With a radius of 69,911 km, Jupiter is the largest planet in the Solar System. But how did it get so massive?

Jupiter: Pebbles or Planetesimals?

Since its formation 4.6 billion years ago, Jupiter has defended this planet against otherworldly threats, acting as a gravitational attractor for comets and asteroids that could have changed the course of evolutionary history.

Without it, we might not be here today.

Since its discovery, scientists have speculated on how the gas giant became so massive. 

As the Solar System was forming, Jupiter was more than likely the first planet to form. But as far as what caused it to get so massive, there are two different theories. One theory is that the gas giant swallowed a lot of “pebbles.”

No, not the kind of pebbles that you might find in your backyard, but rather meteorites around the size of boulders.

I have no idea why they call them pebbles in this case, but I don’t come up with the terms.

But, thanks to a new computer model and paper that sources gravitational data from the Galileo and Juno space probes, the idea that pebbles contributed to Jupiter’s mass may be obsolete.

Jupiter: Too Much Mass

Figuring out how Jupiter’s innards are structured has been a chief concern for scientists. But unlike Earth, where we can use seismic waves to study the mantles and core, Jupiter has no physical surface, but even if it does have a rocky center, it’s highly unlikely that there would be any tectonic activity.

Most data seems to point to the idea that Jupiter has a solid rocky core around 12 to 45 times the mass of our planet. That core is estimated to reach temperatures of 35,000 degrees Celsius. And, if scientists are right and the core is solid, then it is enveloped by a layer of metallic hydrogen as well.

Let’s just say you’d never want to be caught falling into Jupiter.

We have taken a metric ton of photos of Jupiter over the years. Most of these are of its stunning bands of clouds and storms, like the Great Red Spot.

Of the 9 probes we’ve sent to Jupiter, Juno gave us an unprecedented look at the behemoth gas giant both inside and out.

And it is thanks to Juno and Gallileo, that researchers involved with a study published in the journal Astronomy and Astrophysics were able to put together a simulation of Jupiter’s formative years.

Galileo and Juno gathered a lot of data on Jupiter’s gravitational field, and after studying the effects that gravity has on the giant planet, researchers were able to put together a simulation that suggests that not only does Jupiter have way more rocky material in its depths, but that our theories for its structure and formation might be wrong!

Jupiter: Redefining Origins

The second theory of how Jupiter might have expanded to its current size rejects the idea that it got so massive by swallowing boulder-sized meteorites, but rather, that it consumed still forming planets.

And it’s this theory that this new simulation and study seems to support. 

Jupiter takes 12 years to orbit the sun. At key points in that orbit, Juno and Galileo both studied Jupiter’s gravitational field, and the researchers involved with this study were able to use those datasets to create gravitational maps of Jupiter’s surface, establishing where gravity differs on the planet. 

What’s really interesting about this study is that Jupiter seems to have a surprising amount of heavy elements. Like, way more than we originally thought. 

This rocky material exerts a much stronger gravitational pull than the gasses in Jupiter’s other layers. 

The models provided by the researchers suggest that Jupiter’s core is likely somewhere in the ballpark of 11 and 30 Earth masses and that heavier materials could make up as much as 9% of the planet’s mass.

Yamila Miguel, an astrophysicist at Leiden University in The Netherlands says that the idea that Jupiter accumulated all of this mass from smaller meteorites simply can’t explain its massive size.

This means that Jupiter could only have gotten so massive by consuming planetesimals, the building blocks of planets.

Jupiter: Devourer of Planets

In the first couple million years of the Solar System’s life, the still-forming Jupiter was devouring baby planets.

Hypothetically, if Jupiter had formed from smaller, boulder-sized space rocks, then once it had accumulated most of its gaseous layers, it would have stopped absorbing rocky material. However, these new models suggest that Jupiter kept cannibalizing still-forming planets even after it stopped absorbing gas.

This actually makes quite a bit of sense in the early chaotic solar system. Since the discovery of exoplanets, and specifically, hot Jupiters, it’s thought that Jupiter and Saturn had to have formed closer to the sun and slowly migrated to where they are now. 

If that’s the case, then a forming, OG hot Jupiter would have had plenty of material to gobble up on its journey to its current orbit.

So, score, we now have a pretty good idea of how Jupiter formed, right?

But, we’re not done yet.

Before this study, it was thought that Jupiter’s atmosphere caused a lot of convection that would allow for hotter gas to propagate near the core. This would also make it so hotter gasses could rise up to the outer reaches of Jupiter’s layers to cool off a bit before taking the plunge back into the planet.

But, this doesn’t seem to be the case!

This study showed us that Jupiter’s interior layers don’t mix well with its upper gaseous layers.  We’re talking like oil and water here (although that’s probably not fully accurate either). So, no convection.

This flies in the face of what scientists thought we’d find in Jupiter’s core.

It seems that the heavier, rocky, and metallic materials seem to gravitate near the planet’s core, sometimes getting mixed up in the lower atmosphere as well. 

With so much of our understanding surrounding Jupiter changing, it’s only natural that our theories for how the rest of the planets in the Solar System formed would follow that trend. 

And, while it’s going to be a while before new models are produced for the earliest period of the Solar System’s formation, one thing is certain…we’re going to be feeling the after-effects of this study for a while.

Well, at least, all of us space nerds will be.

Sources:

https://www.space.com/jupiter-ate-baby-planets-while-growing

https://iopscience.iop.org/article/10.3847/1538-3881/aa72db https://www.aanda.org/articles/aa/full_html/2022/06/aa43207-22/aa43207-22.html