Transit detection of the long-period volatile-rich super-Earth $\nu^2$ Lupi d with $CHEOPS$

TL;DR

This study reports the detection and characterization of the outer transiting planet d in the $ u^2$ Lupi system using CHEOPS, revealing its size, mass, and composition, and providing insights into low-mass planetary diversity.

Contribution

First detection of transits of the outer planet d in the $ u^2$ Lupi system, with detailed measurements of its properties and implications for planetary formation models.

Findings

Planet d transits its host star, with radius 2.56 R⊕ and mass 8.82 M⊕.

The system shows diverse planetary compositions, from rocky to hydrogen-helium envelopes.

The $ u^2$ Lupi system offers a new laboratory for studying low-mass planet formation.

Abstract

Exoplanets transiting bright nearby stars are key objects for advancing our knowledge of planetary formation and evolution. The wealth of photons from the host star gives detailed access to the atmospheric, interior, and orbital properties of the planetary companions. $\nu^2$ Lupi (HD 136352) is a naked-eye ($V = 5.78$) Sun-like star that was discovered to host three low-mass planets with orbital periods of 11.6, 27.6, and 107.6 days via radial velocity monitoring (Udry et al. 2019). The two inner planets (b and c) were recently found to transit (Kane et al. 2020), prompting a photometric follow-up by the brand-new $CHaracterising\:ExOPlanets\:Satellite\:(CHEOPS)$. Here, we report that the outer planet d is also transiting, and measure its radius and mass to be $2.56\pm0.09$ $R_{\oplus}$ and $8.82\pm0.94$ $M_{\oplus}$, respectively. With its bright Sun-like star, long period, and mild irradiation ($\sim$5.7 times the irradiation of Earth), $\nu^2$ Lupi d unlocks a completely new region in the parameter space of exoplanets amenable to detailed characterization. We refine the properties of all three planets: planet b likely has a rocky mostly dry composition, while planets c and d seem to have retained small hydrogen-helium envelopes and a possibly large water fraction. This diversity of planetary compositions makes the $\nu^2$ Lupi system an excellent laboratory for testing formation and evolution models of low-mass planets.