Dating backwards

Scarcely anything is known about it at all, save for its size (about three kilometers wide) and orbit, which is almost identical to Jupiter’s.Almost, that is, save for one important detail—BZ509’s orbit is backwards, or retrograde, meaning it moves in the opposite direction of the prograde orbits of almost everything else circling the sun.Such interactions with giant planets are thought to be the main way small objects become interstellar in the first place.Perhaps, scientists initially speculated, BZ509’s odd orbit was due to its origins in the Oort cloud—an enormous reservoir of comets ejected by Jupiter and the other giant planets to the outer limits of the solar system, where they can be disturbed by passing stars.

‘Oumuamua was the first of its kind to be observed, and now it may have another newfound counterpart much closer to home.If this were the case with BZ509, however, we would be seeing it at an exceedingly special and rare point in its history: a brief moment of illusory orbital stability that will soon decay into chaos. To find out, Namouni and Morais built a virtual time machine.Yet no comet-like emissions have been detected from BZ509, and it is in resonance with Jupiter, synchronized to periodically swoop within about 175 million kilometers of the giant planet’s cloud tops—just close and often enough for Jupiter to regularly provide gentle tugs that keep its orbit stable for at least a million years. They used a supercomputer to simulate the possible motions of a million digital clones of BZ509, each with slightly different orbital parameters reflecting astronomers’ limited knowledge of the real object’s orbit.Such a star would likely have been a sibling of our sun born alongside us in a “stellar nursery”—a nebula filled with star-forming gas and dust.Galactic motions long ago scattered this progeny far and wide, but it seems remnants may linger on our doorstep today.

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