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.