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.
Cushing, M. C. et al. A Spitzer infrared spectrograph spectral sequence of M, L, and T dwarfs. Astrophys. J. 648, 614–628 (2006).
Saumon, D. & Marley, M. S. The evolution of L and T dwarfs in color-magnitude diagrams. Astrophys. J. 689, 1327–1344 (2008).
Burningham, B. et al. Cloud busting: enstatite and quartz clouds in the atmosphere of 2M2224-0158. Mon. Not. R. Astron. Soc. 506, 1944–1961 (2021).
Gao, P. et al. Aerosol composition of hot giant exoplanets dominated by silicates and hydrocarbon hazes. Nature Astron. 4, 951–956 (2020).
Lothringer, J. D. et al. An HST/STIS optical transmission spectrum of warm Neptune GJ 436b. Astron. J. 155, 66 (2018).
Kitzmann, D. et al. The peculiar atmospheric chemistry of KELT-9b. Astrophys. J. 863, 183 (2018).
Hellier, C. et al. WASP-South hot Jupiters: WASP-178b, WASP-184b, WASP-185b, and WASP-192b. Mon. Not. R. Astron. Soc. 490, 1479–1487 (2019).
Rodr´ıguez Mart´ınez, R. et al. KELT-25 b and KELT-26 b: a hot Jupiter and a substellar companion transiting young a stars observed by TESS. Astron. J 160, 111 (2020).
Matsushima, S. Radiative opacity in stellar atmospheres. II. Effect of ultraviolet continuum on the photospheric radiation field. Astrophys. J. 154, 715 (1968).
Fontenla, J. M., Stancil, P. C. & Landi, E. Solar spectral irradiance, solar activity, and the near- ultra-violet. Astrophys. J. 809, 157 (2015).
Sharp, C. M. & Burrows, A. Atomic and molecular opacities for brown dwarf and giant planet atmospheres. Astrophys. J. Supp. 168, 140–166 (2007).
Lothringer, J. D., Fu, G., Sing, D. K. & Barman, T. S. UV exoplanet transmission spectral features as probes of metals and rainout. Astrophys. J. Lett. 898, L14 (2020).
Hoeijmakers, H. J. et al. Hot exoplanet atmospheres resolved with transit spectroscopy (HEARTS). IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP- 121 b. Astron. Astrophys. 641, A123 (2020).
Stangret, M. et al. Detection of Fe I and Fe II in the atmosphere of MASCARA-2b using a cross- correlation method. Astron. Astrophys. 638, A26 (2020).
Ehrenreich, D. et al. Nightside condensation of iron in an ultrahot giant exoplanet. Nature 580, 597–601 (2020).
Kesseli, A. Y. & Snellen, I. A. G. Confirmation of asymmetric iron absorption in WASP-76b with HARPS. Astrophys. J. Lett. 908, L17 (2021).
Sing, D. K. et al. The Hubble Space Telescope PanCET program: exospheric Mg II and Fe II in the near-ultraviolet transmission spectrum of WASP-121b using jitter decorrelation. Astron. J. 158, 91 (2019).
Gibson, N. P. et al. Detection of Fe I in the atmosphere of the ultra-hot Jupiter WASP-121b, and a new likelihood-based approach for Doppler-resolved spectroscopy. Mon. Not. R. Astron. Soc. 493, 2215–2228 (2020).
Cabot, S. H. C., Madhusudhan, N., Welbanks, L., Piette, A. & Gandhi, S. Detection of neutral atomic species in the ultra-hot Jupiter WASP-121b. Mon. Not. R. Astron. Soc. 494, 363–377 (2020).
Hoeijmakers, H. J. et al. A spectral survey of an ultra-hot Jupiter. Detection of metals in the transmission spectrum of KELT-9 b. Astron. Astrophys. 627, A165 (2019).
Merritt, S. R. et al. An inventory of atomic species in the atmosphere of WASP-121b using UVES high-resolution spectroscopy. Mon. Not. R. Astron. Soc. 506, 3853–3871 (2021).
Wakeford, H. R. et al. Into the UV: a precise transmission spectrum of HAT-P-41b using Hubble’s WFC3/UVIS G280 grism. Astron. J. 159, 204 (2020).
Visscher, C., Lodders, K. & Fegley, J. B. Atmospheric chemistry in giant planets, brown dwarfs, and low-mass dwarf stars. III. Iron, magnesium, and silicon. Astrophys. J. 716, 1060–1075 (2010).
Parmentier, V., Showman, A. P. & Fortney, J. J. The cloudy shape of hot Jupiter thermal phase curves. Mon. Not. R. Astron. Soc. 501, 78–108 (2021).
Roman, M. T. et al. Clouds in three-dimensional models of hot Jupiters over a wide range of temperatures. I. Thermal structures and broadband phase-curve predictions. Astrophys. J. 908, 101 (2021).
Helling, C. et al. Cloud property trends in hot and ultra-hot giant gas planets (WASP-43b, WASP-103b, WASP-121b, HAT-P-7b, and WASP-18b). Astron. Astrophys. 649, A44 (2021).
Thorngren, D., Gao, P. & Fortney, J. J. The intrinsic temperature and radiative–convective boundary depth in the atmospheres of hot Jupiters. Astrophys. J. Lett. 884, L6 (2019).
Hörst, S. M. et al. Haze production rates in super-Earth and mini-Neptune atmosphere experiments. Nature Astron. 2, 303–306 (2018).
Fleury, B., Gudipati, M. S., Henderson, B. L. & Swain, M. Photochemistry in hot H2-dominated exoplanet atmospheres. Astrophys. J. 871, 158 (2019).
Kempton, E. M. R. et al. A framework for prioritizing the TESS planetary candidates most amenable to atmospheric characterization. Publ. Astron. Soc. Pac. 130, 114401 (2018).
Mullally, S. E., Rodriguez, D. R., Stevenson, K. B. & Wakeford, H. R. The Exo.MAST table for JWST exoplanet atmosphere observability. Res. Notes AAS 3, 193 (2019).
Luna, J. L. & Morley, C. V. Empirically determining substellar cloud compositions in the era of the James Webb Space Telescope. Astrophys. J. 920, 146 (2021).
Evans, T. M. et al. An optical transmission spectrum for the ultra-hot Jupiter WASP-121b measured with the Hubble Space Telescope. Astron. J. 156, 283 (2018).
Sing, D. K. et al. A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion. Nature 529, 59–62 (2016).
van Dokkum, P. G. Cosmic-ray rejection by Laplacian edge detection. Publ. Astron. Soc. Pac. 113, 1420–1427 (2001).
Pirzkal, N., Hilbert, B. & Rothberg, B. Trace and Wavelength Calibrations of the UVIS G280 +1/−1 Grism Orders Space Telescope WFC Instrument Science Report (Space Telescope Science Institute, 2017).
Mandel, K. & Agol, E. Analytic light curves for planetary transit searches. Astrophys. J. Lett. 580, L171–L175 (2002).
Pont, F., Zucker, S. & Queloz, D. The effect of red noise on planetary transit detection. Mon. Not. R. Astron. Soc. 373, 231–242 (2006).
Winn, J. N. et al. The Transit Light Curve Project. VII. The not-so-bloated exoplanet HAT-P-1b. Astron. J. 134, 1707–1712 (2007).
Hauschildt, P. H., Allard, F. & Baron, E. The NextGen Model Atmosphere Grid for 3000 ≤ Teff ≤ 10,000 K. Astrophys. J. 512, 377–385 (1999).
Sing, D. K. Stellar limb-darkening coefficients for CoRot and Kepler. Astron. Astrophys. 510, A21 (2010).
Schaller, G., Schaerer, D., Meynet, G. & Maeder, A. New grids of stellar models from 0.8 to 120 M solar at Z=0.020 and Z=0.001. Astron. Astrophys. Suppl. Ser. 96, 269 (1992).
Barman, T. S., Hauschildt, P. H. & Allard, F. Irradiated planets. Astrophys. J. 556, 885–895 (2001).
Lothringer, J. D., Barman, T. & Koskinen, T. Extremely irradiated hot Jupiters: non-oxide inversions, H− opacity, and thermal dissociation of molecules. Astrophys. J. 866, 27 (2018).
Lothringer, J. D. & Barman, T. The influence of host star spectral type on ultra-hot Jupiter atmo- spheres. Astrophys. J. 876, 69 (2019).
Hubeny, I., Burrows, A. & Sudarsky, D. A possible bifurcation in atmospheres of strongly irradiated stars and planets. Astrophys. J. 594, 1011–1018 (2003).
Fortney, J. J., Lodders, K., Marley, M. S. & Freedman, R. S. A unified theory for the atmospheres of the hot and very hot Jupiters: two classes of irradiated atmospheres. Astrophys. J. 678, 1419–1435 (2008).
Diamond-Lowe, H., Stevenson, K. B., Bean, J. L., Line, M. R. & Fortney, J. J. New analysis indicates no thermal inversion in the atmosphere of HD 209458b. Astrophys. J. 796, 66 (2014).
Lewis, N. K. et al. Into the UV: the atmosphere of the hot Jupiter HAT-P-41b revealed. Astrophys. J. Lett. 902, L19 (2020).
Lothringer, J. D. & Barman, T. S. The PHOENIX exoplanet retrieval algorithm and using H− opacity as a probe in ultrahot Jupiters. Astron. J. 159, 289 (2020).
ter Braak, C. J. F. & Vrugt, J. A. Differential evolution markov chain with snooker updater and fewer chains. Stat. Comput. 18, 435–446 (2008).
Lothringer, J. D. et al. A new window into planet formation and migration: refractory-to-volatile elemental ratios in ultra-hot Jupiters. Astrophys. J. 914, 12 (2021).
Wilson, J. et al. Gemini/GMOS optical transmission spectroscopy of WASP-121b: signs of variability in an ultra-hot Jupiter? Mon. Not. R. Astron. Soc. 503, 4787–4801 (2021).
Parmentier, V. & Guillot, T. A non-grey analytical model for irradiated atmospheres. I. Derivation. Astron. Astrophys. 562, A133 (2014).
Gelman, A. & Rubin, D. B. Inference from iterative simulation using multiple sequences. AStat. Sci. 7, 457–511 (1992).
MacDonald, R. J. & Madhusudhan, N. HD 209458b in new light: detection of nitrogen chemistry, patchy clouds and sub-solar water. Mon. Not. R. Astron. Soc. 469, 1979–1996 (2017).
McCullough, P. R., Crouzet, N., Deming, D. & Madhusudhan, N. water vapor in the spectrum of the extrasolar planet HD 189733b. I. The transit. Astrophys. J. 791, 55 (2014).
Rackham, B. V., Apai, D. & Giampapa, M. S. The transit light source effect: false spectral features and incorrect densities for M-dwarf transiting planets. Astrophys. J. 853, 122 (2018).
Rackham, B. V., Apai, D. & Giampapa, M. S. The transit light source effect. II. The impact of stellar heterogeneity on transmission spectra of planets orbiting broadly Sun-like stars. Astron. J. 157, 96 (2019).
Kirk, J. et al. ACCESS and LRG-BEASTS: a precise new optical transmission spectrum of the ultrahot Jupiter WASP-103b. Astron. J. 162, 34 (2021).
Kochanek, C. S. et al. The All-Sky Automated Survey for Supernovae (ASAS-SN) Light Curve Server v1.0. Publ. Astron. Soc. Pac. 129, 104502 (2017).
Jayasinghe, T. et al. The ASAS-SN catalogue of variable stars – II. Uniform classification of 412 000 known variables. Mon. Not. R. Astron. Soc. 486, 1907–1943 (2019).
Lecavelier Des Etangs, A., Pont, F., Vidal-Madjar, A. & Sing, D. Rayleigh scattering in the transit spectrum of HD 189733b. Astron. Astrophys. 481, L83–L86 (2008).
Ohno, K. & Kawashima, Y. Super-Rayleigh slopes in transmission spectra of exoplanets generated by photochemical haze. Astrophys. J. Lett. 895, L47 (2020).
Powell, D. et al. Transit signatures of inhomogeneous clouds on hot Jupiters: insights from micro- physical cloud modeling. Astrophys. J. 887, 170 (2019).
Espinoza, N. & Jones, K. Constraining mornings and evenings on distant worlds: a new semianalytical approach and prospects with transmission spectroscopy. Astron. J. 162, 165 (2021).
Mikal-Evans, T. et al. Diurnal variations in the stratosphere of an ultrahot planet. Nat. Astron. https://doi.org/10.1038/s41550-021-01592-w (2021).
Showman, A. P., Fortney, J. J., Lewis, N. K. & Shabram, M. Doppler signatures of the atmospheric circulation on hot Jupiters. Astrophys. J. 762, 24 (2013).
Woitke, P. et al. Equilibrium chemistry down to 100 K. Impact of silicates and phyllosilicates on the carbon to oxygen ratio. Astron. Astrophys. 614, A1 (2018).
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