Dynamical Stability of the LHS 1903 Exoplanetary System: Constraining the Orbital Parameter Space of Planet e and a Putative Fifth Body

Abstract

Kalash Garg, Tanishq Limbasiya and Ravindran Swamy

We present a comprehensive N-body dynamical stability analysis of the LHS 1903 exoplanetary system, a compact four-planet system orbiting a 0.538Mâ?? M-dwarf star recently confirmed by [1]. Using the rebound N-body integration package with the waist simplistic integrator and a timestep of dt = 0.0003 yr, we construct high resolution 50 × 50 survival time maps across 80,000 years — equivalent to approximately 106 orbital periods of the outermost confirmed planet — for key orbital parameter combinations centered on the dynamically interesting planet e [2,3]. The nominal system is robustly stable: at the observed mass of planet e (me = 5.79M⊕), the stability boundary lies at ed ≈ 0.296, giving a stability margin of â??ed ≈ 0.184. A companion grid varying the masses of planets d and e independently at nominal eccentricities produces complete stability across all 400 simulations, confirming eccentricity as the dominant driver of instability rather than mass. Analysis of all five resonance angles associated with the 7:3 mean motion commensurability between planets d and e reveals uniform circulation in each case (range ≈ 360â?¦), directly challenging the resonance identification of Wilson and indicating a near-resonant rather than formally reso- nant dynamical state. For the putative fifth body identified as an unconfirmed radial velocity signal at P ≈ 53.9 days, we derive dynamical constraints over 147,600 years (106 orbital periods of the candidate planet), finding that long-term stability requires ef â?² 0.12 for most of the tested mass range (0.5–20M⊕). A pronounced instability stripe at ef ≈ 0.296, with mean survival times below 820 years across all tested masses, is consistent with a mean motion resonance between the candidate planet and planet e near the 11:6 commensurability. These results establish the first dedicated dynamical characterization of the LHS 1903 system and provide constraints relevant to the interpretation of its anomalous inside-out planetary architecture.

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