. For example, an atmosphere with high altitude clouds and the molecular species CO2 and NH3 is compatible with current datasets. Simulating 20 observations with Twinkle highlights that these features could be detected at longer wavelengths. By conducting a survey over multiple years, Twinkle will allow scientists to study smaller planets in depth in search of atmospheric signatures.
Twinkle’s catalogue includes a number of bright super-Earths which could be observed. These are some of the most challenging targets for Twinkle, but also some of the most exciting given their relevance as a vital stepping stone in the search for habitable exoplanets.
STARS & BROWN DWARFS
In addition to its core science cases, Twinkle will observe a large number of stars, enabling scientists to monitor stellar activity and variability, observe stellar discs in various stages of planet formation and study brown dwarfs.