The Sight of Jupiter and Saturn Together Is a Beautiful Thing

The Sight of Jupiter and Saturn Together Is a Beautiful Thing

A combination is a good time to reflect on our cosmic heritage, and on the long shadows cast by the gas giants. Illustration by SN VFX/ Shutterstock

T oday, December 21, Jupiter and Saturn will appear incredibly near each other in the sky, less than a tenth of a degree apart, about one fifth the width of the full Moon. 1 “Conjunctions” like this happen all the time but, for us, here in the world, it’s special because– though the planets will still be more than a half-billion kilometers apart– from our point of view, the worlds might almost touch. 2

A combination is a great time to review our cosmic heritage, and on the long shadows cast by the gas giants. After all, Jupiter and Saturn affected Earth’s development and advancement. Evidence suggests there were at least three occasions on which they (Jupiter specifically) chose Earth’s evolutionary course, some 4.5 billion years back, assisting to shape the planet we know and love today.

The wee baby Earth was just sitting there growing
Some rocky things flying by, crashing and glowing.
However far from the Sun, way out in the cold
The giants were stirring. A force to witness! 3

Planets grow within swirling disks of gas and dust around young stars. We have pictures of these disks taken with our most significant telescopes, specifically the giant radio telescope ALMA. Here is one of ALMA’s classics:

ALMA picture of the planet-forming disk around the close-by star TW Hydrae. The disk as a whole is comparable in size to the planetary system, and the inner hole in the inset has to do with the size of Earth’s orbit. S. Andrews (Harvard-Smithsonian CfA), ALMA (ESO/ NAOJ/ NRAO)

Jupiter starved the growing Earth. This may sound like a bad thing, but without Jupiter, Earth would probably have become an enormous “super-Earth” and moved inward, onto a roasting-hot orbit close to the Sun. Not terrific for life.

Planets outgrow dust. It drifts around in a disk, then accumulate to form 100 kilometer-scale “planetesimals,” the real building blocks of the planets. Think of these like the potato-shaped asteroids from Star Wars— just bigger, farther apart, and without the giant space slugs. The rings in the ALMA image are pile-ups of sand-sized dust, where these objects might be forming today.

Planetesimals grow further by slamming into each other, and also by sweeping up dust drifting inward. When a growing planetesimal gets enormous enough, it blocks the drifting dust, not just starving itself however also obstructing the dust from getting to any planetesimal closer to the Sun. These planetesimals can keep growing by self-collisions but not by any brand-new dust originating in the outer planetary system.

Jupiter’s growing core– a super-big planetesimal about 10 times the mass of Earth– may have done precisely that. It grew huge enough, fast enough, to stop the flow of dust. Scientists do not know precisely why Jupiter’s core formed so much faster than Earth’s, but the factor likely has something to do with the fact that Jupiter started forming further from the Sun, where (sticky) ices might exist and add to its development.

Meteorites corroborate this concept. The thousands of meteorites in NASA collections can be divided into 2 main classes, based upon their isotopes– the relative quantities of the very same components with various varieties of neutrons. These meteorites are constructed out of dust that’s just the best size to drift truly fast within the disk. Yet the two kinds of meteorites didn’t blend. Why not? Jupiter’s dust-blocking core.

Our system was nicely just sliced up right in 2
As Jupiter’s core stopped the dust flowing through.
If not for this, Earth would have simply kept growing
And wound up so close to the Sun we ‘d be glowing!

Jupiter and Saturn likewise stunted Mars’ growth. The Red World gets a great deal of press, however it’s a runt. Mars is nine times less huge than its closest next-door neighbor, Earth. Decades of computer simulations (starting in the late 1970 s) have actually shown that, without an outdoors perturbation, Mars would have grown to be about as huge as Earth– even with Jupiter obstructing the circulation of dust into the inner planetary system.

The real inner solar system (top), and what the inner solar system would appear like today if not for a strong outdoors perturbation throughout the planets’ development (bottom; according to computer system simulations).

So why isn’t Mars larger? There are a couple of various ideas out there. One circumstance– called the Grand Tack— has Jupiter’s orbit moving, or “moving,” as it grew. On its own, Jupiter would have migrated inward and probably ended up someplace near to Earth’s orbit, which is where the majority of the Jupiter-mass worlds around other stars are found.

This is where Saturn is available in. Jupiter and Saturn together move outside rather of inward. This is a cool hydrodynamical result that takes place when two gas giants are close to each other in the same disk, and the inner one is more massive than the external one. The circulation of gas changes and produces a pileup interior to the inner world’s orbit, which acts to press both worlds external. Like two buddies, they have the wherewithal to go against the flow and move outside together, whereas each would migrate inward by itself.

A hydrodynamical simulation of Jupiter and Saturn (circled) migrating in the sun’s disk. Arnaud Pierens

Jupiter and Saturn’s migration cleared out Mars’ feeding zone, stunting its growth. As long as their migration turned around (” added” in cruising jargon) early enough, the gas giants would not have disrupted Earth’s development.

Like dinosaurs, Saturn and Jupiter wandered
They shaped this system that human beings call home
Their orbits initially diminished, then moved and grew.
Our Earth was built from rocks they raked through.

We may be observing something comparable in another system almost 400 light-years away. The disk around the star PDS70 seems shaped by 2 huge baby planets, secured an orbital resonance, and moving around right now!

Artist’s impression of the exoplanets PDS70 b and c as they are moving around within their birth disk. J. Olmsted (StSci)

The gas giants eventually became unstable, showering all of the other, more inner worlds with roaming rocks and ice. There are numerous pieces of inconclusive evidence leading researchers to think that our planetary system suffered a dynamical instability, an occasion during which the huge worlds’ orbits changed in unexpected, significant fashion. The huge planets’ orbits are not circles, but modestly stretched out ellipses. The Kuiper belt is nearly empty, yet Pluto and Eris, another dwarf world (found in 2005), needed a lot more mass to grow as big as they are. The giant worlds are surrounded by clouds of “irregular satellites” that appear to have been recorded from somewhere else. The orbits of Jupiter’s Trojan asteroids are oddly tilted. The list goes on.

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The Nice design, a situation of how our planetary system dynamically evolved, developed in the French city of the same name, proposes that the giant planets’ orbits went unstable hundreds of millions of years after the planets formed (more on that later). The instability may looked something like this:

Snapshots of the giant planet’s instability (from a computer system simulation). The colored lines represent the orbits of the giant planets, and the white dots are leftover planetesimals. Gomes et al. 2005/ Wikicommons

During the instability, the ice giants, Uranus and Neptune, were kicked all over the place. The external disk of planetesimals– essentially, a substantial stack of remaining comets– was practically totally cleaned out. Those leftovers rained down on the planets and a few were captured (like the irregular satellites).

And in the future there was a big instability
This time the giants expressed their hostility
The ice giants’ orbits were highly affected
An extra one might have been completely ejected!
( The Earth was bombarded; no world secured.)

There is a brand-new twist in this story associated to the timing of these events. As you know, the Moon is covered in craters. Since the 1970 s, the ages of craters were thought to reveal a huge spike in the rate of impacts hundreds of millions of years after the planets formed (remember, that was still billions of years ago). This was called the “late heavy bombardment.”

Brand-new analysis shows that this spike is probably just the tail end of an early bombardment, the one that grew the planets in the first place. Not a “late” heavy bombardment. This doesn’t call the past instability into concern, since it can explain a lot of features of the planetary system at the same time. But it should have taken place much previously, possibly throughout the growth of the rocky worlds. There’s a cherry on top: New simulations show that an early giant-planet instability can even explain why Mars is smaller sized than Earth.

In modeling occasions billions of years back, we are truly only constrained by 2 things: the laws of physics (and offered evidence) and our creativities.

We don’t have a time device, can’t see the past
However designs can answer some questions we have actually asked
To test our ideas, we use simulations
On huge computer systems, with big computations.

Some individuals like to call Jupiter and Saturn the “architects” of the solar system, as though they had a clear plan for how our system should end up. Not us. We consider the gas giants as bullies that pressed Earth around and made a lot of choices for us.

Some even call Jupiter Earth’s “protector.” This is because, over the past 4 billion years, its huge gravity has actually spread comets and asteroids away that may otherwise have actually struck our planet. But on closer inspection, this does not hold up. Jupiter only secures Earth from comets that Jupiter put on Earth-crossing orbits in the first place. It’s like a bully– albeit, quite a lovely one– asking for a handout to safeguard you from his fist.

Sean Raymond is an American astrophysicist operating at the Bordeaux Astrophysical Laboratory in France. He also composes a blog site at the user interface of science and fiction (, and recently released a book of astronomy poems The poems in this post were written by him.

Sebastiaan Krijt is a Speaker in Astrophysics at the University of Exeter in the U.K.

We hope we have actually given you something to think about as you gaze up at Jupiter and Saturn throughout this grand combination. If you’re trying to find more details on how to enjoy the conjunction yourself or wish to find out more about Jupiter and Saturn, have a look at http://jupitersaturn2020 org(including this video we made).


1. For how to see the combination, see here

2. For a history of conjunctions, see here

3. For more astronomy poems by Sean Raymond, see here

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