The Dark Planets of the WASP-47 Planetary System

Abstract

Exoplanet discoveries have demonstrated a vast range of planetary system architectures. The demographic of compact planetary systems are especially interesting from the perspective of planetary formation and the evolution of orbital dynamics. Another interesting demographic is that of giant planets in eccentric orbits, since these planets have likely had a dynamical history involving planetplanet scattering events. The WASP-47 system is particularly fascinating since it combines these two demographics, having both compact planetary orbits and a giant planet on an eccentric orbit within the system Habitable Zone. Here we provide an analysis of the WASP-47 system from the perspective of atmospheric detection and characterization. We discuss the system architecture and the potential for additional long-period planets. We simulate expected phase variations as a function of planet orbital phase for the system due to the combined effect of the planets. We present an analysis of precision photometry of WASP-47 from the K2 mission, phased on each of the planets. The analysis rules out the detection of phase signatures for the two inner-most planets, enabling constraints upon their albedos and atmospheric properties. Our study concludes that WASP-47b is an example of a "dark" planet with a tentative geometric albedo of 0.016 and a 1σ upper limit of 0.17. The WASP-47e data are consistent with a broad range of albedos, but also show early evidence of having a relatively low albedo. The growing number of dark, short-period giant planets provide the framework of an ideal sample for studying low albedo dependence on atmospheric composition.

Aerosols in Exoplanet Atmospheres

“…For example, dayside near-infrared photometry and spectra can be explained without the need for optically thick aerosols down to the pressure levels probed, suggesting either a lack of aerosols altogether or that aerosols form at pressures higher than the planets' photosphere (Barstow et al, 2014;Kataria et al, 2015; J. M. Lee et al, 2012;Line et al, 2014). Similarly, the observed optical geometric albedos are low (≤0.1) for nearly all giant transiting planets (Figure 6; Angerhausen et al, 2015;Bell et al, 2017;Coughlin & López-Morales, 2012;Dai et al, 2017;Heng & Demory, 2013;Kane et al, 2020;Mallonn et al, 2019;Močnik et al, 2018;Niraula et al, 2018), consistent with significant gas absorption without substantial reflective aerosols (Marley et al, 1999;Sudarsky et al, 2000). One outlier, Kepler-7b, which possesses a geometric albedo ∼0.3 (Demory et al, 2011), also exhibits a maximum in its optical phase curve west of the substellar point; this has been interpreted as the presence of highly reflective aerosols covering a small fraction of the western limb (Demory et al, 2013;Garcia Munoz & Isaak, 2015;Webber et al, 2015).…”

The GALAH Survey: improving our understanding of confirmed and candidate planetary systems with large stellar surveys

“…(iv) WASP-47 e: Contained within a four-planet system is a super-Earth exoplanet known as WASP-47 e, first discovered in 2015 (Becker et al 2015 ;Dai et al 2015 ). Orbiting around an ironrich ([Fe/H] = 0.45 ± 0.09 dex) G9V dwarf every 0.78 d, this exoplanet and its companions have been characterized by numerous studies since its disco v ery (Weiss et al 2017 ;Dorn et al 2019 ;Kane, Fetherolf & Hill 2020 ). With our refined stellar parameters (1.13 ± 0.02 R , 1.06 ± 0.05 M ) and combining multiple independent archi ve observ ations, we have determined the radius and mass of WASP-47 e to be 1.79 ± 0.04R ⊕ and 7.21 ± 0.61M ⊕ , respectively.…”

“…But if there is an atmosphere, with an escape velocity of 22.44 ± 0.99 km s −1 , it may contain ionic calcium, silicon, magnesium, and maybe even aluminium with its extreme equilibrium temperature (Dorn et al 2019 ). The albedo of such worlds is still a field of active research, with Kane et al ( 2020 ) being able to constrain the geometric albedo of its sibling, WASP-47 b, to be 0.0016 but were unable to constrain WASP-47 e's. Ho we ver, there was some evidence to suggest that WASP-47 e's geometric albedo could also be as low as WASP-47 b, but a wide range of geometric albedo solutions were also found.…”

The GALAH Survey: Improving our understanding of confirmed and candidate planetary systems with large stellar surveys

“…(iv) WASP-47 e: Contained within a four-planet system is a super-Earth exoplanet known as WASP-47 e, first discovered in 2015 (Becker et al 2015;Dai et al 2015). Orbiting around a iron-rich ([Fe/H] = 0.45 ± 0.09 dex) G9V dwarf every 0.78 days, this exoplanet, and its companions have been characterised by numerous studies since its discovery (Weiss et al 2017;Dorn et al 2019;Kane et al 2020). With our refined stellar parameters (1.13 ± 0.02R , 1.06 ± 0.05M ) and combining multiple independent archive observations, we have determined the radius and mass of WASP-47 e to be 1.79 ± 0.04R ⊕ and 7.21 ± 0.61M ⊕ respectively.…”

“…But if there is an atmosphere, with an escape velocity of 22.44 ± 0.99 𝑘𝑚𝑠 −1 , it may contain ionic calcium, silicon, magnesium, and maybe even aluminium with its extreme equilibrium temperature (Dorn et al 2019). The albedo of such worlds is still a field of active research, with Kane et al (2020) being able to constrain the geometric albedo of its sibling, WASP-47 b, to be 0.0016 but were unable to constrain WASP-47 e's. However there was some evidence to suggest that WASP-47 e's geometric albedo could also be as low as WASP-47 b, but a wide range of geometric albedo solutions were also found.…”