[astro-ph.EP] Compact multi-planet systems orbiting ultracool dwarfs, such as TRAPPIST-1, may be a major population of terrestrial worlds with poorly constrained demographics.
Individual close-in planets are expected to escape microlensing detection, but whether a compact system can produce a detectable collective signal remains to be quantified. We calculate the microlensing signal from each TRAPPIST-1 planet, and of all seven combined, if the system were a lens toward the bulge.
For a 0.09 M⊙ lens at 4 kpc, RE≈1.21 AU, placing the planets at s=0.010–0.051 θE, 12–63 times interior to the lensing zone. We measure each signal as the residual after fitting a single-lens model, which accounts for the barycentric dipole. Because the quadrupole term cancels between the two images to first order, the residual scales as the octupole (qs3), reaching |δA/A|∼10−6.
The seven planets combined yield Δχ2∼10−3 even at 1~mmag precision, five orders of magnitude below the standard detection threshold. Detection requires increasing all planet masses by a factor of ∼300, whereas the resonant chain loses its guarantee of Hill stability at a factor of ∼7. Only a lens within ∼15 pc of the source or the Sun places the outer planets in the lensing zone.
The Nancy Grace Roman Space Telescope is therefore not expected to detect a TRAPPIST-1-like system through microlensing, whereas its transit channel in the same fields can detect such planets in large numbers, marking a sharp boundary in the Galactic census between microlensing and transit sensitivity.
Stephen R. Kane
Comments: 18 pages, 6 figures, 2 tables, accepted for publication in PASP
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2611.05535 [astro-ph.EP](or arXiv:2611.05535v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2611.05535
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Submission history
From: Stephen Kane
[v1] Tue, 3 Nov 2026 19:29:35 UTC (132 KB)
https://arxiv.org/abs/2611.05535
Astrobiology, exoplanet,
