Unveiling exoplanets through their atmospheres

Exoplanets are everywhere in our Galaxy. We now know more than 4,000 of them, and new ones are discovered every month thanks to dedicated space telescopes (such as TESS) and ground-based surveys.

Yet, we know so little about their properties. Since they are outshined by their host star, their spectra are difficult to measure, and so their physio-chemical properties. Thanks to technological advance, newly developed techniques and dedicated missions, the field is now living a revolution, and we are going to observe the atmospheres of hundreds of exoplanets in the next 10 years.

In Arcetri, a newly established, yet internationally recognized team aims at combining observations of planets with multiple techniques to answer some of the fundamental questions about them:

1) What is the climate and chemistry in hot gas giants?

2) How do exoplanets form and evolve?

3) What do the atmospheres of rocky exoplanets look like?

Unveiling climate and chemistry in hottest gas giants, and their interplay

Ultra-hot Jupiters (UHJs) are tidally locked gas giants having dayside temperatures up to more than 2,200 K. At these temperatures, the atmospheres of these extreme planets start looking more like those of stars than those traditionally attributed to planets, and metals like iron and titanium are observable in their spectra (Hoeijmakers et al., 2019). UHJs thus constitute a unique opportunity to observe the chemistry of refractory elements in exoplanets.

The climate of UHJs may differ as well. Due to tidal-locking, UHJs have day-night temperature contrasts reaching up to more than a thousand Kelvin. They thus often have relatively cold, cloudy-night sides where iron and other metals rain out (Ehrenereich et al. 2020), and cloud-free day-sides with dissociated molecules and ionized metals. The resulting free electrons in the day-side, coupled with the planetary magnetic field, interact with the Alfven waves which produce the strong equatorial jets witnessed in “regular” hot Jupiters. The result is that day-night winds are strongly suppressed in this class of planets. Chemistry and climate are thus intrinsically linked in this extreme class of planets.

In Arcetri, within an international collaboration spanning Europe, the UK, the USA and Japan, we employ a combination of high spectral resolution observations from the ground and space-borne observations with HST and JWST to unveil the secrets of the climate and chemistry of the hottest gas giants.

Using optical, high spectral resolution HARPS-N data, Pino et al. (2020) measured iron in the day-side of UHJ KELT-9b for the first time. They showed that (1) the atmosphere of this planet has a thermal inversion, (2) assuming that the planet has the same metallicity of the star, the data can be reproduced. This technique is also sensitive to the presence of winds, through the resolved shape of planetary lines, and is thus ideally suited to understand the link between chemistry and climate in the hottest gas giants. Another indication comes from KELT-20b, where Rainer et al. (under review) find a time-varying signal in metal lines, suggesting that a varying climate in the planet could alter their abundances.

Figure 1: From Pino et al. (2020). HARPS-N observations of KELT-9b reveal iron in the emission spectrum of the planet. By spectrally resolving the iron lines, it is possible to trace the motion of the planet during one night and observe that the line appears in emission. This is the proof that the atmosphere of KELT-9b is thermally inverted.

These techniques, pioneered by the Arcetri team, will be applied to tens of other hot gaseous giants, in the context of several programs that the Arcetri team is leading or is involved in. We will use all the best available facilities: TNG HARPS-N (through the GAPS project), VLT ESPRESSO, GEMINI-N MAROON-X, LBT PEPSI (through the PETS Large Program), and complement with HST and JWST infrared observations.

Tracing formation and evolution of exoplanets though their atmospheric chemistry

One product of our observations are the elemental abundances in exoplanets. These can be compared to the chemistry of protoplanetary disks to understand the formation location of exoplanets (see the Astrochemistry of young Solar-analogs team page).

The Arcetri exoplanet atmosphere team is at the center of several national and international efforts aimed at the extraction of atmospheric tracers of planet formation, such as the C/O ratio. The GAPS project is targeting tens of planets in the course of a 5 year long effort. Thanks to the large near-infrared coverage (J, H and K band) of the state-of-the-art instrument GIANO-B, for every observed planet it is possible to cover absorption bands from CO, CO₂, H₂O, CH₄, NH₃, C₂H₂, HCN among others (see Fig. 2).

Figure 2: A typical transmission spectrum of a hot Jupiter. By studying the stellar light transmitted through the atmosphere of exoplanets we can measure their apparent size at different wavelengths (y-axis), and thus their chemical make-up. In the optical wavelengths (up to 8000 Angstroms), we see an enhanced Rayleigh scattering slope due to the presence of small aerosols (hazes) in the planet atmosphere, on top of which strong lines from atomic species (sodium, potassium, ... ) emerge. In the near-Infrared, billions of lines belonging to molecular species can be targeted with space telescopes (HST, JWST, ...), but also from the ground at high-spectral resolution.

The whole field is going to be transformed with the advent of JWST first (2021), and ARIEL next (2029). In particular, Arcetri is heavily involved in the ARIEL satellite mission, which will extract elemental abundances of thousands of exoplanets (see also the Properties of stars hosting exoplanets team page).

Towards understanding the atmospheres of rocky exoplanets

Ultimately, one of the goals of exoplanet atmosphere sciences is to study the atmospheres of rocky planets, including those in the habitable zone. This task is outside of the capabilities of current instrumentation, but the Arcetri team is well placed to play a decisive role in this sub-field.

The pioneering techniques developed by the team to study hot gaseous giants will be applied to smaller, rocky planets using next generation spectrographs such as HIRES at the E-ELT (2025+; PI: Alessandro Marconi, associated researcher at INAF Arcetri). Thanks to the large collecting area, a handful of transits will already provide detections of O₂ in a few transiting planets around the habitable zone of M-dwarfs. In combination with JWST observations, this kind of observations offer the most promising short-medium term effort to characterize the atmospheres of potentially habitable planets.

In the longer term, an instrument dedicated to study the atmosphere of rocky planets in the habitable zone will be the only viable option to provide a statistical sample of their atmospheres. This is a necessary step to assess the reliability of bio-markers as tracers of life on other planets. In this context, the Arcetri team takes part to the efforts of the LIFE space-born interferometer.