Martin & Livio (2015) statistically compared the Solar System to the observed exoplanet population and concluded that its two most unusual characteristics are the absence of close-in super-Earth planets (roughly 1-10 Earth masses with orbital periods of days to months) and the general lack of planets interior to Mercury's orbit, while Jupiter's semi-major axis is only a mild outlier plausibly caused by observational selection effects. They left open whether these anomalies are intrinsic, how often planet formation naturally produces such inner-depleted systems, and whether close-in super-Earths would preclude habitable Earth analogues. Later work has settled parts of this: giant planets are now measured to be common at 1-10 au (Jupiter's orbit is not special), and Kepler-based population modeling shows that fewer than about 8% of planetary systems lack planets interior to Mercury's orbit, so the inner paucity is real. The open core is the intrinsic rarity of Solar-System-like architectures - systems with cold giant planets but no close-in super-Earths - currently constrained only indirectly (estimates near 1% from super-Earth-cold-Jupiter correlation studies, in tension with population synthesis), together with the formation pathways and frequency of inner-depleted systems and the dynamical impact of close-in super-Earths on terrestrial-planet formation and habitability.

Background

Exoplanet surveys have shown that the most common planets in the Galaxy are ones the Solar System does not contain. A super-Earth is a planet of roughly 1-10 Earth masses (11-10 M⊕10\,M_\oplus), intermediate between Earth and Neptune; such planets orbit a large fraction of Sun-like stars at orbital periods of days to months. Occurrence analyses of NASA's Kepler transit survey (Fressin et al. 2013) find close-in small planets to be ubiquitous, and population simulations calibrated on them (Mulders et al. 2018) estimate that 36±14%36\pm14\% of Sun-like stars host a habitable-zone Earth-size planet, while fewer than 8%8\% of planetary systems lack any planet interior to Mercury's orbit. The habitable zone is the radial range of distances from a star at which a rocky planet can maintain liquid water on its surface. The four main detection methods have strongly heterogeneous selection functions: the transit method favors short periods and large radii; the radial-velocity (RV) method, which measures the Doppler wobble of the host star, favors massive, short-period planets and requires decade-long baselines to reach Jupiter-like orbital periods; gravitational microlensing, in which a planet perturbs the brightening of a distant background star, is most sensitive to planets at separations of a few astronomical units (au) around faint stars; and direct imaging resolves only giant planets at tens of au. Any statement about typical planetary architectures must therefore correct for these selection effects. This is precisely what Martin & Livio (2015) did when they asked whether the Solar System, viewed as one exoplanetary system among the hundreds then known, is special: applying a Box-Cox power transformation (a statistical device that normalizes skewed distributions) to exoplanet parameter distributions so the Solar System could be compared quantitatively to the observed sample, they found the masses and densities of the Solar System's giant planets typical, its age and eccentricities unremarkable, and Jupiter's semi-major axis only a mild outlier most likely produced by selection effects; the two genuinely unusual characteristics were the lack of close-in super-Earths and the general lack of planets inside Mercury's orbit. They left open whether these characteristics are intrinsically rare or survey artifacts, noting that planet-formation theory could at most offer qualitative arguments and caveated theoretical estimates of how often systems end up inner-depleted: gas-driven migration of giant planets (the Grand Tack scenario of Walsh et al. 2011, in which Jupiter migrated inward to about 1.5 au1.5\,\mathrm{au} before reversing course, and the model of Batygin & Laughlin 2015, in which Jupiter's early migration cleared the inner disk of the material that would otherwise have formed super-Earths), disk dead zones (quiescent mid-plane regions of low turbulence) whose edges can act as planet traps halting migration, and the observed tendency of super-Earths to form from inwardly migrating cores (Terquem & Papaloizou 2007; Cossou et al. 2014) all bear on the question, as does the possibility that a close-in super-Earth would destabilize or otherwise alter terrestrial-planet formation in the habitable zone (Izidoro et al. 2014). More recent work proposes instead that the Solar System's inner planets formed from a narrow, low-mass ring of planetesimals (rocky bodies of roughly kilometer size, the building blocks of planets) bounded by pressure bumps (local maxima in the gas disk's pressure profile where drifting material accumulates), rather than from a spatially extended disk (Izidoro et al. 2022). Since 2015, giant-planet occurrence at few-au separations has been measured by legacy RV surveys (Wittenmyer et al. 2016, 2020; the California Legacy Survey of Fulton et al. 2021), direct imaging (Nielsen et al. 2019), and microlensing (Suzuki et al. 2016), and a strong empirical correlation between inner super-Earths and outer cold giant planets has been discovered (Zhu & Wu 2018; Bryan et al. 2019) and debated against population-synthesis predictions (Schlecker et al. 2021; Mishra et al. 2023). What still does not exist is a direct, selection-corrected measurement of the frequency of Solar-System-like architectures, a formation theory calibrated to produce them at a definite rate, or a systematic quantification of super-Earth dynamical effects on habitable terrestrial planets.

Problem Statement

Treating the Solar System as one member of the exoplanet population, determine whether its distinctive architecture - giant planets at few-au separations but no close-in super-Earth planets (the Solar System's additional lack of any planet interior to Mercury's orbit is descriptive of the Solar System, not a further condition of the target class) - is intrinsically rare or an artifact of heterogeneous observational selection effects, and establish how often planet formation produces systems like ours. The single overarching objective is a defensible, quantitative statement of the intrinsic (selection-corrected) rarity of Solar-System-like architectures together with its physical explanation, supported by three linked components, all three of which are required for a complete solution: (i) intrinsic occurrence rates for Solar-System-analog architectures - in particular, as one measurable proxy for this architecture, the conditional frequency of systems that host cold giant planets at Jupiter- to Saturn-like separations but no close-in super-Earths - combining transit, radial-velocity, microlensing, and direct-imaging constraints; (ii) identification, within planet-formation and migration theory (including giant-planet migration, disk dead zones and planet traps, super-Earth formation from inward-migrating cores, and planetesimal-ring disk structures), of the pathways that produce inner-depleted systems, with a calibrated prediction of how frequently such systems arise; and (iii) quantification of the dynamical impact of close-in super-Earths on the formation and long-term habitability of terrestrial planets in the habitable zone. Component (i) supplies the empirical rarity, component (ii) its physical origin, and component (iii) whether that rarity matters for the emergence of Earth-like planets; together they address whether the Solar System is intrinsically rare or typical, and why.

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