Highly Reflective White Clouds On The Western Dayside Of An Exo-Neptune
Highly-irradiated gas giant exoplanets are predicted to show circulation patterns dominated by day-to-night heat transport and a spatial distribution of clouds that is driven by advection and local heating.
Hot-Jupiters have been extensively studied from broadband phase-curve observations at infrared and optical wavelengths, but spectroscopic observations in the reflected light are rare and the regime of smaller and higher-metallicity ultra-hot planets, such as hot-Neptunes, remains largely unexplored to date.
Here we present the phase-resolved reflected-light and thermal-emission spectroscopy of the ultra-hot Neptune LTT 9779b, obtained through observing its full phase-curve from 0.6 to 2.8 μm with JWST NIRISS/SOSS. We detect an asymmetric dayside in reflected light (3.1σ significance) with highly-reflective white clouds on the western dayside (A = 0.79±0.15) and a much lower-albedo eastern dayside (A = 0.41±0.10), resulting in an overall dayside albedo of A = 0.50±0.07. The thermal phase curve is symmetric about the substellar point, with a dayside effective temperature of Teff,day = 2,260+40−50 K and a cold nightside (Teff,night <1,330 K at 3-σ confidence), indicative of short radiative timescales.
We propose an atmospheric circulation and cloud distribution regime in which heat is transported eastward from the dayside towards the cold nightside by an equatorial jet, leading to a colder western dayside where temperatures are sufficiently low for the condensation of silicate clouds.
Longitudinal transition of LTT 9779b from cloudy to cloud-free. Retrieved median (full lines) temperature-pressure profiles for the planetary emission spectra extracted at six orbital phases. Longitude φ = 180◦ corresponds to the nightside whereas φ = 0◦ is the dayside. The 1 and 2-σ confidence intervals are shown by the shaded regions. The MgSiO3 (enstatite, dashed black line) and Mg2SiO4 (forsterite, dash-dotted blue line) condensation curves from ref.44 are shown. These species can condense out of the atmosphere and form clouds when the local temperature is lower than the corresponding condensation curve. In the center of the figure is a schematic depicting the temperature and cloud coverage of LTT 9779b as seen from above the orbital plane. The white dashed lines indicate the orbital phases analyzed by the six phase-resolved atmospheric retrievals. The black arrow indicates the direction of the equatorial jet caused by the day-night temperature gradient. There is an asymmetry in cloud coverage, as heat is advected from the dayside to the nightside by an eastward equatorial jet, leading to a colder western dayside where silicate clouds can form. — astro-ph.EP
Louis-Philippe Coulombe, Michael Radica, Björn Benneke, Élyse D’Aoust, Lisa Dang, Nicolas B. Cowan, Vivien Parmentier, Loïc Albert, David Lafrenière, Jake Taylor, Pierre-Alexis Roy, Stefan Pelletier, Romain Allart, Étienne Artigau, René Doyon, Ray Jayawardhana, Doug Johnstone, Lisa Kaltenegger, Adam B. Langeveld, Ryan J. MacDonald, Jason F. Rowe, Jake D. Turner
Comments: Accepted for publication
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.14016 [astro-ph.EP] (or arXiv:2501.14016v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.14016
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From: Louis-Philippe Coulombe
[v1] Thu, 23 Jan 2025 19:00:00 UTC (18,801 KB)
https://arxiv.org/abs/2501.14016
Astrobiology,
