Astronomy - USA (2020-01)

Mْ= 1 Earth Mass

Jupiter

Mass: 318 Mْ Saturn

Mass: 95.2 Mْ

Uranus

Mass: 14.5 Mْ

Neptune

Mass: 17.1 Mْ

Pluto

Mass: 0.003 Mْ

Planet 9

Mass: ~5–15 Mْ

Mercury
Mass: 0.055 Mْ

Venus

Mass: 0.815 Mْ

Earth

Mass: 1 Mْ

Mars

Mass: 0.107 Mْ

WWW.ASTRONOMY.COM 47

at nearly the same point in

space as a handful of other

noteworthy objects. These

include the team’s earlier dis-

covery of 2012 VP 113 (which

they nicknamed Biden as a

nod to the “VP” in its desig-

nation) and Sedna, a dwarf

planet roughly half the diam-

eter of Pluto that was discov-

ered in 2003.

It’s this recently identified

pack of perihelion-clustered

objects, which Sheppard refers

to as “extreme TNOs,” that

suggests an unseen and mas-

sive planet is hiding in the

outskirts of our solar system.

this peculiar class of extreme

TNOs indicates that dwarf

planets may not be the kings

and queens of the Kuiper

Belt. Instead, there’s a grow-

ing dossier of evidence that

suggests a colossal world

some five to 15 times the

mass of our own planet

is lurking far beyond,

playing puppeteer to the

oddly clustered collection

of distant bodies.

In 2016, Caltech

astronomers Kostantin

Batygin and Mike Brown

published a paper titled

“Evidence for a Distant Giant

Planet in the Solar System.”

In it, they discuss a unique

population of TNOs that

make their closest approaches

to the Sun at roughly the

same point in space, despite

having different orbital trajec-

tories. According to that

study, the odds that such clus-

tering is the result of chance

is about 0.007 percent (or 1 in

14,000). Furthermore, at the

beginning of 2019, Batygin

and Brown also showed

there’s only about a 0.2 per-

cent chance (or 1 in 500) that

the clustering is the result

of observational biases.

“Based on the clustering of

the objects we are finding that

are way out there like The

Goblin,” Sheppard says, “a

super-Earth planet that is a

few hundred AU away” could

be shepherding them into

position. Thanks to cutting-

edge simulations, the

researchers expect Planet

Nine (sometimes referred to

as Planet X), to be in a some-

what eccentric orbit that takes

it about 400 AU from the Sun.

But, Sheppard says, “the

planet could be up to some

1,500 AU away in the more

massive planet models.”

But if Planet Nine really

exists, why does it orbit so far

from the Sun?

That’s still a heavily

debated question. “There’s

two main theories for how

the planet got out there,”

Sheppard says. “The most

likely scenario I like to call

the ‘runt of the giant planet

family.’ ” In this scenario,

Planet Nine formed alongside

Jupiter, Saturn, Uranus, and

Neptune, which Sheppard

The hunt for

Planet Nine

With the advancement of

powerful survey telescopes, as

well as sophisticated software

that can sift through massive

amounts of data like never

before, we are just now begin-

ning to scour the distant

reaches of the solar system

for icy dwarf planets and

the like. But the discovery of

Planet Nine is expected to be about five to 15 times as

massive as Earth, which would make it pretty similar to the

large super-Earths and small sub-Neptunes astronomers are

increasingly finding around other stars. ASTRONOMY: ROEN KELLY AFTER KARL TATE

THE DISCOVERY OF THIS PECULIAR CLASS OF

EXTREME TNOS INDICATES THAT DWARF PLANETS MAY NOT

BE THE KINGS AND QUEENS OF THE KUIPER BELT.

SIZING UP

PLANET NINE

In recent years, astronomers have

identified a mysterious group of

extreme TNOs with distant, clustered

perihelia, as well as relatively aligned

orbits that seem decoupled from the

gravitational influence of the solar

system’s giant planets. Based on the

orbits of these objects — which

include Sedna (orange), “The Goblin”

(green), “Biden” (blue), and a handful

of other bodies with less-colorful

names (shown in gray) — some

researchers think a massive world

called Planet Nine is shepherding the

extreme TNOs into position. ASTRONOMY:

ROEN KELLY AFTER CALTECH/R. HURT (IPAC)