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We present the validation of a transiting low-density exoplanet orbiting theM2.5 dwarf TOI 620 discovered by the NASA TESS mission. We utilize photometricdata from both TESS and ground-based follow-up observations to validate theephemerides of the 5.09-day transiting signal and vet false positive scenarios.High-contrast imaging data are used to resolve the stellar host and excludestellar companions at separations $\gtrsim 0.2''$. We obtain follow-upspectroscopy and corresponding precise radial velocities (RVs) with multiplePRV spectrographs to confirm the planetary nature of the transiting exoplanet.We calculate a 5$\sigma$ upper limit of $M_P < 7.1$ M$_\oplus$ and $\rho_P <0.74$ g cm$^{-3}$, and we identify a non-transiting 17.7-day candidate. We alsofind evidence for a substellar (1-20 M$_{\rm J}$) companion with a projectedseparation $\lesssim 20$ au from a combined analysis of Gaia, AO imaging, andRVs. With the discovery of this outer companion, we carry out a detailedexploration of the possibilities that TOI 620 b might instead be acircum-secondary planet or a pair of eclipsing binary stars orbiting the hostin a hierarchical triple system. We find, under scrutiny, that we can excludeboth of these scenarios from the multi-wavelength transit photometry, thusvalidating TOI 620 b as a low-density exoplanet transiting the central star inthis system. The low density of TOI 620 b makes it one of the most amenableexoplanets for atmospheric characterization, such as with JWST and Ariel,validated or confirmed by the TESS mission to date.

A close-in puffy Neptune with hidden friends: The enigma of TOI 620