A CHEOPS-enhanced view of the HD 3167 system

Abstract

Much remains to be understood about the nature of exoplanets smaller than Neptune, most of which have been discovered in compact multi-planet systems. With its inner ultra-short period planet b aligned with the star and two larger outer planets d-c on polar orbits, the multi-planet system HD 3167 features a peculiar architecture and offers the possibility to investigate both dynamical and atmospheric evolution processes. To this purpose we combined multiple datasets of transit photometry and radial velocimetry (RV) to revise the properties of the system and inform models of its planets. This effort was spearheaded by CHEOPS observations of HD 3167b, which appear inconsistent with a purely rocky composition despite its extreme irradiation. Overall the precision on the planetary orbital periods are improved by an order of magnitude, and the uncertainties on the densities of the transiting planets b and c are decreased by a factor of 3. Internal structure and atmospheric simulations draw a contrasting picture between HD 3167d, likely a rocky super-Earth that lost its atmosphere through photo-evaporation, and HD 3167c, a mini-Neptune that kept a substantial primordial gaseous envelope. We detect a fourth, more massive planet on a larger orbit, likely coplanar with HD 3167d-c. Dynamical simulations indeed show that the outer planetary system d-c-e was tilted, as a whole, early in the system history, when HD 3167b was still dominated by the star influence and maintained its aligned orbit. RV data and direct imaging rule out that the companion that could be responsible for the present-day architecture is still bound to the HD 3167 system. Similar global studies of multi-planet systems will tell how many share the peculiar properties of the HD3167 system, which remains a target of choice for follow-up observations and simulations.

Rocky Planet or Water World? Observability of Low-density Lava World Atmospheres

“…The second case study that we consider is HD 3167 b (Vanderburg et al 2016;Christiansen et al 2017;Bourrier et al 2022). With a mass of 4.73 M ⊕ and a radius of 1.63 R ⊕ , HD 3167 b's bulk density is consistent with a 100% silicate interior composition (Figure 1).…”

“…This can be explained by a core-less silicate interior (Elkins-Tanton & Seager 2008; Lichtenberg 2021), a volatile-rich iron core (Li et al 2019b;Schlichting & Young 2022), and/or a volatile-rich mantle (Dorn & Lichtenberg 2021). Bourrier et al (2022) rule out a hydrogen-rich atmosphere for this planet using atmospheric evolution models. With a substellar temperature of ∼2500 K, a purely rocky surface would be expected to vaporize and form an optically thick, ∼10 mbar atmosphere (Zilinskas et al 2022).…”

“…We note that while the MIRI LRS observation of 55 Cnc e is expected to saturate the detector at shorter wavelengths, and therefore reduce the usable wavelength range, we have not accounted for this in our simulated observations and consequently present the full 5-12 μm wavelength range. HD 86226 c, HD 3167 b, and 55 Cnc e have very short transit durations of 3.1, 1.6, and 1.6 hr, respectively (Demory et al 2011;Teske et al 2020;Bourrier et al 2022). Consequently, observing ∼5 eclipses for one of these targets is feasible within a small to medium JWST proposal, making them excellent targets for investigating super-Earth interior compositions.…”

“…where R å is the stellar radius, a is the semimajor axis of the planet, and T eff is the effective temperature of the star. Notable low-density lava worlds with favorable observability metrics and high substellar temperatures include HD 86226 c (T sub = 1854 K; Teske et al 2020), HD 3167 b (T sub = 2513 K, Vanderburg et al 2016;Bourrier et al 2022), 55 Cnc e (T sub = 2773 K; McArthur et al 2004;Bourrier et al 2018), and TOI-561 b (T sub = 3218 K; Lacedelli et al 2021;Weiss et al 2021;Brinkman et al 2023), as shown in Figure 1. Given their extremely high irradiation, the dayside atmospheres of these planets are expected to consist of evaporated surface material.…”

“…Low-density lava worlds are not expected to host primordially accreted and hydrogen-rich atmospheres due to their extremely high irradiation levels (e.g., Bourrier et al 2022). Their low bulk densities must therefore be a result of their interior compositions.…”