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L91’s 20,000-year orbit may have been shaped by an external force.

L. Calçada/ESO

New icy world with 20,000-year orbit could point to Planet Nine

PASADENA, CALIFORNIA—The solar system has gained a new extreme object: L91, a small, icy world with one of the longest known orbits, taking more than 20,000 years to go around the sun. Researchers have yet to pin down the object’s size or mass, but they can add it to the growing list of frozen bodies circling well beyond Neptune in strange orbits that imply gravitational disruptions from outside the sun and the known giant planets. In the case of L91, some astronomers say that external disrupter could be a ninth giant planet, as yet undiscovered. However, L91’s discovery team favors a scenario in which the disturbance is more mundane: a passing star, or the Milky Way’s gravity.

“It’s right at the limit of what we can detect,” said astrophysicist Michele Bannister of Queen’s University Belfast, who described the result today at the American Astronomical Society’s Division for Planetary Science meeting here.

L91 never comes closer to the sun than 50 astronomical units (AU), or 50 times the Earth-sun distance. From there, it slowly crawls all the way out to 1430 AU. This means it has a more elongated orbit than Sedna, another distant Pluto-sized object, whose closest approach is 76 AU and whose estimated far point reaches 937 AU. L91 was found using the Canada-France-Hawaii Telescope at Mauna Kea in Hawaii, as part of the Outer Solar System Origins Survey.

Astronomers once thought the solar system was relatively static, with the planets’ current configurations roughly unchanged since their birth in a gigantic cloud of dust and gas more than 4 billion years ago. But during the past decade or so, researchers have realized that planetary history is full of chaotic movements, with gas giants like Jupiter and Saturn drifting inward and outward from the sun. As these gargantuan masses moved, their gravitational influence sent other objects careening around, in some cases getting jettisoned entirely.

L91 is thought to be another wanderer, except the ice giant Neptune might be responsible for its movements. Bannister sketched a scenario in which the icy object was born with a more regular elliptical orbit. Back then, its closest and furthest points from the sun would have been roughly similar.

Over billions of years, Neptune’s gravitational influence might have given it little kicks that stretched out its orbital far point all the way to the inner part of the Oort cloud—a cluster of frozen bodies thought to start 2000 or more AU from the sun. Then a passing star or gravitational interactions with our Milky Way galaxy could have retracted L91’s orbit down to the less elongated but still extreme shape we see today.

“It’s a story that’s not implausible, but I also think it’s not needed,” said planetary scientist Konstantin Batygin of the California Institute of Technology (Caltech) here, who wasn’t part of the recent discovery.

His preferred explanation is gravitational tugging from Planet Nine, an as-yet-unseen Neptune-sized world that he and Mike Brown, another Caltech astronomer, came up with in January to explain the strange stretched-out orbits of a half-dozen objects, including Sedna. Bannister and her team modeled scenarios in which a Planet Nine–mass world could have provided the gravitational kicks necessary to elongate L91’s orbit but found that that would have tilted L91 into a different orbit. But Batygin says that galactic gravitational tugging is an inefficient process and that the Planet Nine explanation remains a less convoluted way of achieving the same result.