A Solar-System Window for Hidden Stellar Companions

Could the closest stellar or substellar object to the Sun be not an ordinary star at parsec distance, but a hidden-brane companion at hundreds or thousands of astronomical units? We do not perform a new Solar-System dynamics analysis; instead we construct a phenomenological mass-distance map using an illustrative ephemeris tidal envelope calibrated to Planet-Nine-like constraints. A smooth dark matter halo cannot supply such an object: the local density contains only a sub-Pluto mass within 1000 AU. A nearby hidden-brane companion must therefore be a structured, gravitationally bound object rather than a typical halo draw. The illustrative envelope still allows Earth to sub-Saturn masses at 300-1000 AU, rising to Jovian mass near 2000 AU. In a simple QCD-scaled hidden sector with a confinement scale about ten times larger than ordinary QCD, the minimum hidden stellar mass overlaps the upper part of this window, providing a benchmark for a genuine hidden-sector star: bright in dark photons, electromagnetically dark to us, and visible only through gravity. We also derive an Earth-based source-strength proxy for brane-to-brane channels and show that, along the ephemeris envelope, it grows as the square of the companion distance: the largest received source scale comes from the most massive companion still allowed, not the nearest one. A probe sent to the companion's gravitational projection would reduce the ordinary source-receiver separation by two to three orders of magnitude relative to Earth-based operation. This is not a detection forecast; the excitation of KK modes, the compact-direction brane-to-brane transfer factor, and the detector response remain model-dependent.