The K2-3 System Revisited: Testing Photoevaporation and Core-powered Mass Loss with Three Small Planets Spanning the Radius Valley

• Published in: The Astronomical journal Vol. 164; no. 5; pp. 172 - 186
• Main Authors: Diamond-Lowe, Hannah, Kreidberg, Laura, Harman, C. E., Kempton, Eliza M.-R., Rogers, Leslie A., Joyce, Simon R. G., Eastman, Jason D., King, George W., Kopparapu, Ravi, Youngblood, Allison, Kosiarek, Molly R., Livingston, John H., Hardegree-Ullman, Kevin K., Crossfield, Ian J. M.
• Format: Journal Article
• Language: English
• Published: Madison The American Astronomical Society 01-11-2022; IOP Publishing
• Subjects: Astronomical models; Astronomy; Atmospheric models; Composition; Computer architecture; Energy spectra; Exoplanet atmospheric composition; Exoplanet evolution; Exoplanet systems; Extrasolar planets; Hubble Space Telescope; Low mass stars; M dwarf stars; Planet formation; Planetary evolution; Planetary mass; Radial velocity; Red dwarf stars; Solar composition; Spectral energy distribution; Spectroscopy; Spectrum analysis; Stellar models; Stellar planets; Stochastic processes; Ultraviolet astronomy; Valleys; X-ray astronomy; XMM (spacecraft)
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ac7807

Abstract
Multiplanet systems orbiting M dwarfs provide valuable tests of theories of small-planet formation and evolution. K2-3 is an early M dwarf hosting three small exoplanets (1.5–2.0
R
⊕
) at distances of 0.07–0.20 au. We measure the high-energy spectrum of K2-3 with HST/COS and XMM-Newton and use empirically driven estimates of Ly
α
and extreme-ultraviolet flux. We use
EXOFASTv2
to jointly fit radial velocity, transit, and spectral energy distribution data. This constrains the K2-3 planet radii to 4% uncertainty and the masses of K2-3b and c to 13% and 30%, respectively; K2-3d is not detected in radial velocity measurements. K2-3b and c are consistent with rocky cores surrounded by solar composition envelopes (mass fractions of
0.36
−
0.11
+
0.14
%
and
0.07
−
0.05
+
0.09
%
), H
2
O envelopes (
55
−
12
+
14
%
and
16
−
10
+
17
%
), or a mixture of both. However, based on the high-energy output and estimated age of K2-3, it is unlikely that K2-3b and c retain solar composition atmospheres. We pass the planet parameters and high-energy stellar spectrum to atmospheric models. Dialing the high-energy spectrum up and down by a factor of 10 produces significant changes in trace molecule abundances, but not at a level detectable with transmission spectroscopy. Though the K2-3 planets span the small-planet radius valley, the observed system architecture cannot be readily explained by photoevaporation or core-powered mass loss. We instead propose that (1) the K2-3 planets are all volatile-rich, with K2-3d having a lower density than typical of super-Earths, and/or (2) the K2-3 planet architecture results from stochastic processes such as planet formation, planet migration, and impact erosion.