This Exoplanet Has the Weirdest Orbit We’ve Ever Seen
Nature & Science

This Exoplanet Has the Weirdest Orbit We’ve Ever Seen

  • The planet, HR 5183 b, takes 77 years to complete its “comet-like” orbit. But that orbit is extremely eccentric, spanning essentially the equivalent of Jupiter’s orbit at its closest point to its home star, and past Neptune at its furthest.
  • The planet, which is about 102 light years away, likely got this way in an incident that ejected another planet in the system.
  • It’s both one of the longest orbital periods known, and one of the most eccentric.

    We know of 4,043 planets outside our solar system. Of those, a small handful could be considered weird. And now there’s a new member to that strange planetary pantheon: HR 5183 b.

    First, there’s its size: It has the more mass than three Jupiter-size planets put together. But we’ve come across high mass planets before, up to the 13 Jupiter limit between planets and a class of object called brown dwarfs. It also has an extremely long year, circling its sun-like home star in 77 years. That’s an entire human lifetime, and makes it easily in the top 10 longest planetary orbits yet detected.

    But what makes HR 5183 b go from “a little different” to “truly weird” is its orbital path. If you were to overlay it in our solar system, it would sweep in as close as Jupiter and as far out as Neptune in that 77-year span. It’s covering the distance, in other words, of the entire outer solar system.

    This animation compares the eccentric orbit of HR 5183 b to the more circular orbits of the planets in our own solar system. Credit: W. M. Keck Observatory/Adam Makarenko

    It also thus far seems to be alone out there. But that could be part of its origin story.

    “This is definitely a first-of-its-kind planet,” says Sarah Blunt, a Caltech NSF graduate fellow and coauthor of a new paper about HR 5183 b. “It’s very eccentric—it’s almost in a comet-like orbit. In our solar system, comets tend to have pretty eccentric orbits, but planets tend to be very circular, with very low eccentricity.”

    In the paper, accepted for publication in The Astronomical Journal, the researchers outline this new weird wor

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