Study finds Roman Space Telescope should not detect TRAPPIST-1-like planets by microlensing

Astrobiology Web reported Nov. 6, 2026 that Stephen R. Kane’s paper, accepted for publication in PASP, finds the Nancy Grace Roman Space Telescope should not detect a TRAPPIST-1-like system through gravitational microlensing when modeled as a lens toward the galactic bulge. The study was submitted to arXiv on Nov. 3, 2026 under the identifier 2611.05535 [astro-ph.EP] and runs to 18 pages with 6 figures and 2 tables. The paper carries the arXiv identifier 2611.05535 in the Earth and Planetary Astrophysics category.

Gravitational microlensing uses a foreground object’s gravity to magnify light from a more distant source.

Using a lens mass of 0.09 solar masses at a distance of 4 kiloparsecs, they found an Einstein radius of approximately 1.21 AU, the characteristic angular scale of the lens’s magnification. That places every planet at 0.010 to 0.051 times the angular Einstein radius, or 12 to 63 times interior to the lensing zone, the region where a planet’s gravity normally creates a detectable anomaly. Consequently, every TRAPPIST-1 planet sits far inside the region where microlensing is normally sensitive.

Worlds orbiting close to their host stars should escape microlensing detection, and the detectability of a full compact system’s combined signal was previously unmeasured. The authors note that single planets are expected to be invisible to microlensing, while the combined effect of an entire compact system had not been measured before this work.

Even assuming photometric precision of one millimagnitude, a thousandth of a magnitude in brightness, the full seven-planet system yields a Delta chi-squared of about 10 to the minus 3, landing well short of the usual detection cutoff by five orders of magnitude. The combined statistical significance spans five orders beneath what surveys require to claim a detection.

The authors calculate that boosting every planet’s mass by about 300 times would be necessary to push the microlensing signal above the standard detection threshold.

The outer planets would enter the lensing zone only if the lens lay within roughly 15 parsecs of either the background source or the Sun, an exceptionally close alignment. To isolate each planetary signal, the researchers subtracted a single-lens model from the light curve and accounted for the barycentric dipole, a shift tied to the system’s center of mass. Because the quadrupole contributions from both images cancel at first order, the remaining signal depends on the octupole term, a higher-order part of the signal, and reaches roughly one part in a million.

Roman’s transit channel is expected to uncover large numbers of these planets in the same survey fields. The result draws a clear dividing line in the galaxy’s planetary census between what microlensing can find and what transits can find.