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DISCOVERMAGAZINE.COM
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Centered about 3.7 billion miles from the sun, the
Kuiper Belt — a doughnut of icy rocks and rocky ice
past Neptune’s orbit — serves as a cosmic map bound-
ary. “It’s hard to assess what’s beyond that,” says
astronomer Scott Sheppard of the Carnegie
Institution for Science. There’s just too
little sunlight at such distances. “Even
big things are very faint.”
Sheppard wants to find those faint
things anyway. In 2012, his team used a
new, car-sized camera to discover a dwarf
planet nicknamed Biden. It seemed to have an
oddly stretched-out orbit, suspiciously similar to the
only other known dwarf planet in the area, called Sedna.
In 2014, Sheppard’s team proposed an explana-
tion: A super-Earth-sized planet might have, ahem,
shepherded the dwarfs into their unusual orbits around
the sun with its gravitational crook. More evidence for
this idea came last year, when Sheppard published the
similarly tricky orbit of the Goblin, another dwarf planet
discovered around Halloween 2015.
“I think the planet is more likely real than not,”
Sheppard says. “But it’s definitely not a slam dunk.”
Ann-Marie Madigan, an astrophysicist at the University of Colorado
Boulder, didn’t even set out to study the solar system. She focused
on far-out stars and how they move around the supermassive black
holes frequently found at the centers of galaxies. Over time, she
realized that although they were individually small, together
these stars could flex some serious gravitational muscle.
Their influence on each other could skew their orbits,
tilting them in the same direction.
Graduate student Alexander Zderic worked with
Madigan over the last couple of years to take this galac-
tic model of “collective gravity” down to a smaller scale:
the solar system. Here, he says, dwarf planets like Biden,
Sedna and the Goblin could end up with similar orbits because a
bunch of smaller objects — which would collectively weigh about
the same as the hypothetical Planet Nine — did the gravitational
shepherding instead. To find out if they’re right, and potentially
convince the Planet Nine enthusiasts, Zderic’s team will have to
spot more of the hard-to-find objects.
During the current stalemate, at least both groups of researchers
can agree on one thing: Determining how distant objects tug on one
another is complicated. As Zderic puts it, “Gravity is one of the more
mysterious physical forces.”
GIANT SQUID are tough to find
because they’re rare and live deep
in the ocean. Similarly, if a giant
planet lives deep in the solar system,
telescopes will have a hard time
spotting it. But despite no physical
evidence, some scientists still believe
such a hypothetical world lurks about
56 billion miles from the sun.
The presence of this world, up to
10 times as massive as Earth and
known as “Planet Nine,” would solve
the orbital mysteries of other way-out
objects. But is this “planet” just a
gravitational illusion?
In Science Smackdown, we
let experts argue the evidence.
— SARAH SCOLES
A Whole New World?
PLANET NINE LIVES MORE LIKE PLANET NEIN
SCIENCE SMACKDOWN
The Claim The Counterpoint
Is Planet Nine still hiding somewhere in our solar system?
THE CRUX