July 30, 2020 | David F. Coppedge

Forcing Planets into Line

You can’t make a planet do what your model says it must do to exist, unless it cooperates in your imagination.

Writer and novelist Stephen Crane (1871-1900) once quipped,

A man said to the universe: Sir, I exist!”
“However,” replied the universe, “The fact has not created in me a sense of obligation.”

We might revise the lines to fit modern cosmologists and planetary scientists:

A man said to the universe [or solar system]: “Sir, my models exist!”
“However,” replied the universe [or solar system], “The fact has not created in me a sense of obligation.”

The planets don’t seem to care about what scientists come up with to explain their origins. They just exist, defying the models.

Model of the planets with sizes relative to Jupiter.

Inner Workings: Was Jupiter born beyond the current orbits of Neptune and Pluto? (PNAS). In this entry of a series called “Inner Workings,” Kenneth Croswell describes some of the new models trying to explain the birth of Jupiter. Planetary scientists thought they had the gas giants figured out in the 1990s, until some inconvenient facts got in the way. The discovery of “hot Jupiters” around other stars threw the models into disarray; instead of slow accretion out beyond the “frost line,” they had to invoke gravitational billiards to get them to migrate in close to the parent stars. It also threw the models of our Jupiter’s origin into a cocked hat.

But how did such a behemoth arise? Conventional theory says that Jupiter formed more or less where it is now, about five times farther from the Sun than Earth is. At that distance, the disk of gas and dust that swirled around the young Sun was dense enough to give birth to the planetary goliath.

Everything they thought they knew was wrong, and it was time to start over. They invented new models, like “pebble accretion” and disk instability, with type-II migration to move the gas giants closer in after they formed.

In 2019, however, two groups of researchers unaware of each other’s work—one in America, the other in Europe—proposed a literally far-out alternative: Jupiter got its start in the solar system’s hinterlands, probably beyond the current orbits of Neptune and Pluto, and then moved inward.

To some scientists, this is just fun and games. It keeps them employed.

“It’s the most fun I’ve had with a paper for some time,” says Karin Öberg, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, and one of the theory’s originators. “You can explain it to almost anyone in a couple of minutes.” The theory may be straightforward, but its consequences are profound: If it’s right, the solar system’s biggest planet was born some 10 times farther from the Sun than it now is, which means that some of the other giant worlds in our solar system and beyond likely arose at vast distances from their stars and then moved to their current locations.

It’s far out, all right. But is it right? Croswell spends some time discussing possible evidences for the “radical new theory,” but they seem more and more contrived as he goes. The American was glad to read about the A. D. Bosnan’s similar idea in Europe; “It sort of gave me some confidence that maybe this idea wasn’t so strange after all,” Karin Öberg said. Other planetary scientists have had to suspend disbelief to look at the new model with any seriousness.

Other researchers have noticed the radical new theory. “Ten years ago, people would have just rolled their eyes,” says Jonathan Fortney, a planetary scientist at the University of California at Santa Cruz. But thanks to pebble accretion theory, he says, that’s no longer the case. Although it “sounds a little crazy,” it’s not unreasonable that you could create the core of a giant planet beyond Pluto’s current orbit, Fortney says.

Lemmings, by JB Greene. Used by permission.

Thank goodness one critic is not following the lemmings off the cliff. He has some good advice for all lemmings in all fields.

David Stevenson, a planetary scientist at the California Institute of Technology in Pasadena, is more skeptical. “It’s not that difficult to come up with a variety of stories, given our lack of understanding,” he says. Furthermore, even if Jupiter’s core arose far from the Sun and thus acquired high levels of nitrogen and argon, he says it’s hard to get those elements up to the planet’s atmosphere, where the Galileo probe detected them; after all, the core constitutes only a small fraction of the planet. To solve this problem, Öberg and Wordsworth invoke a giant impact on Jupiter to lift the elements from the core to the atmosphere. “Well,” Stevenson says, “you can use a giant impact to do anything.”

This is like Step Two, “Then a miracle occurs” in Sidney Harris‘s famous cartoon of a grad student at the blackboard describing his derivation to his adviser, who replies, “I think you should be more explicit here in Step Two.” Once you allow one miracle of chance, others are sure to follow. They become like additional lies that are needed to support the first lie. Croswell goes on to say that with the new model, why, Saturn might have formed that way, too!

Meanwhile, Öberg is pleased with the theory’s reception. “I met much less resistance than I expected,” she says. “The vast majority of reactions have been ‘Yeah, that kind of makes sense.’” And she can even explain the theory to her nonscientist neighbors.

Could it be that the majority of reactions are less resistant because no other model makes any sense?

“Kind of makes sense.” That means it doesn’t make sense; it just “kinda” does. It pretends to make sense, when you aren’t thinking about it.

We can anticipate the nonscientist version in two words: “Stuff Happens.” Then a miracle occurs. (You may now roll your eyes.)

Editor’s Anecdote: I remember a college astronomy class in the late 1980s where the professor admitted that in models of the solar system, you almost need a miracle to keep the model going. It appears not much has changed since then.




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