Aerosols have been found to be nearly ubiquitous in substellar atmospheres1,2,3. The precise temperature at which these aerosols begin to form in exoplanets has yet to be observationally constrained. Theoretical models and observations of muted spectral features indicate that silicate clouds play an important role in exoplanets between at least 950 and 2,100 K (ref. 4). Some giant planets, however, are thought to be hot enough to avoid condensation altogether5,6. Here we report the near-ultraviolet transmission spectrum of the ultra-hot Jupiter WASP-178b (approximately 2,450 K), which exhibits substantial absorption. Bayesian retrievals indicate the presence of gaseous refractory species containing silicon and magnesium, which are the precursors to condensate clouds at lower temperatures. SiO, in particular, has not previously, to our knowledge, been detected in exoplanets, but the presence of SiO in WASP-178b is consistent with theoretical expectations as the dominant Si-bearing species at high temperatures. These observations allow us to re-interpret previous observations of HAT-P-41b and WASP-121b that did not consider SiO, to suggest that silicate cloud formation begins on exoplanets with equilibrium temperatures between 1,950 and 2,450 K.
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The raw data from this study, HST Program 16068, is publicly available via the Space Science Telescope Institute’s Mikulski Archive for Space Telescopes (https://archive.stsci.edu/).
The raw data was reduced with the available STScI CALWF3 pipeline and spectra were extracted with the public IRAF apall routines. The light curve fitting used custom routines that we opt not to make public due to undocumented intricacies. Model and retrievals were generated using PHOENIX, which is a proprietary code but described in many publications, for example, refs. 66,67.
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We thank the UV-SCOPE team for relevant discussions. We thank T. Barman for the use of the computing resources used in the calculation of the atmospheric retrievals. Support for this work was provided by NASA through grant number HST-GO-16086 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. This research has made use of the NASA Astrophysics Data System and the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program.
The authors declare no competing interests.
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Extended data figures and tables
Error bars show the 1-σ uncertainties. The left column (a, c, e) shows the +1 spectral order, while the right column (b, d, f) shows the −1 spectral order. The top row (a, b) are the raw light curves, the middle row (c, d) are the light curves with systematics removed and a transit fit, and the bottom row (e, f) are the residuals with the standard deviation of the residuals also shown (dotted lines). Plots of the binned residual RMS are also shown as insets.
Two example fitted light curves with 1-σ uncertainties from the +1 spectral order from spectroscopic bins covering 0.2412 (a, c, e) and 0.5875 μm (b, d, f), with transit depths of 1.48 ± 0.04% and 1.16 ± 0.03%, respectively. The rows are the same as in Extended Data Fig. 1.
WFC3/UVIS G280 transmission spectrum of WASP-178b (with 1-σ uncertainties) from the +1 (blue) and −1 order (red). The −1 order shows larger uncertainties due to a reduced throughput, but the transmission spectra show good agreement including an enhanced NUV absorption between 0.2 and 0.3 µm.
a, The 0.18–0.28 µm NUV light curve of WASP-178 b (with 1-σ uncertainties), with the best-fitting symmetric light curve, and an asymmetric light curve representing a scenario with a hotter/larger trailing terminator, and a colder/smaller leading terminator. The radius of the leading terminator was set to the optical value, and the trailing terminator was fixed to the value that fits the NUV transit depth. The inset shows the RMS scatter of the residuals as a function of number of points per bin, N. b, Residuals to the symmetric and asymmetric light curve fits.
2-D cross- sections of the retrieved posterior distribution with 1-D marginalized distribution for the fitted parameters. The quoted quantities are the mean and 1-σ retrieved values. The first five parameters are the temperature structure parameterization from ref. 57, the sixth is the reference radius, and the final eight are the various atomic and molecular abundances.
Same as Fig. 1, but for the retrieval without SiO. Note the combined ability of Mg I and Fe II absorption to generate the large short-wavelength transit depths.
Partial pressures of important silicon- bearing species (a) and iron-bearing species (b) at 1 mbar as a function of temperature. Equilibrium chemical abundances were calculated using GGchem69.
NUV high-resolution transit spectra of WASP-178b (with 1-σ uncertainties) compared to WASP-121b around the Fe II (a) and Mg II (b) lines. Shown are the spectra from STIS E230M for WASP-178b (red), WASP-121b17 (grey), and the low resolution UVIS spectra (blue). While WASP-121b shows strong Fe II and Mg II absorption features, the WASP-178b E230M spectra is consistent with the broadband NUV continuum with no Fe II or Mg II.
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Lothringer, J.D., Sing, D.K., Rustamkulov, Z. et al. UV absorption by silicate cloud precursors in ultra-hot Jupiter WASP-178b. Nature 604, 49–52 (2022). https://doi.org/10.1038/s41586-022-04453-2