The planets that stayed are the detector
A planet can orbit its star ten thousand times farther out than Earth does. At that distance it is barely held, a whisper of gravity away from being unbound. A passing clump of invisible mass could knock it loose.
Which means the ones still there are evidence.
Two astronomers at Montpellier, Théo Paré and Julien Lavalle, took 128 wide-orbit planets and companion objects from the exoplanet catalogue and asked the question backwards. If these worlds are still bound today, then whatever dark matter drifts through the galaxy cannot be too dense or too heavy. The survivors set the limit on what could have passed.
Their target is one specific candidate: ultra-compact minihalos, dense dark clumps that could form if the universe's earliest fluctuations ran larger than the standard picture allows. From planets that simply stayed, they derive new limits on those fluctuations, competitive with limits from other methods, and sensitive to clumps above roughly a hundred thousand solar masses.
They also name a signature to look for. In a system with both a distant planet and a close one, repeated invisible passes should slowly tilt the far planet's orbit out of the inner system's plane, more with distance. A trail left by something that leaves no other mark.
No planet was found here, and no clump was seen. It is a limits paper built on a catalogue and statistics, a preprint not yet peer-reviewed, from one lab. But it points a new kind of instrument at the dark universe: not one we build, but a planet we watch for staying.
The loneliest planets are dark-matter detectors.
Sources: arXiv:2606.14827v2 (Paré & Lavalle, LUPM 26-002). Preprint, not yet peer-reviewed. First published on iLands.
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