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Register a new userDetecting ring systems around exoplanets using high resolution spectroscopy: the case of 51Pegb
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In this paper we explore the possibility that the recently detected reflected light signal of 51,Peg,b could be caused by a ring system around the planet. We use a simple model to compare the observed signal with the expected signal from a short-period giant planet with rings. We also use simple dynamical arguments to understand the possible geometry of such a system. We provide evidence that, to a good approximation, the observations are compatible with the signal expected from a ringed planet, assuming that the rings are non-coplanar with the orbital plane. However, based on dynamical arguments, we also show that this configuration is unlikely. In the case of coplanar rings we then demonstrate that the incident flux on the ring surface is about 2% the value received by the planet, a value that renders the ring explanation unlikely. The results suggest that the signal observed cannot in principle be explained by a planet+ring system. We discuss, however, the possibility of using reflected light spectra to detect and characterize the presence of rings around short-period planets. Finally, we show that ring systems could have already been detected by photometric transit campaigns, but their signal could have been easily misinterpreted by the expected light curve of an eclipsing binary.
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The technique of transmission spectroscopy allows us to constrain the chemical composition of the atmospheres of transiting exoplanets. It relies on very high signal-to-noise spectroscopic (or spectrophotometric) observations and is thus most suited for bright exoplanet host stars. In the era of TESS, NGST and PLATO, more and more suitable targets, even for mid-sized telescopes, are discovered. Furthermore, a wealth of archival data is available that could become a basis for long-term monitoring of exo-atmospheres. We analyzed archival HARPS spectroscopic time series of four host stars to transiting bloated gas exoplanets, namely WASP-76b, WASP-127b, WASP-166b and KELT-11b, searching for traces of sodium (sodium doublet), hydrogen (H$alpha$, H$beta$), and lithium (670.8 nm). The archival data sets include spectroscopic time series taken during transits. Comparing in- and out-of-transit spectra we can filter out the stellar lines and investigate the absorption from the planet. Simultaneously, the stellar activity is monitored using the Mg I and Ca I lines. We independently detect sodium in the atmosphere of WASP-76b at a 7-9 $sigma$ level. Furthermore, we report also at 4-8 $sigma$ level of significance the detection of sodium in the atmosphere of WASP-127b, confirming earlier result based on low-resolution spectroscopy. The data show no sodium nor any other atom at high confidence levels for WASP-166b nor KELT-11b, hinting at the presence of thick high clouds.
Observations to characterize planets larger than Earth but smaller than Neptune have led to largely inconclusive interpretations at low spectral resolution due to hazes or clouds that obscure molecular features in their spectra. However, here we show that high-resolution spectroscopy (R $sim$ 25,000 to 100,000) enables one to probe the regions in these atmospheres above the clouds where the cores of the strongest spectral lines are formed. We present models of transmission spectra for a suite of GJ1214b-like planets with thick photochemical hazes covering 1 - 5 $mu$m at a range of resolutions relevant to current and future ground-based spectrographs. Furthermore, we compare the utility of the cross-correlation function that is typically used with a more formal likelihood-based approach, finding that only the likelihood based method is sensitive to the presence of haze opacity. We calculate the signal-to-noise of these spectra, including telluric contamination, required to robustly detect a host of molecules such as CO, CO$_{2}$, H$_{2}$O, and CH$_{4}$, and photochemical products like HCN, as a function of wavelength range and spectral resolution. Spectra in M band require the lowest S/N$_{res}$ to detect multiple molecules simultaneously. CH$_{4}$ is only observable for the coolest models ($T_{rm{eff}} =$ 412 K) and only in the L band. We quantitatively assess how these requirements compare to what is achievable with current and future instruments, demonstrating that characterization of small cool worlds with ground-based high resolution spectroscopy is well within reach.
Current observations of the atmospheres of close-in exoplanets are predominantly obtained with two techniques: low-resolution spectroscopy with space telescopes and high-resolution spectroscopy from the ground. Although the observables delivered by the two methods are in principle highly complementary, no attempt has ever been made to combine them, perhaps due to the different modeling approaches that are typically used in their interpretation. Here we present the first combined analysis of previously-published dayside spectra of the exoplanet HD 209458b obtained at low resolution with HST/WFC3 and Spitzer/IRAC, and at high resolution with VLT/CRIRES. By utilizing a novel retrieval algorithm capable of computing the joint probability distribution of low- and high-resolution spectra, we obtain tight constraints on the chemical composition of the planets atmosphere. In contrast to the WFC3 data, we do not confidently detect H2O at high spectral resolution. The retrieved water abundance from the combined analysis deviates by 1.9 sigma from the expectations for a solar-composition atmosphere in chemical equilibrium. Measured relative molecular abundances of CO and H2O strongly favor an oxygen-rich atmosphere (C/O<1 at 3.5 sigma) for the planet when compared to equilibrium calculations including O rainout. From the abundances of the seven molecular species included in this study we constrain the planet metallicity to 0.1-1.0x the stellar value (1 sigma). This study opens the way to coordinated exoplanet surveys between the flagship ground- and space-based facilities, which ultimately will be crucial for characterizing potentially-habitable planets.
The characterization of planetary atmospheres is a daunting task, pushing current observing facilities to their limits. The next generation of high-resolution spectrographs mounted on large telescopes -- such as ESPRESSO@VLT and HIRES@ELT -- will allow us to probe and characterize exoplanetary atmospheres in greater detail than possible to this point. We present a method that permits the recovery of the colour-dependent reflectivity of exoplanets from high-resolution spectroscopic observations. Determining the wavelength-dependent albedo will provide insight into the chemical properties and weather of the exoplanet atmospheres. For this work, we simulated ESPRESSO@VLT and HIRES@ELT high-resolution observations of known planetary systems with several albedo configurations. We demonstrate how the cross correlation technique applied to theses simulated observations can be used to successfully recover the geometric albedo of exoplanets over a range of wavelengths. In all cases, we were able to recover the wavelength dependent albedo of the simulated exoplanets and distinguish between several atmospheric models representing different atmospheric configurations. In brief, we demonstrate that the cross correlation technique allows for the recovery of exoplanetary albedo functions from optical observations with the next generation of high-resolution spectrographs that will be mounted on large telescopes with reasonable exposure times. Its recovery will permit the characterization of exoplanetary atmospheres in terms of composition and dynamics and consolidates the cross correlation technique as a powerful tool for exoplanet characterization.
We investigate directly imaging exoplanets around eclipsing binaries, using the eclipse as a natural tool for dimming the binary and thus increasing the planet to star brightness contrast. At eclipse, the binary becomes point-like, making coronagraphy possible. We select binaries where the planet-star contrast would be boosted by $>10times$ during eclipse, making it possible to detect a planet that is $gtrsim10times$ fainter or in a star system that is $sim2$-$3times$ more massive than otherwise. Our approach will yield insights into planet occurrence rates around binaries versus individual stars. We consider both self-luminous (SL) and reflected light (RL) planets. In the SL case, we select binaries whose age is young enough so that an orbiting SL planet would remain luminous; in U Cep and AC Sct, respectively, our method is sensitive to SL planets of $sim$4.5$M_J$ and $sim$9$M_J$ with current ground- or near-future space-based instruments, and $sim$1.5$M_J$ and $sim$6$M_J$ with future ground-based observatories. In the RL case, there are three nearby ($lesssim50$ pc) systems -- V1412 Aql, RR Cae, RT Pic -- around which a Jupiter-like planet at a planet-star separation of $gtrsim20$ mas might be imaged with future ground- and space-based coronagraphs. A Venus-like planet at the same distance might be detectable around RR Cae and RT Pic. A habitable Earth-like planet represents a challenge; while the planet-star contrast at eclipse and planet flux are accessible with a 6-8m space telescope, the planet-star separation is 1/3 - 1/4 of the angular separation limit of modern coronagraphy.
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