Evidence mounts for hidden ninth planet
that orbits the Sun every 20,000 years
A century after observatory founder Percival Lowell
speculated that a 'Planet X' lurks at the fringes of the
Solar System, astronomers say that they have the
best evidence yet for such a world. They call it Planet
Nine.
Orbital calculations suggest that Planet Nine, if it
exists, is about ten times the mass of Earth and
swings an elliptical path around the Sun once every
10,000–20,000 years. It would never get closer than
about 200 times the Earth–Sun distance, or 200
astronomical units (au). That range would put it far
beyond Pluto, in the realm of icy bodies known as the
Kuiper belt.
No one has seen Planet Nine, but researchers have
inferred its existence from the way several other
Kuiper belt objects (KBOs) move. And given the
history of speculation about distant planets, Planet
Nine may end up in the dustbin of good ideas gone
wrong.
Astronomers have long speculated about the
existence of additional large planets in the outer
Solar System, but none has yet been confirmed.
1846 Johann Gottfried Galle discovers Neptune,
guided by predictions from perturbations of Uranus's
orbit.
1905 Percival Lowell starts hunting for a 'Planet X',
which he predicted would lie beyond Neptune, just as
Neptune lies beyond Uranus. His calculations led
astronomers at Lowell's namesake observatory to
find Pluto in 1930, but the object is not massive
enough to be Planet X.
1984 On the basis of periodic extinctions in the fossil
record, scientists propose that a dwarf star, later
named Nemesis, passes through the Solar System
every 26 million years, flinging comets on a path to
impact Earth.
1999 Perturbations in comet orbits lead astronomers
to propose that a brown dwarf (bigger than a planet
but smaller than a star) exists in the outer Solar
System. It is named Tyche, the good sister of
Nemesis.
2014 A search with the Wide-Field Infrared Survey
Explorer satellite rules out the existence of both
Nemesis and Tyche. But the discovery of an object in
the distant Kuiper belt prompts Chadwick Trujillo and
Scott Sheppard to propose a large planet in the
Kuiper belt.
2016 Orbital calculations by Konstantin Batygin and
Mike Brown strengthen the concept of this unseen
planet, which they name 'Planet Nine'.
"If I read this paper out of the blue, my first reaction
would be that it was crazy," says Mike Brown, an
astronomer at the California Institute of Technology
in Pasadena who was part of the research team. "But
if you look at the evidence and statistics, it's very hard
to come away with any other conclusion."
Brown and his colleague Konstantin Batygin propose
Planet Nine in a paper published on 20 January in the
Astronomical Journal.
Alessandro Morbidelli, an orbital-dynamics specialist
at the University of the Côte d'Azur in Nice, France,
who has reviewed the paper in detail, says he is
"quite convinced" that the planet exists. Others are
not so sure.
"I have seen many, many such claims in my career,"
says Hal Levison, a planetary scientist at the
Southwest Research Institute in Boulder, Colorado.
"And all of them have been wrong."
Claims of Planet Nine's existence recall a period in
the nineteenth century when astronomers predicted
and then discovered Neptune by studying tiny
perturbations in the orbit of Uranus. The gravity of
some unseen body must be tugging on Uranus, they
said- and they were right. "In some sense we're
hoping to relive history a little bit," says Batygin.
*Culled from Nature
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