2023MNRAS.525.6303G

Very young massive planets are sufficiently luminous by their internal heat of formation to permit detailed studies, including spectroscopy of their atmospheres with large telescopes at sufficient resolution (λ/Δλ ≳ 1000) to identify major constituents to inform models of planet formation and early evolution. We obtained 1-2.4 μm (YJHK) spectra of the planetary-mass 'b' companion of 2MASS J04372171+2651014, a 1-3 Myr-old M dwarf member of the Taurus star-forming region, and one of the youngest such objects discovered to date. These indicate the presence of CO and possibly H₂O and CH₄ in the atmosphere, all suggesting a T_eff of around 1200 K, characteristic of a L-T transition spectral type and consistent with previous estimates based on its luminosity and age. The absence or attenuation of spectral features at shorter wavelengths suggests the presence of micron-size dust, consistent with the object's red colour. The spectrum of 2M0437b resembles those of the HR 8799 planets, especially the innermost 'b' planet, with the exception of a pronounced flux deficit in the H-band of uncertain origin.