Astronomy - USA (2020-01)

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the planet, and nailing down
their clustering trends much
better will better help us
locate the planet and further
show it is real.”
By even doubling the
known number of extreme
TNOs orbiting far beyond
Neptune (which is currently a
sample of about a dozen),
researchers think they can
tease out whether Planet Nine
is really there or not. And,
more importantly, an
increased sample size may
help them further constrain
exactly where Planet Nine is
in its orbit. But for now,
Sheppard says, “None of the
most distant perihelia objects
with large semimajor axes
obviously buck the
clustering trend, but
again, we are talk-
ing about only a
little more than a
handful of objects.”

Although it’s fascinating to

ponder whether a hidden

planet many times the mass of

Earth is slinking in the outer

solar system, there are some

alternative theories that

attempt to explain the clus-

tered orbits of extreme TNOs

without employing Planet

Nine. For example, some

researchers suggest the orbit-

perturbing effects of Planet

Nine instead could be caused

by a distant and massive disk

of small, icy objects — or pos-

sibly even a baseball-sized

primordial black hole

of similar mass

that was cap-

tured by the

solar system.

However, “the planet

hypothesis is the simplest and

best explanation,” Sheppard

says. “A massive disk might be

possible, but we don’t see any

massive disk out there, and if

there were one, it would be

harder to explain than just

having one planet.” And as

for the black hole theory,

Batygin says, “the important

thing to understand here is

that all the calculations can tell

us is the mass of Planet Nine,

not its composition. So in

principle, Planet Nine can be a

planet, a potato, a black

hole, a hamburger, etc.,

as long as the orbital

parameters are right.”

So, although the

existence and exact

nature of Planet Nine

is still up for

debate,

Sheppard’s

team is

currently

carrying out the widest and

deepest survey of the solar

system ever to help find out

for sure. “I always like to say

it’s more likely than not that

[Planet Nine] exists out there.

I would say somewhere in the

80 to 90 percent range.”

“The primary reason why

I’m so excited about this work,”

Batygin says, “is because

there’s a near-term opportu-

nity for observational determi-

nation one way or another.”

But for now, Sheppard says,

“It’s exciting to be looking at

sky that no one has ever

imaged as deeply as we are. It’s

like Forrest Gump said: Each

image we take is like a box of

chocolates, you never know

what you are going to find.”

Jake Parks is an associate

editor of Astronomy and is

excited to learn what surprises

the solar system is still hiding

from us after all these years.

Konstantin Batygin (left)
and Mike Brown (right)
also have been pivotal
to progressing the
Planet Nine
hypothesis.
LANCE HAYASHIDA/CALTECH

“SO IN PRINCIPLE, PLANET NINE CAN BE

A PLANET, A POTATO, A BLACK HOLE, A HAMBURGER, ETC.,

AS LONG AS THE ORBITAL PARAMETERS ARE RIGHT.”