Astronomy - USA (2020-01)

500 AU

Beginning of

Oort Cloud

2,000 AU

Top down view

2012 VP 113

“Biden”

Planet 9

Sedna

48 ASTRONOMY • JANUARY 2020

Saturn and got tossed out-

ward.” This ended up pushing

the still-forming Planet Nine

out of the feeding zone, which

stunted its growth before it

could reach the size of a true

giant planet.

Alternatively, Sheppard

says, “it could be a captured

object from another star sys-

tem.” Because stars form out

of massive clouds of condens-

ing gas and dust, they are

born in litters, not in isolation.

So, early in the solar system’s

history, the Sun was sur-

rounded by many other

nearby stars that all gravita-

tionally interacted with one

another. According to

Sheppard, this means other

stars “were tossing objects out

of their solar system, so there

was probably a lot of free-

f loating planets being ejected

from all these different stars.”

And if one of the rogue plan-

ets strayed too close to the

Sun, there’s a chance it was

seized by our own solar sys-

tem, where it’s since remained.

However, for the Sun to

capture such a planet, “you’d

have to slow it down some-

how,” Sheppard says. “So

that’s the questionable factor.”

Although capturing an ejected

planet isn’t the easiest thing to

do, Sheppard points out it

could happen. For example, he

says, there was probably a lot

more gas and dust around in

the early solar system, mean-

ing an approaching planet

“could have been frictionally

slowed down and captured.”

There’s also a third possi-

bility: the solar system was

shaken up in its past by a

wandering star. In research

published last year, astrono-

mers announced the discovery

of an exoplanet named

HD 106906 b, which they

think was exiled into a large,

perturbed orbit thanks to a

pair of passing stars that

strayed too close to its system.

According to Batygin, it’s fea-

sible that a similar process

could have happened when

the Sun was still near other

stars from its birth cluster.

Proving Planet

Nine’s existence

The problem with Planet Nine

is that in order for us to be

100 percent sure it exists, we

need to see it with our own

eyes. Fortunately, Sheppard

says, “our survey is designed

to not simply find the planet,

but to triple the known [num-

ber of] very distant objects.

These very distant objects are

the ones that are sensitive to

says all likely began their lives
within about 5 to 15 AU of
one another. “There’s prob-
ably several Earth-mass-sized
objects and bigger which

formed in that area,” he says.

As Planet Nine was forming,

“it got to several Earth masses

in size, but then it probably

got too close to Jupiter and

ABOVE AND RIGHT: To carry out their

survey of the most distant objects in

the solar system, Sheppard and his

colleagues rely on the Subaru

Telescope (above), perched atop the

summit of Mauna Kea in Hawaii, as

well as the Victor Blanco Telescope

(right), located at Cerro Tololo Inter-

American Observatory in Chile. SUBARU:

ROBERT LINSDELL/FLICKR; BLANCO: NOAO/AURA/NSF

MORE EVIDENCE

FOR PLANET NINE

If Planet Nine exists, there should be a set of objects with orbits that are
perpendicular to the ecliptic plane of the solar system. Five such objects
(shown in magenta) are known, and according to Batygin, they perfectly
fit the models. ASTRONOMY: ROEN KELLY AFTER CALTECH/R. HURT (IPAC)