Astronomy

Asteroids Uranus Neptune Comets

Jupiter

Saturn

Jupiter forms

Jupiter migrates inward

Saturn migrates to 3:2 resonance; formation of the terrestrial planets

Outward migration

End of the “Grand Tack”

After “Nice scenario”

Sun 2 AU 4 AU 6 AU 8 AU 10 AU

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thinks a five-planet model with another ice
giant is sufficient.
These movements also would explain
the Late Heavy Bombardment. As the ice
giants moved out, they disrupted the early
Kuiper Belt and sent volatile-rich bodies
careening into the inner system.
All this planetary dancing was crazy
enough — and then came Planet Nine.

Out in the cold
The first inklings that there might be a
large planet beyond the Kuiper Belt came
in 2003, when Caltech astronomer Mike
Brown noticed that Sedna — an object
roughly 600 miles (1,000 km) in diameter
that never gets closer to the Sun than 76 AU
— has a particularly elongated orbit. With
Batygin, he found five other objects whose
orbits seemed similarly aligned. Brown
and Batygin proposed that the only thing
that could cause that was a planet — but
Neptune was too far away to be the culprit.
It had to be a world many times Earth’s
mass in a millennia-long orbit.
If Planet Nine is real, it couldn’t have
formed in the outer reaches of the solar
system, says Kat Volk of the University of
Arizona. There simply isn’t enough mass in
that region — recent estimates put the total
for all KBOs at about that of the Moon.
The zones where Planet Nine would lurk
are home to even less material.
That means if it does exist, Planet Nine
either was ejected from closer in to the Sun
or it was stolen from a passing star. Each
hypothesis has its difficulties. Bannister
says it’s hard to imagine capturing a planet
from a passing star because it already
would have to be relatively far away from

its parent, raising the question of how it got

there. If it were an ejected ice giant, then

the issue is what kind of orbit it might end

up in — still a big unknown. Bannister

notes that an ejected super-Earth-sized or

ice-giant-sized planetary core would be as

likely to leave the solar system as to hang

around its fringes. To get something that

massive out there would require a lot of

moving parts to work together.

Weird and rare, but home

Rare events might be why our solar system

is as odd as it is compared with what we

see elsewhere in the galaxy. Jackson says

that for Jupiter and Saturn to migrate in

the right way means a lot of things had to

play out just so. “It’s probably not going to

happen around a large fraction of other

stars,” he says.

Just because our system is hard to make

doesn’t mean it can’t happen. “If you want

to describe planetary systems in general,

you want broad evolutionary models,” says

Morbidelli. “When you want to describe a

specimen, you’ve got to reconstruct its his-

tory like an archaeologist. A general pat-

tern needs some exceptions, like describing

the specific life of each one of us.”

Jesse Emspak is a science writer who lives

and works in New York City.

Debris disks, like this one around the star Beta
Pictoris, should be a lot more common if planets
take millions of years to form. Planet migration
can clear out debris quickly, however, leaving
such disks as rare commodities. ESO/A.-M. LAGRANGE ET AL.

Astronomers developed the Grand Tack model in 2011 as a way to explain Mars’ low mass. It contends

that Jupiter, after forming some 3.5 astronomical units (AU) from the Sun, migrated inward to 1.5 AU

before tacking back out to its current spot 5.2 AU away. The critical stages in the process encompass

the birth of Jupiter through 600,000 years later. ASTRONOMY: ROEN KELLY

Grand Tack model