test 1 Astronomy - May 2018 USA | Page 27

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. W W W.ASTR ONOMY.COM 27