Do Outer Giants Inflate Neptune-sized Planets? An Architecture-Dependent Mass-Radius Relation

TL;DR

Do outer giants inflate Neptune-sized planets? An Architecture-Dependent Mass-Radius Relation analyzes whether outer giant companions induce a measurable shift in the M–R relation for inner Neptune-sized planets. Using a uniform sample of close-in Neptune-sized planets with precise masses and radii, the authors compare systems with and without a confirmed outer giant. A total-least-squares MCMC fit to the M–R relation yields an architecture offset $\delta \approx 0.068$ and a mean radius increase of about $16.9\%$ at fixed mass for inner planets in OG systems; metallicity cannot explain the offset, and the signal persists under homogeneous parameterization though it remains limited by small-number statistics. If confirmed, this architecture-dependent M–R relation provides a new constraint on planet formation and evolution, linking outer giants to enhanced inner-envelope radii.

Abstract

Exoplanet demographics increasingly reveal that planetary properties depend not only on local irradiation and composition but also on the wider system architecture. We analyse a sample of Neptune-sized short-period planets with well-measured masses and radii, identifying those whose host stars harbour at least one confirmed outer-giant (OG) companion. On the mass-radius (M-R) plane, the two populations diverge modestly: inner planets in OG systems cluster at systematically larger radii than their counterparts in no-giant (NG) systems, a result that remains suggestive after controlling for planet and stellar properties. Bayesian modelling quantifies the offset, revealing an average radius enhancement of $17 \pm 4 \%$ for inner planets in OG systems relative to NG systems at fixed mass. Alternative cuts, including the use of a homogeneous set of parameters, confirm the robustness of the signal, though the result still relies on small-number statistics. Possible mechanisms for the observed inflation include prolonged inner-disc gas supply that boosted envelope accretion, and volatile enrichment by the outer giant. If upheld, this empirical link between outer giants and inflated inner-planet radii offers a new constraint on coupled formation and evolution in planetary systems.

Figures (4)

• Figure 1: Gaia CMD for the 4,374 confirmed planet hosts used to define the stellar sample. The red dashed polygon encloses the unevolved FGK main-sequence locus, removing evolved or late-type stars. The 3,952 planets (2,838 hosts) that survive this cut form the parent population for all subsequent analysis.
• Figure 2: M--R diagram for inner ($P< 50$ days) Neptune-sized planets. Red circles represent inner planets in systems with no detected giant (NG), and blue squares are inner planets in systems with at least one outer ($P=300-10,000$ days) giant ($M > 80 M_{\oplus}$; OG). Blue and red bands indicate the median radii in mass bins for each subsample. The black dashed line shows the best-fit (posterior) linear trend $\alpha + \beta \mathcal{M}_i$, with the shaded purple region marking the offset $\delta$ associated with OG systems. Shaded grey bands illustrate the typical uncertainties of the fit. For reference, the faint grey lines indicate theoretical composition models of pure H$_{\mathrm{2}}$O ice and pure MgSiO$_{\mathrm{3}}$ rock.
• Figure 3: Empirical cumulative-distribution functions (CDFs) for planetary properties of inner (P < 50 d) Neptune-sized planets in systems with and without confirmed outer giants. Left-to-right panels show orbital period, planet mass, planet radius (upper panel), planet density, equilibrium temperature and incident flux (bottom panel). Red curves correspond to NG systems, blue curves to OG systems. The inset labels give the two-sample KS and AD p-values for each parameter.
• Figure 4: CDFs for host-star properties of the same planet sample as Fig \ref{['fig:CDF_planets']}. Panels show effective temperature ($T_{\rm eff}$), surface gravity ($\log g$); metallicity [Fe/H].