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Artist’s concept of “Planet Nine,” a hypothesized world about 10 times more massive than Earth that may orbit far from the sun.
Credit: Caltech/R. Hurt (IPAC) |
April 6, 2016 - SPACE - The
hunt is on to find "Planet Nine" — a large undiscovered world, perhaps
10 times as massive as Earth and four times its size — that scientists
think could be lurking in the outer solar system. After Konstantin
Batygin and Mike Brown, two planetary scientists from the California
Institute of Technology,
presented evidence for its existence
this January, other teams have searched for further proof by analyzing
archived images and proposing new observations to find it with the
world's largest telescopes.
Just this month, evidence from the Cassini spacecraft orbiting Saturn
helped close in on the missing planet. Many experts suspect that within
as little as a year someone will spot the unseen world, which would be a
monumental discovery that changes the way we view our solar system and
our place in the cosmos. "Evidence is mounting that something unusual is
out there — there's a story that's hard to explain with just the
standard picture," says David Gerdes, a cosmologist at the University of
Michigan who never expected to find himself working on Planet Nine. He
is just one of many scientists who leapt at the chance to prove — or
disprove — the team's careful calculations.
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Researchers say an anomaly in the orbits of distant Kuiper Belt objects points to the existence of an unknown planet orbiting the sun.
Credit: by Karl Tate, Infographics artist |
Batygin
and Brown made the case for Planet Nine's existence based on its
gravitational effect on several Kuiper Belt objects — icy bodies that
circle the sun beyond Neptune's orbit. Theoretically, though, its
gravity should also tug slightly on the planets, moons and even any
orbiting spacecraft. With this in mind, Agnès Fienga at the Côte d'Azur
Observatory in France and her colleagues checked whether a theoretical
model (one that they have been perfecting for over a decade) with the
new addition of Planet Nine could better explain slight perturbations
seen in Cassini's orbit. Without it, the eight planets in the solar
system, 200 asteroids and five of the most massive Kuiper Belt objects
cannot perfectly account for it. The missing puzzle piece might just be a
ninth planet.
So
Fienga and her colleagues compared the updated model, which placed
Planet Nine at various points in its hypothetical orbit, with the data.
They found a sweet spot—with Planet Nine 600 astronomical units (about
90 billion kilometers) away toward the constellation Cetus — that can
explain Cassini's orbit quite well. Although Fienga is not yet convinced
that she has found the culprit for the probe's odd movements, most
outside experts are blown away. "It's a brilliant analysis," says Greg
Laughlin, an astronomer at Lick Observatory, who was not involved in the
study. "It's completely amazing that they were able to do that so
quickly." Gerdes agrees: "That's a beautiful paper."
The good news does not end there. If Planet Nine is located toward the constellation Cetus, then it could be picked up by the
Dark Energy Survey,
a Southern Hemisphere observation project designed to probe the
acceleration of the universe. "It turns out fortuitously that the
favored region from Cassini is smack dab in the middle of our survey
footprint," says Gerdes, who is working on the cosmology survey. "We
could not have designed our survey any better." Although the survey was
not planned to search for solar system objects, Gerdes has discovered
some (including one of the icy objects that led Batygin and Brown to
conclude Planet Nine exists in the first place).
Laughlin thinks this survey has the best immediate chance of success.
He is also excited by the fact that Planet Nine could be so close.
Although 600 AUs—roughly 15 times the average distance to Pluto—does
sound far, Planet Nine could theoretically hide as far away as 1,200
AUs. "That makes it twice as easy to get to, twice as soon," Laughlin
says. "And not just twice as bright but 16 times as bright."
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| Mini-Neptunes like "Planet Nine" outnumber other types of planets found by astronomers. Credit: by Karl Tate, Infographics artist |
And
the Dark Energy Survey is not the only chance to catch the faint world.
It should be possible to look for the millimeter-wavelength light the
planet radiates from its own internal heat. Such a search was
proposed by
Nicolas Cowan, an exoplanet astronomer at McGill University in
Montreal, who thinks that Planet Nine might show up in surveys of the
cosmic microwave background (CMB), the pervasive afterglow of the big
bang. "CMB experiments have historically used solar system giant planets
to calibrate their instruments, so we know that current and planned CMB
experiments are sensitive enough to measure the flux from Planet Nine
if it is as bright as we think it is," Cowan says.
Already, cosmologists have started to comb through data from existing
experiments, and astronomers with many different specialties have also
joined in on the search. "I love that we can take this four-meter
telescope and find a rock 100 kilometers in diameter that is a billion
kilometers past Neptune with the same instrument that we are using to do
extragalactic stuff and understand the acceleration of the universe,"
Gerdes says.
In the meantime Batygin and Brown are proposing a dedicated survey of their own. In a
recent study
they searched through various sky maps to determine where Planet Nine
cannot be. "We dumpster-dived into the existing observational data to
search for Planet Nine, and because we didn't find it we were able to
rule out parts of the orbit," Batygin says. The zone where the planet
makes its farthest swing from the sun as well as the small slice of sky
where Fienga thinks the planet could be now, for example, have not been
canvassed by previous observations. To search the unmapped zones,
Batygin and Brown have asked for roughly 20 observing nights on the
Subaru Telescope on Mauna Kea in Hawaii. "It's a pretty big request
compared to what other people generally get on the telescope," Brown
says. "We'll see if they bite." If they do, Brown is convinced he will
have his planet within a year.
"I really want to see what it looks like," says Batygin, who adds that
his aspiration drives him to search for the unseen world. But Laughlin
takes it a step further: "I think [the discovery] would provide amazing
inspiration for the next stage of planetary exploration," he says. We
now have another opportunity to see one of the worlds of our own solar
system for the first time. "If Planet Nine isn't out there, we won't
have that experience again." -
SPACE.
NOTE: Thanks to Joann Mckeon-Chan for contributing to this post.
