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Jupiter, Saturn and the Early Solar System

The days when scientists assumed our Solar System would be something of a template for planetary systems elsewhere are long past. The issue now is to delve deeper into system architectures to figure out what happens in their infancy and how they evolve. Working backward from today’s Solar System is one way to approach the problem. Thus Matt Clement (Carnegie Institution for Science), who has led a recent study into the formation of Jupiter and Saturn, hoping to determine how they wound up in their present orbits. Says Clement:

“We now know that there are thousands of planetary systems in our Milky Way galaxy alone. But it turns out that the arrangement of planets in our own Solar System is highly unusual, so we are using models to reverse engineer and replicate its formative processes. This is a bit like trying to figure out what happened in a car crash after the fact–how fast were the cars going, in what directions, and so on.”

Image: New work led by Carnegie’s Matt Clement reveals the likely original locations of Saturn and Jupiter. Credit: Saturn image is courtesy of NASA/JPL-Caltech/Space Science Institute.

As you would imagine, work like this involves numerous computer simulations, some 6,000 of which are described in the paper in Icarus. The Solar System we were born into emerges in these simulations from conditions that were markedly dissimilar in the early days of planet formation, when the larger bodies in the system began to spring gravitational surprises on a tidy early arrangement, with a re-shuffling that gave shape to today’s orbital stability.

Our models for how all this happened are now being tweaked, as exemplified by Clement and team’s work. Whereas Jupiter was once believed to orbit the Sun three times for every two orbits completed by Saturn — this has evolved within the ‘Nice Model’ of system formation — the new computer simulations show that the arrangement of planets we have today is better produced by starting with a ratio of two Jupiter orbits to one Saturn orbit.

As the authors put it, “adequately exciting Jupiter’s eccentricity without exceeding Jupiter and Saturn’s modern orbital spacing is extremely challenging.” The Nice Model (named after the city in France, where it was originally developed at the Observatoire de la Côte d’Azur) considers the migration of the giant planets after the dissipation of the early protoplanetary disk. Clement’s simulations show the effect of the Kuiper Belt on the positions of Uranus and Neptune, with signs of an early ice giant that was ejected during the roiling period of system formation.

Image: Jupiter in its infancy was thought to orbit the Sun three times for every two orbits that Saturn completed. But this arrangement is not able to satisfactorily explain the configuration of the giant planets that we see today. Matt Clement and his co-authors showed that a ratio of two Jupiter orbits to one Saturnian orbit more consistently produced results that look like our familiar planetary architecture. Credit: NASA.

In the Nice Model, the planets emerge from the protoplanetary disk in a compact, resonant configuration, with subsequent perturbations breaking the resonance of one or more planets, creating a phase of dynamical instability that reshapes the outer Solar System. The Nice Model is widely accepted in the field because of its ability to predict the orbital shape of the Kuiper Belt and main asteroid belt, and properties of some gas giant moons.

The Nice Model seems strong, and as the paper points out, its simulations replicate the capture of irregular moons in the outer Solar System and allow the survival of the asteroid belt while explaining the trojan asteroids. As the authors go on to say, “A major potential pitfall of the primordial 2:1 version of the Nice Model presented in this manuscript is its effects on the asteroid belt and terrestrial-forming regions.”

Hence the need to investigate the consequences of the proposed scenario on the inner Solar System. But the paper discusses the problems resolved by assuming an initial 2:1 resonance. The authors acknowledge that some aspects of the system’s present architecture are still low-probability outcomes. A major issue is eccentricity. Were the gas giants actually born in a 2:1 resonance and in orbits that were at origin somewhat eccentric?

That scenario, described as “somewhat of a paradigm shift,” does produce the modern Jupiter-Saturn orbits and is consistent with the position of Uranus and Neptune, with this addition: “…we show that Uranus and Neptune’s final orbits are determined by a combination of the mass in the primordial Kuiper belt and that of an ejected ice giant.” Adds Clement:

“This indicates that while our Solar System is a bit of an oddball, it wasn’t always the case. What’s more, now that we’ve established the effectiveness of this model, we can use it to help us look at the formation of the terrestrial planets, including our own, and to perhaps inform our ability to look for similar systems elsewhere that could have the potential to host life.”

As we tighten the Nice Model further, the paper describes the research path ahead:

Though our work shows that the primordial 2:1 Jupiter–Saturn resonance is a viable evolutionary path for the solar system, future work is still required to fully validate our presumed initial conditions, and robustly analyze the consequences of such a scenario on the solar system’s fragile populations of small bodies. In particular, follow-on investigation of the giant planets’ instability evolution with Jupiter and Saturn in a primordial 2:1 MMR with enhanced eccentricities must consider longer integration times (≳100 Myr), higher resolution disks (≳10,000 particles), and account for the dissipating gaseous nebula.

Clement presented these results at the American Astronomical Society’s Division for Planetary Sciences virtual meeting, which ended on the 30th of October.

The paper is Clement et al., “Born eccentric: Constraints on Jupiter and Saturn’s pre-instability orbits,” Icarus Vol. 355 (February 2021), 114122 (abstract).


Comments on this entry are closed.

  • James franklin November 3, 2020, 8:28

    I have read many of these “predictions” over the years, and so far I am unconvinced by all of these hypothesies, but respect the work of the researchers involved.
    There is more we do not know about planet formation than we think we do, some 40 years ago I had a discussion with Patrick Moore in which I suggested that planets may form before the central star, Pat actually poured cold water on this idea from a 13 year old but it would appear that recent research is pointing the way that the precocious teen was correct – at least in part.

    It is likely there are many paths taken for planets to form inside acretion disks, the fact that the first exoplanets ever confirmed were actually orbiting a Pulsar is evidence that nature will always find a way.

    The issue with out solar system is that people are looking to solve a problem that may not exist, people are finding hot Jupiter’s, Neptunes etc and using this as a reason to state the Solar System has reformed to it’s current layout, and they could potentially be right, but nature abhores the complex, it generally takes the easy route and my thinking is that the solar system formed, ostensibly, as it is, the planets having not really moved that much, although it is likely there were multpile collisions in the early part of the solar system as planets and proto-planets merged in titanic encounters.

    Were bodies ejected from the system, highly problable, with others captured to become Moons, such as Titan and certainly Triton, I would suggest the Jovian moons were formed in situe as Jupter is a mini solar system all by itself, we will likely find this common as we are able to detect Moons around other gas giants in other systems.

    Clearly collisions took place, Uranus is evidence that something catastrophic happened in the outer solar system, just as Venus and Mercury point to dramatic events in the inner solar system.

    We now know the Moon orbits inside Earth’s exosphere, the outermost and most tenuous part of the atmsophere that extends up to 400,000 miles from the surface, this begs the question as to whether the panet’s atmopsphere was once a lot denser than it currently is and that it thinned by a combination of solar activety as the Sun became true star and then grew in activity as it developed, we know it is around 30% more luminous than 4 billion years ago, and other processes, the Moon may have acted to thin Earth’s atmosphere as well as chemical processes.

    We can all make assumptions, but we simply need more data, extrapolating from a few is not more helpful that extrapolating from the one, hopefully, the JWST will start to give us more data that will allow researchers to star to nail down predictions and models to give us a better understanding. If we can better staudy protoplanetary discs around young stars, then compare these to dics around similar stars a few hundred millions years old and then to older stars, we should then be able to build up an evolutionary picture that will give us greater information.

    JWST has the ability to give data on proto-stellar objects, things that are not yet a star of any description, and does not seem to be studied to much at the moent due to technical issues, but once we can study planetary systems from “cradle to the grave”, so to speak, then models as discussed can give us much clearer insights.

    Until then, we need to take these models with a pinch of salt.

    Paul – Not getting emails again!!

    • Paul Gilster November 3, 2020, 9:01

      Sorry to hear about the emails. I have seen no sign of trouble at this end. If others are suddenly having a problem, please let me know.

  • Thomas Mazanec November 3, 2020, 14:32

    Could the ejected gas giant have become the putative Planet Nine?

  • Geoffrey Hillend November 3, 2020, 16:12

    The end of the exosphere is 6200 miles. A 2:1 Jupiter Saturn resonance is a lot considering it’s now almost a 3:1 resonance. The idea that Jupiter migrated inwards outwards is dubious to me considering that the Sun is loosing mass due to the thermonuclear fusion of hydrogen into helium and solar wind, CME etc. which would cause less gravity and an outward migration. How does the nice theory explain Saturn which would have only Jupiter migrating inwards, but not Saturn or maybe Saturn migrated inwards more slowly in order to explain the supposed original a 2:1 ratio?

    There is also momentum transfer of the effects of the Sun’s gravitational field on the planets including the Earth which causes the planets to move further away from the Sun.

    • James Franklin November 4, 2020, 9:47

      Your information on the exosphere is out of date, research has confirmed it extends out to well beyond the Moon’s orbit.


    • Alex Tolley November 4, 2020, 12:38

      The charts in the paper seem to indicate that Jupiter maintains its orbit and it is Saturn that migrates outwards.

      As the sun loses mass, shouldn’t all the planets be very slowly migrating outwards? Does this create new unstable resonances?

      • Bruce D Mayfield November 5, 2020, 19:21

        Yes, all the planets do VERY, very slowly migrate outward in response to the Sun’s mass loss, but they all do so in complete unison such that the orbital period ratios remain unchanged. The extremely slow rate of orbital expansion is due to how tiny the Sun’s mass loss is when compared to its enormous total mass at all points during its main sequence lifetime.

  • Geoffrey Hillend November 3, 2020, 16:17

    The exosphere extends up to 120,000 miles or halfway to the Moon, but any particles further away from that would have escape velocity or not remain around the Earth.

  • ljk November 3, 2020, 17:42

    Nov. 2, 2020

    NASA Contacts Voyager 2 Using Upgraded Deep Space Network Dish

    The only radio antenna that can command the 43-year-old spacecraft has been offline since March as it gets new hardware, but work is on track to wrap up in February.

    On Oct. 29, mission operators sent a series of commands to NASA’s Voyager 2 spacecraft for the first time since mid-March. The spacecraft has been flying solo while the 70-meter-wide (230-foot-wide) radio antenna used to talk to it has been offline for repairs and upgrades. Voyager 2 returned a signal confirming it had received the “call” and executed the commands without issue.

    The call to Voyager 2 was a test of new hardware recently installed on Deep Space Station 43, the only dish in the world that can send commands to Voyager 2. Located in Canberra, Australia, it is part of NASA’s Deep Space Network (DSN), a collection of radio antennas around the world used primarily to communicate with spacecraft operating beyond the Moon.

    Since the dish went offline, mission operators have been able to receive health updates and science data from Voyager 2, but they haven’t been able to send commands to the far-flung probe, which has traveled billions of miles from Earth since its 1977 launch.

    Among the upgrades to DSS43, as the dish is known, are two new radio transmitters. One of them, which is used to talk with Voyager 2, hasn’t been replaced in over 47 years. Engineers have also upgraded heating and cooling equipment, power supply equipment, and other electronics needed to run the new transmitters.

    The successful call to Voyager 2 is just one indication that the dish will be back online in February 2021.

    “What makes this task unique is that we’re doing work at all levels of the antenna, from the pedestal at ground level all the way up to the feedcones at the center of the dish that extend above the rim,” said Brad Arnold, the DSN project manager at NASA’s Jet Propulsion Lab in Southern California. “This test communication with Voyager 2 definitely tells us that things are on track with the work we’re doing.”

    Full article here:


  • EricSECT November 3, 2020, 21:05

    The Grand Tack hypothesis seems to make sense and explain a lot about the uniqueness of our Sol system. Our under mass Mars, our weird asteroid Belt, our low mass terrestrial planets which include NO Super Earths. And them Hot Jupiter systems? They musta ran a different course then we did.

    Sorry, but one thing I am still hung up on?

    Jupiter, the first formed, starts to spiral inward ….but newly formed Saturn then coalesces…. and then the both giants entered a 2:1 resonance, and that resonance somehow moves both gas giants OUT several AU. HOW does that occur, physically? That is a LOT of energy to dissipate. Somehow! Where did that energy go?

    And then the mutual embrace for some reason…. stops cold, Jupiter stays where it is, Saturn may or may not! ….kick out some nameless ice giant …..but otherwise stays put where it is now. I guess Uranus and Neptune maybe swap orbits. Whatever!

    This random transfer of angular momentum betwixt our 2 gas giants mainly and MAYBE betwixt our 2 (or 3….) ice giants…. just seems to defy physical sense.

  • AlexTru November 4, 2020, 3:06

    it turns out that the arrangement of planets in our own Solar System is highly unusual

    As well, as I know , our present astronomical instruments has obvious limitations, so total most of detected planets are orbiting Red dwarfs, even in the case of star with detected planets in most cases there is possibility that there are undetected planets. So I suppose the quote from article – is pure speculation, because modern science absolutely do not have any data about planetary systems around sun like G dwarfs (excluding our own).
    The statement that structure of our planetary system is unusual , do not have any scientific support meanwhile.
    Can accept discussed hypothesis as white noise…

    • ljk November 4, 2020, 12:36

      AlexTru said on November 4, 2020, 3:06:

      “So I suppose the quote from article – is pure speculation, because modern science absolutely do not have any data about planetary systems around sun like G dwarfs (excluding our own).”

      Modern science would highly disagree with you on that comment…


      and this…


      “Some of the nearest G-type stars known to have planets include the Sun, 61 Virginis, HD 102365, HD 147513, 47 Ursae Majoris, Mu Arae, and Tau Ceti.”

      • AlexTru November 4, 2020, 14:39

        Dear ljk,
        Do not understand why you the link to list of known nearby G-dwarfs… it was not issue that I am talking about, sorry.
        I am afraid you again understand me wrongly.
        I do not write about existence of G-type stars :-) It will be very funny , if I could deny this obvious fact.
        But I am taking about discovered planetary system that known G dwarfs own, our current instruments simply does not have required resolution. Meanwhile, astronomical resolution allows only to find Super Jupiter candidates that potentially rotating around G-dwarfs. When we are talking about smaller worlds like Earth of Super Earth size – it is out of our astronomy ability present moment.

        • ljk November 5, 2020, 11:27

          Just for the record, you said “planetary systems” not Earth-size or Earth-type worlds. I was just going on what you said here, not what you were apparently implying.

          And are you sure we have not found any such worlds circling G class suns?

          • AlexTru November 5, 2020, 16:52

            Yes, I am talking about planetary systems, that means knowledge about ALL planets huge and Earth sized , that are potentially rotating around the distant G stars. We detected only few Hot Super Jupiters – that was possible to detect near G-dwarfs by our modern astronomical instruments, current astronomical resolution allows to detect only planet’s huge mass and fast period.
            Author of discussed article try to compare our solar system with not detected yet worlds :-) when in reality Earth’s astronomers do not have enough data to compare and postulate what planetary structure is usual and what is extraordinary .
            By the way we even do not know exact composition of our own system… still looking for a planet 9 etc…

  • Geoffrey Hillend November 4, 2020, 16:14

    I stand corrected on the exosphere’s end, and I’ve never heard that before, It is interesting that according to that paper the Earth’s exospheric hydrogen is loose atoms with a air density hardly more than empty space so it makes sense to me that hydrogen atoms might get caught there. The evidence is still arguable.

  • ljk November 11, 2020, 12:05

    Earth’s oldest asteroid strike linked to ‘big thaw’

    Scientists have discovered Earth’s oldest asteroid strike occurred at Yarrabubba, in outback Western Australia, and coincided with the end of a global deep freeze known as a Snowball Earth.

    Full article here:


    Have to wonder if any of these ancient space rocks were knocked around by Jupiter or Saturn first?

  • ljk January 9, 2021, 16:20

    The Nuclear-Powered Aircraft That We’ll Use to Explore Jupiter

    This engine will let us stay there for months, if not years.


    JAN 8, 2021


    The paper online here:


    Those who remember the Daedalus interstellar fusion probe concept by the BIS in the 1970s may recall the idea of aerostats in the atmosphere of Jupiter mining deuterium. Well now there is a concept vehicle for getting around that planet’s atmosphere presuming future missions to the stars might need to utilize Jupiter in that manner. Or just for exploring the gas giant itself, as it were.

  • ljk January 26, 2021, 22:59

    Jan. 26, 2021

    Unique Solar System Views from NASA Sun-Studying Missions

    Illustration showing Solar Orbiter’s perspective on the planets as it orbits the Sun

    This computer-generated view shows the perspective of the Solar Orbiter spacecraft on Nov. 18, 2020, illustrating why Solar Orbiter’s view shows — from left to right — Venus, Earth, and Mars, with Mercury and the Sun off camera to the right.
    Credits: ESA

    Though they focus on the star at the center of our solar system, three of NASA’s Sun-watching spacecraft have captured unique views of the planets throughout the last several months. Using instruments that look not at the Sun itself, but at the constant outflow of solar material from the Sun, the missions — ESA and NASA’s Solar Orbiter, NASA’s Parker Solar Probe, and NASA’s Solar and Terrestrial Relations Observatory — have sent home images from their distinct vantage points across the inner solar system.

    All three missions carry instruments to study the Sun and its influence on space, including cameras that look out the sides of the spacecraft to study the Sun’s outer atmosphere, the solar wind, and the dust in the inner solar system. It’s these instruments that, at various points in 2020, saw several planets pass through their fields of view.

    Each of the three missions has a distinct orbit, so their perspectives are different from both ours here on Earth and from each other. This is reflected in each spacecraft’s view of the planets, which show the bodies in different positions than what would have been seen from Earth and from the other spacecraft on those dates.

    Full article and beautiful images and videos here: