Astrobiologists want one thing above all else: to find proof of alien life. That’s easier said than done, of course. Space is vast. Stars, planets, and moons are innumerable. Our probes are few and far between and our telescopes can see only so far and with so much fidelity.
But it helps to narrow the search. To that end, a team of astrobiologists led by Artem Aguichine, from Aix-Marseille University in France, has turned its attention to Jupiter, one of the weirder planets in our solar system. “Understanding planetary formation is key to predict the habitability of planets and the search for extraterrestrial life, which constitutes the main goal of astrobiology,” Aguichine told The Daily Beast. Aguichine’s new study, recently peer-reviewed and submitted to The Planetary Science Journal, appeared online on May 2.
Jupiter’s atmosphere probably can’t support life—at least, not life as we currently know it. So it stands to reason that planets like Jupiter also shouldn’t support life. If we can figure out where the planet came from and how it got so unlivable, we might be able to rule out—in our search for extraterrestrials—some of the roughly 5,000 nearby “exoplanets” in star systems other than our own.
No one is quite sure exactly how the fifth planet from the sun formed and how it got where it is. The oldest and biggest of our seven neighboring planets, it’s mostly gas by volume—with a roiling, hydrogen-and-helium atmosphere 2,000 miles thick and crisscrossed by clouds laced with ammonia crystals. It’s an atmosphere most astrobiologists believe is inhospitable to living things.
Only two probes have paid a close visit to the massive, gassy planet in the past 25 years. NASA’s Galileo probe between 1995 and 2003 and the Juno mission from 2016 to 2018. Using data from those probes as well as other fly-by missions, Aguichine and his team built a sophisticated computer model that they hope can accurately trace Jupiter’s formation starting around 4.6 billion years ago—and also chart its movement across the solar system.
The model combines a lot of findings from a range of space sciences. There are rules for how ice and vapor behave in space, especially in close vicinity to a star. There are also rules for how primordial space rocks clump together to form the cores of planets, and for how those rocky cores gather up gas from nebulas they pass through before settling, producing magnetic fields, and making up a grown-up planet.
Applying their model to the data from Galileo and Juno, Augichine and company concluded that Jupiter may have been a restless planet in its youth—moving toward and then away from the sun as it gradually transformed into the gassy behemoth that’s likely hostile to life that we now know.
If the team can confirm their conclusions by way of ever-better telescopes and future probes, they can apply the same model to other big, gassy planets scattered across the galaxy. If those planets match Jupiter, we might be able to rule them out as possible homes for E.T..
The gas giant, for all its mass, is light on its feet so to speak. The consensus is that the planet started out as a rocky core hundreds of million miles from the sun. Tugged on by a gassy nebula that once surrounded the star, as well as by the core of Saturn, which began forming shortly after Jupiter, the rock that would become the planet moved closer and closer to the sun over a period of millions of years. Astronomers call it migration. Having grown to its bloated size, the planet eventually settled in its current orbit, 470 million miles from the sun.
Scientists agree Jupiter’s passage across the solar system may have been one of the formative events in the formation of, well, everything else around us. It swept aside countless asteroids, scooped out much of the sun’s nebula, and left a trail of rocky debris that eventually coalesced into the smaller planets including Earth. “The big implication is that Jupiter as the ‘King of the Planets’ played the defining role for shaping the appearance of our solar system,” Christoph Burkhardt, an astronomer at the Institute for Planetology at Germany’s Munster University, told The Daily Beast.
That’s where the consensus ends, however. “It is extremely hard to discriminate among different formation scenarios,” Ravit Helled, a University of Zurich planetary scientist, told The Daily Beast. Another popular theory has Jupiter forming 1.7 billion miles or so from the sun before slowly closing the distance.
Aguichine and his team disagree with that theory. Their model supports a more elaborate idea called the “Grand Tack,” a term that refers to a maneuver in sailing known as tacking where a boat changes directions as it sails against the wind. In that origin story, the planet formed at a distance between 300 and 450 million miles from the sun and, over a period of millions of years, migrated up to a couple hundred million miles closer.
That’s where Saturn—a younger and speedier planet—caught up to Jupiter. Yanking on each other with the swelling force of their respective gravity, the two huge planets cast each other away from the sun. They both migrated back outward to the orbits where we now find them.
“Our results are compatible with some theories where Jupiter was not subject to strong migration,” Aguichine said. The main reason why: The planet has a lot of heavy elements in its atmosphere other than hydrogen and helium, which suggests it was born a lot closer to home. After all, those elements had to come from somewhere. The going theory is that they were concentrated in the nebula that once shrouded the sun before planets like Jupiter scooped it up.
If the gas giant was born nearly 2 billion miles from the sun, it would have fewer heavy elements, according to Aguichine’s model, because there were never many heavy elements that far out. Thus Aguichine and company’s assertion is that Jupiter formed much closer to the sun—a starting position that aligns with the Grand Tack theory.
This isn’t settled science. Aguichine and his coauthors stressed that we need more data before concluding Jupiter took a reversing, near-sun jog during its youth. “Further developments in the modeling of Jupiter’s deep interior will provide a better knowledge of its interior structure and composition, which will help discriminating between various formation scenarios,” they wrote.
It would help to look more closely at the data from the Juno probe, Jonathan Lunine, chair of the astronomy department at Cornell University and one of Aguichine’s coauthors, told The Daily Beast. We could also send a new probe to Saturn, which may have traveled a path similar to Jupiter’s.
In the meantime, it can’t hurt to start comparing Jupiter to some faraway exoplanets. We might find a few that look like they formed by way of back-and-forth tacks around their stars, the way Jupiter may have. Doing so could potentially eliminate some of those planets from our search for alien life.
Lucky for us, we’ve got just the thing: NASA’s new $10-billion James Webb Space Telescope, which launched in 2021 after decades of development. “JWST will help us out here with detailed compositional measurements of atmospheres of giant planets around other stars,” Lunine said.
The new telescope has just begun capturing its first, super-sharp images of distant planets. Planets that might be just as weird, and hostile to life, as Jupiter.