Born in deep shadows? That could explain Jupiter’s strange makeup

A frigid nursery might have helped shape this gas giant’s peculiar atmosphere

The atmosphere of Jupiter (shown) has seemingly too much nitrogen, argon, krypton and xenon. This suggests the giant planet formed in a very cold place.

NASA, ESA, A. Simon/GSFC, M.H. Wong/UCB

Jupiter may have formed in a shadow — one colder than Pluto. Such a frigid birthplace could explain the giant planet’s unusual abundance of certain gases. That’s the conclusion of a new study.

Jupiter consists mostly of hydrogen and helium. Those were the most common elements in a planet-spawning disk that spun around our newborn sun. Other elements that were gases near Jupiter’s birthplace became part of the planet, too. And they would be present in the same proportions as existed in the disk of planet-forming materials. It’s known as the protoplanetary (Proh-toh-PLAN-eh-tair-ee) disk.

Astronomers think the sun’s composition largely reflects the protoplanetary disk’s. So Jupiter’s elemental recipe should resemble the sun’s — at least for elements that were gases. But the gases nitrogen, argon, krypton and xenon are some three times as common on Jupiter (relative to hydrogen) as they are on the sun. Why?

“This is the main puzzle of Jupiter’s atmosphere,” says Kazumasa Ohno. He’s a planetary scientist at the University of California, Santa Cruz.

If Jupiter was born at its current distance from the sun, its birthplace would have been a frosty 60 kelvins. That’s –213˚ Celsius (-351.4˚ Fahrenheit). And at that temperature, those elements should be gases. Below about 30 kelvins, however, they would freeze solid. It’s easier to build up a planet from solids than from gases. So if Jupiter somehow arose in a place much colder than its current home, it could have acquired an icy mass containing bonus amounts of those otherwise gassy elements.

Two years ago, in fact, two different research teams each offered up this radical idea: that Jupiter originated in a deep freeze beyond the current orbits of Neptune and Pluto. Later, they suggested, it could have spiraled in toward the sun.

Ohno has now teamed up with astronomer Takahiro Ueda at the National Astronomical Observatory of Japan in Tokyo to propose a different idea. They argue that Jupiter could have formed where it is. But the region would have been a lot colder back then. They think a pileup of dust could have formed between the planet’s orbit and the sun. This would have blocked the sun’s warming light.

That would have cast a long shadow, one that imposed a deep freeze on Jupiter’s birthplace. The ultracold temps would have made nitrogen, argon, krypton and xenon freeze solid. And this would have allowed them to become a larger part of the planet.

The scientists describe their idea in a new study. It appears in the July Astronomy & Astrophysics.

Enter snowballs

Where would that dust have come from? Ohno and Ueda think it could have been debris left when rocky objects closer to the sun collided and shattered.

Farther from the sun — where the protoplanetary disk was colder — water froze. This would have given rise to objects that resembled snowballs. When they collided, they were more likely to stick together than shatter. Thus, they wouldn’t cast much of a shadow, the researchers say.

“I think it’s a clever fix” to explain what would otherwise be difficult to explain, says Alex Cridland. He’s an astrophysicist. He works at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Cridland was one of the scientists who had suggested Jupiter likely formed beyond Neptune and Pluto. But that theory, he says, means Jupiter had to move much closer to the sun after its birth. The new scenario, he says, nicely avoids that complication.

the planet Saturn
Knowing what Saturn’s atmosphere is made of might help pinpoint the birthplace of Jupiter.NASA, ESA, A. Simon/GSFC, M.H. Wong/UCB, the OPAL Team

How to test the new idea? “Saturn might hold the key,” Ohno says. Saturn is nearly twice as far from the sun as Jupiter is. The dust shadow that could have chilled Jupiter’s birthplace would barely have reached Saturn’s, Ohno and Ueda have calculated.

If true, Saturn would have arisen in a warmer region. So this gas giant should not have acquired nitrogen, argon, krypton or xenon ice. In contrast, if both Jupiter and Saturn really formed in the cold beyond the present orbits of Neptune and Pluto, then like Jupiter, Saturn should have lots of those elements.

Astronomers know the composition of Jupiter. They learned when NASA’s Galileo probe dove into Jupiter’s atmosphere in 1995. What’s needed, Ohno and Ueda say, is a similar mission to Saturn. NASA’s Cassini spacecraft did orbit Saturn from 2004 to 2017. However, it measured only an uncertain level of nitrogen in the Ringed Planet’s atmosphere. It found no argon, krypton or xenon.

About Ken Croswell

Ken Croswell earned his Ph.D. in astronomy at Harvard University for studying the Milky Way's halo. He has authored eight books on astronomy, including The Alchemy of the Heavens: Searching for Meaning in the Milky Way.

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