Grand Tack model
Jupiter forms
Jupiter
Asteroids
Saturn
Uranus
Neptune
Comets
Jupiter migrates inward
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. Saturn migrates to 3:2 resonance; formation of the terrestrial planets
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. Outward migration
End of the “Grand Tack”
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
After “Nice scenario”
Sun
2 AU
4 AU
6 AU
8 AU
10 AU
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
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.
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