A Star’s Companion Hides a Secret Twin express
The companion of the star HIP 53005 has stumped astronomers. Its color and brightness and spectrum point to a mass of about 80 Jupiters — right at the hydrogen-burning limit, where an object is no longer a planet but not yet a star. Yet its orbital motion demands nearly twice that mass: 185 Jupiters. It’s like a suitcase that looks light but feels heavy, as if a second one is hidden inside.
Where is the missing mass? Either another, completely invisible companion is circling nearby, or the object itself is a tight pair. In that case, their combined light is faint, but their total gravity is strong. A similar illusion was revealed with Gliese 229B: what was thought to be a single brown dwarf turned out to be two, orbiting each other every 12 days. Such discoveries are rare, and they shed light on the birth of stellar families, where suns often live in pairs or triples.
Original abstract
as written by the authors arXiv:2604.03767
We present the discovery of a low-mass companion located at $$\rho$$ $$\sim$$ 0\farcs{}85 ($$r_{\rm proj} \approx 62~au$$) from the early-type 1.2 Gyr-old star HIP 53005 using direct imaging data from the Subaru and Keck Telescopes and astrometry from the Hipparcos-Gaia Catalog of Accelerations. The companion, HIP 53005 C, is a component of a multiple system also including a $$\approx$$ 12\farcs{}4-separation M dwarf companion inducing a negligible proper motion acceleration. HIP~53005 C's position on color-magnitude diagrams, the fit of its spectral energy distribution to atmosphere models, and its location on an empirical mass-magnitude diagram all suggest that it lies at the M/L transition and near the hydrogen-burning limit ($$\sim80~M_{\rm Jup}$$). However, our orbital fitting combining direct-imaging relative astrometry with proper motion acceleration favors a much higher dynamical mass of $$\sim185\ M_{\rm Jup}$$. An additional unseen, more closely-orbiting companion below the detection limit (at $$\rho\lesssim0\farcs2$$)) may explain this discrepancy. Alternatively, HIP~53005C could be a low-mass binary like Gliese~229Bab, making this system an intriguing laboratory for studying multiple star formation.
Brown dwarfs are failed stars: too lightweight to ignite nuclear fusion, they merely glow dimly, occupying the gap between planets and true stars.
Like Tatooine with its two suns, only here one of the lights might itself be a binary, and then there are even more celestial dancers.