Thermal inversions have long been predicted to exist in the atmospheres of ultra-hot Jupiters. However, detection of two species thought to be responsible -- TiO and VO -- remain elusive. We present ...a search for TiO and VO in the atmosphere of the ultra-hot Jupiter WASP-121b (\(T_\textrm{eq} \gtrsim 2400\) K), an exoplanet already known to show water features in its dayside spectrum characteristic of a temperature inversion as well as tentative evidence for VO at low-resolution. We observed its transmission spectrum with UVES/VLT and used the cross-correlation method -- a powerful tool for the unambiguous identification of the presence of atomic and molecular species -- in an effort to detect whether TiO or VO were responsible for the observed temperature inversion. No evidence for the presence of TiO or VO was found at the terminator of WASP-121b. By injecting signals into our data at varying abundance levels, we set rough detection limits of \(\text{VO} \lesssim -7.9\) and \(\text{TiO} \lesssim -9.3\). However, these detection limits are largely degenerate with scattering properties and the position of the cloud deck. Our results may suggest that neither TiO or VO are the main drivers of the thermal inversion in WASP-121b, but until a more accurate line list is developed for VO, we cannot conclusively rule out its presence. Future work will search for finding other strong optically-absorbing species that may be responsible for the excess absorption in the red-optical.
We perform atmospheric retrievals on the full optical to infrared (\(0.3-5 \, \mu \mathrm{m}\)) transmission spectrum of the inflated hot Jupiter WASP-52b by combining HST/STIS, WFC3 IR, and ...Spitzer/IRAC observations. As WASP-52 is an active star which shows both out-of-transit photometric variability and starspot crossings during transits, we account for the contribution of non-occulted active regions in the retrieval. We recover a \(0.1-10\times\) solar atmospheric composition, in agreement with core accretion predictions for giant planets, and a weak contribution of aerosols. We also obtain a \(<3000\) K temperature for the starspots, a measure which is likely affected by the models used to fit instrumental effects in the transits, and a 5% starspot fractional coverage, compatible with expectations for the host star's spectral type. Such constraints on the planetary atmosphere and on the activity of its host star will inform future JWST GTO observations of this target.
We present new observations of the transmission spectrum of the hot Jupiter
WASP-6b both from the ground with the Very Large Telescope (VLT) FOcal Reducer
and Spectrograph (FORS2) from 0.45-0.83 ...$\mu$m, and space with the Transiting
Exoplanet Survey Satellite (TESS) from 0.6-1.0 $\mu$m and the Hubble Space
Telescope (HST) Wide Field Camera 3 from 1.12-1.65 $\mu$m. Archival data from
the HST Space Telescope Imaging Spectrograph (STIS) and Spitzer is also
reanalysed on a common Gaussian process framework, of which the STIS data show
a good overall agreement with the overlapping FORS2 data. We also explore the
effects of stellar heterogeneity on our observations and its resulting
implications towards determining the atmospheric characteristics of WASP-6b.
Independent of our assumptions for the level of stellar heterogeneity we detect
Na I, K I and H$_2$O absorption features and constrain the elemental oxygen
abundance to a value of O/H $\simeq -0.9\pm0.3$ relative to solar. In
contrast, we find that the stellar heterogeneity correction can have
significant effects on the retrieved distributions of the Na/H and K/H
abundances, primarily through its degeneracy with the sloping optical opacity
of scattering haze species within the atmosphere. Our results also show that
despite this presence of haze, WASP-6b remains a favourable object for future
atmospheric characterisation with upcoming missions such as the James Webb
Space Telescope.
Non-rocky sub-jovian exoplanets in high irradiation environments are rare. LTT 9979b, also known as TESS Object of Interest (TOI) 193.01, is one of the few such planets discovered to date, and the ...first example of an ultra-hot Neptune. The planet's bulk density indicates that it has a substantial atmosphere, so to investigate its atmospheric composition and shed further light on its origin, we obtained {\it Spitzer} IRAC secondary eclipse observations of LTT 9979b at 3.6 and 4.5 \(\mu\)m. We combined the {\it Spitzer} observations with a measurement of the secondary eclipse in the {\it TESS} bandpass. The resulting secondary eclipse spectrum strongly prefers a model that includes CO absorption over a blackbody spectrum, incidentally making LTT 9979b the first {\it TESS} exoplanet (and the first ultra-hot Neptune) with evidence of a spectral feature in its atmosphere. We did not find evidence of a thermal inversion, at odds with expectations based on the atmospheres of similarly-irradiated hot Jupiters. We also report a nominal dayside brightness temperature of 2305 \(\pm\) 141 K (based on the 3.6 \(\mu\)m secondary eclipse measurement), and we constrained the planet's orbital eccentricity to \(e < 0.01\) at the 99.7 \% confidence level. Together with our analysis of LTT 9979b's thermal phase curves reported in a companion paper, our results set the stage for similar investigations of a larger sample of exoplanets discovered in the hot Neptune desert, investigations which are key to uncovering the origin of this population.
Recent years have seen increasing interest in the characterization of sub-Neptune sized planets because of their prevalence in the Galaxy, contrasted with their absence in our solar system. HD 97658 ...is one of the brightest stars hosting a planet of this kind, and we present the transmission spectrum of this planet by combining four HST transits, twelve Spitzer/IRAC transits, and eight MOST transits of this system. Our transmission spectrum has higher signal to noise ratio than that from previous works, and the result suggests that the slight increase in transit depth from wavelength 1.1 to 1.7 microns reported in previous works on the transmission spectrum of this planet is likely systematic. Nonetheless, our atmospheric modeling results are not conclusive as no model provides an excellent match to our data. Nonetheless we find that atmospheres with high C/O ratios (C/O >~ 0.8) and metallicities of >~ 100x solar metallicity are favored. We combine the mid-transit times from all the new Spitzer and MOST observations and obtain an updated orbital period of P=9.489295 +/- 0.000005 d, with a best-fit transit time center at T_0 = 2456361.80690 +/- 0.00038 (BJD). No transit timing variations are found in this system. We also present new measurements of the stellar rotation period (34 +/- 2 d) and stellar activity cycle (9.6 yr) of the host star HD 97658. Finally, we calculate and rank the Transmission Spectroscopy Metric of all confirmed planets cooler than 1000 K and with sizes between 1 and 4 R_Earth. We find that at least a third of small planets cooler than 1000 K can be well characterized using JWST, and of those, HD 97658b is ranked fifth, meaning it remains a high-priority target for atmospheric characterization.
We present HST near-ultraviolet (NUV) transits of the hot Jupiter WASP-121b, acquired as part of the PanCET program. Time series spectra during two transit events were used to measure the ...transmission spectra between 2280 and 3070 Angstroms at a resolution of 30,000. Using HST data from 61 STIS visits, we show that data from HST's Pointing Control System can be used to decorrelate the instrument systematic errors (Jitter Decorrelation), which we used to fit the WASP-121b light curves. The NUV spectrum show very strong absorption features, with the NUV white light curve found to be larger than the average optical and near-infrared value at 6-\(\sigma\) confidence. We identify and spectrally resolve absorption from the Mg ii doublet in the planetary exosphere at a 5.9-\(\sigma\) confidence level. The Mg ii doublet is observed to reach altitudes of \(R_{pl}/R_{star}=0.284\pm0.037\) for the 2796 Angstrom line and \(0.242\pm0.0431\) in the 2804 Angstrom line, which exceeds the Roche lobe size as viewed in transit geometry (\(R_{\rm eqRL}/R_{star}\) = 0.158). We also detect and resolve strong features of the Fe ii UV1 and UV2 multiplets, and observe the lines reaching altitudes of \(R_{pl}/R_{star}\approx0.3\). At these high altitudes, the atmospheric Mg ii and Fe ii gas is not gravitationally bound to the planet, and these ionized species may be hydrodynamically escaping or could be magnetically confined. Refractory Mg and Fe atoms at high altitudes also indicates that these species are not trapped into condensate clouds at depth, which places constraints on the deep interior temperature.
Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability. However, no unambiguous photochemical products have been detected in ...exoplanet atmospheres to date. Recent observations from the JWST Transiting Exoplanet Early Release Science Program found a spectral absorption feature at 4.05 \(\mu\)m arising from SO\(_2\) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M\(_J\)) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of \(\sim\)1100 K. The most plausible way of generating SO\(_2\) in such an atmosphere is through photochemical processes. Here we show that the SO\(_2\) distribution computed by a suite of photochemical models robustly explains the 4.05 \(\mu\)m spectral feature identified by JWST transmission observations with NIRSpec PRISM (2.7\(\sigma\)) and G395H (4.5\(\sigma\)). SO\(_2\) is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H\(_2\)S) is destroyed. The sensitivity of the SO\(_2\) feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of \(\sim\)10\(\times\) solar. We further point out that SO\(_2\) also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.
Planets occur most frequently around cool dwarfs, but only a handful of specific examples are known to orbit the latest-type M stars. Using TESS photometry, we report the discovery of two planets ...transiting the low-mass star called LP 791-18 (identified by TESS as TOI 736). This star has spectral type M6V, effective temperature 2960 K, and radius 0.17 R_Sun, making it the third-coolest star known to host planets. The two planets straddle the radius gap seen for smaller exoplanets; they include a 1.1 R_Earth planet on a 0.95 day orbit and a 2.3 R_Earth planet on a 5 day orbit. Because the host star is small the loss of light during these planets' transits is fairly large (0.4% and 1.7%). This has allowed us to detect both planets' transits from ground-based photometry, refining their radii and orbital ephemerides. In the future, radial velocity observations and transmission spectroscopy can both probe these planets' bulk interior and atmospheric compositions, and additional photometric monitoring would be sensitive to even smaller transiting planets.
The chemical abundances of exoplanet atmospheres may provide valuable information about the bulk compositions, formation pathways, and evolutionary histories of planets. Exoplanets with large, ...relatively cloud-free atmospheres, and which orbit bright stars provide the best opportunities for accurate abundance measurements. For this reason, we measured the transmission spectrum of the bright (V ∼ 10.2), large (1.37 RSUBJ/SUB), sub-Saturn mass (0.19 MSUBJ/SUB) exoplanet WASP-127b across the near-UV to near-infrared wavelength range (0.3-5 μm), using the Hubble and Spitzer Space Telescopes. Our results show a feature-rich transmission spectrum, with absorption from Na, HSUB2/SUBO, and COSUB2/SUB, and wavelength-dependent scattering from small-particle condensates. We ran two types of atmospheric retrieval models: one enforcing chemical equilibrium, and the other which fit the abundances freely. Our retrieved abundances at chemical equilibrium for Na, O, and C are all supersolar, with abundances relative to solar values of 9 $^{+15}_{-6}$ , 16 $^{+7}_{-5}$ , and 26 $^{+12}_{-9}$ , respectively. Despite giving conflicting C/O ratios, both retrievals gave supersolar COSUB2/SUB volume mixing ratios, which adds to the likelihood that WASP-127b's bulk metallicity is supersolar, since COSUB2/SUB abundance is highly sensitive to atmospheric metallicity. We detect water at a significance of 13.7σ. Our detection of Na is in agreement with previous ground-based detections, though we find a much lower abundance, and we also do not find evidence for Li or K despite increased sensitivity. In the future, spectroscopy with James Webb Space Telescope will be able to constrain WASP-127b's C/O ratio, and may reveal the formation history of this metal-enriched, highly observable exoplanet.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We present new observations of the transmission spectrum of the hot Jupiter WASP-6b both from the ground with the Very Large Telescope (VLT) FOcal Reducer and Spectrograph (FORS2) from 0.45-0.83 ...\(\mu\)m, and space with the Transiting Exoplanet Survey Satellite (TESS) from 0.6-1.0 \(\mu\)m and the Hubble Space Telescope (HST) Wide Field Camera 3 from 1.12-1.65 \(\mu\)m. Archival data from the HST Space Telescope Imaging Spectrograph (STIS) and Spitzer is also reanalysed on a common Gaussian process framework, of which the STIS data show a good overall agreement with the overlapping FORS2 data. We also explore the effects of stellar heterogeneity on our observations and its resulting implications towards determining the atmospheric characteristics of WASP-6b. Independent of our assumptions for the level of stellar heterogeneity we detect Na I, K I and H\(_2\)O absorption features and constrain the elemental oxygen abundance to a value of O/H \(\simeq -0.9\pm0.3\) relative to solar. In contrast, we find that the stellar heterogeneity correction can have significant effects on the retrieved distributions of the Na/H and K/H abundances, primarily through its degeneracy with the sloping optical opacity of scattering haze species within the atmosphere. Our results also show that despite this presence of haze, WASP-6b remains a favourable object for future atmospheric characterisation with upcoming missions such as the James Webb Space Telescope.
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