Astronomy Without A Telescope – Our Unlikely Solar System


Recent modeling of Sun-like stars with planetary systems, found that a system with four rocky planets and four gas giants in stable orbits – and only a sparsely populated outer belt of planetesimals – has only a 15 to 25% likelihood of developing. While you might be skeptical about the validity of a model that puts our best known planetary system in the unlikely basket, there may be some truth in this finding.

This modeling has been informed by the current database of known exoplanets and otherwise based on some prima facie reasonable assumptions. Firstly, it is assumed that gas giants are unable to form within the frost line of a system – a line beyond which hydrogen compounds, like water, methane and ammonia would exist as ice. For our Solar System, this line is about 2.7 astronomical units from the Sun – which is roughly in the middle of the asteroid belt.

Gas giants are thought to only be able to form this far out as their formation requires a large volume of solid material (in the form of ices) which then become the cores of the gas giants. While there may be just as much rocky material like iron, nickel and silicon outside the frost line, these materials are not abundant enough to play a significant role in forming giant planets and any planetesimals they may form are either gobbled up by the giants or flung out of orbit.

However, within the frost line, rocky materials are the dominant basis for planet forming – since most light gas is blown out of the region by force of the stellar wind and other light compounds (such as H2O and CO2) are only sustained by accretion within forming planetesimals of heavier materials (such as iron, nickel and silicates). Appreciably-sized rocky planets would probably form in these regions within 10-100 million years after the star’s birth.

So, perhaps a little parochially, it is assumed that you start with a system of three regions – an inner terrestrial planet forming region, a gas giant forming region and an outer region of unbound planetesimals, where the star’s gravity is not sufficient to draw material in to engage in further accretion.

From this base, Raymond et al ran a set of 152 variations, from which a number of broad rules emerged. Firstly, it seems that the likelihood of sustaining terrestrial inner planets is very dependent on the stability of the gas giants’ orbits. Frequently, gravitational perturbations amongst the gas giants results in them adopting more eccentric elliptical orbits which then clears out all the terrestrial planets – or sends them crashing into the star. Only 40% of systems retained more than one terrestrial planet, 20% had just one and 40% had lost them all.

The Moon has retained a comprehensive record of the Late Heavy Bombardment from 4.1 to 3.8 billion years ago - resulting from a reconfiguration of the gas giants. As well as clearing out much of debris disk of the early Solar System, this reconfiguration flung material into the inner solar system to bombard the rocky planets.

Debris disks of hot and cold dust were found to be common phenomena in matured systems which did retain terrestrial planets. In all systems, primal dust is largely cleared out within the first few hundred million years – by radiation or by planets. But, where terrestrial planets are retained, there is a replenishment of this dust – presumably via collisional grinding of rocky planetesimals.

This finding is reflected in the paper’s title Debris disks as signposts of terrestrial planet formation. If this modeling work is an accurate reflection of reality, then debris disks are common in systems with stable gas giants – and hence persisting terrestrial planets – but are absent from systems with highly eccentric gas giant orbits, where the terrestrial planets have been cleared out.

Nonetheless, the Solar System appears as unusual in this schema. It is proposed that perturbations within our gas giants’ orbits, leading to the Late Heavy Bombardment, were indeed late with respect to how other systems usually behave. This has left us with an unusually high number of terrestrial planets which had formed before the gas giant reconfiguration began. And the lateness of the event, after all the collisions which built the terrestrial planets were finished, cleared out most of the debris disk that might have been there – apart from that faint hint of Zodiacal light that you might notice in a dark sky after sunset or before dawn.

Further reading: Raymond et al Debris disks as signposts of terrestrial planet formation.

16 Replies to “Astronomy Without A Telescope – Our Unlikely Solar System”

  1. This is in line with the chaotic dynamics of solar system stability I did over 10 years ago, and reworked again 5 years ago. This was in association with my calculations on the drift of the Earth’s orbital radius due to the perturbation of Jupiter. There is a chaotic dynamics to this codified by the Lyapunov exponent. I ran some analyses of putative 1AU planets around known stars with jovian planets. The results were largely stunning, where the exponent averaged about 4 times that for Earth. This means the drift would be about 50 times that of the Earth. Most solar systems would not hold a planet such as Earth in a stable orbit. I reworked this for my book in 2006, with more extra solar system data available, and the results on average were about the same. I did some Bayesian statistics and found that out of the approximately 10,000 solar systems that might be similar to ours in the entire galaxy with 3 billion G-class stars.


  2. Though I am not an expert my any measure, a few lingering questions come to mind that are unexplained.

    Has the data taken to account the limitations in our observational capabilities? As far as I understand it, detection of planetary bodies Super-Earth sized and smaller pushes current resolving limits. Would this difficulty not be compounded by the detection and gravitational influence of companion gas giants? It stands to reason that systems with known existing giants would provide more of a challenge when attempting to resolve terrestrial sized worlds. Correct me if I am wrong on this. Also, gas giants that are further out from their host stars would become less gravitationally influential. Consequently, they become difficult to detect and/or we must rely on planetary transits.

    Secondly, have changes in the frost line radius over time been taken into account? I would image this line varies a bit in the early stages of a star’s life before the star settles down, leaving the frost line to a more stable gradual expansion as the star heats up.

    Thirdly, what about the variability in planetary orbits? Previous articles have discussed the possibility that our gas giants did not form where they presently are, and LC has mentioned that planets gradually drift outward with time.

    Lots of questions tonight.


    1. The orbit of Earth drifts outwards very slowly due to the gravitational perturbation of Jupiter. In fact Jupiter moves inwards a very tiny amount. The orbits of planets do rattle each other, with gas giants being the dominant gravitational players.

      There is probably an observational bias involved. Clearly close in or “torch” jovian planets are more readily observed by either Doppler methods.


      1. Thanks for this. I look forward to further planetary modelling. I’m sure there will be plenty of surprises as more observational data comes in.

    2. have changes in the frost line radius over time been taken into account?

      I was going to make a more considered comment on this, but my references turned out to be in transit so here goes from memory:

      Good catch, I think. There is a considerable uncertainty involved here as I have come to understand it, even if I don’t know how the discussion trends. At least in the early protoplanetary nebula you can, IIRC, find work questioning the relevance of it in some ways. Massive photo-dissociation can possibly create a self-protecting shield, at least for water, quite a ways from the first-order theoretical snow line (that comet behavior test, I assume).

      [It is one of those “get out of jail” effects, like when you read about atmosphere escape and then comes to hydrogen atmosphere massive outflow in effect short-circuiting the usual mechanisms and you have to go “WTF came _that_ from?”]

      It may not affect planetary formation as such, but water content and early chemistry of nebula and its forming worlds. I get the feeling the snow line behavior is one of the assumptions that can be questioned, if not models start to converge on observations sooner or later.

  3. “Firstly, it is assumed that gas giants are unable to form within the frost line of a system – a line beyond which hydrogen compounds, like water, methane and ammonia would exist as ice.”

    This statement makes me wonder how this ‘frost line’ might have migrated over time? When nearest Earth, do we experience an Ice Age? Great floods? Or, in what other energy realms such as the cryogenic or plasmagenic(?), might similar condensation ‘frost lines’ occur? A weather prediction for Mercury might be? Hot and sunny with a high of 875 degrees F There will also be a slight chance of chromium frost and associated ion storm near the solar leading terminator this evening.

    1. The frost line is depending how much energy the sun outputs and has nothing to do with Ice age or so.

      1. Broadly agree with Olaf – although you could say that the frost line is very slowly creeping outwards as the Sun’s luminosity increases due to stellar aging.

        Periods of intense glaciation are more correlated with perturbations in the Earth’s orbit – see for example here:

        … although, over the really long term, genuine ice ages are more about continental drift – see for example here:

  4. This is an interesting subject to come to grips with now, at least for a layman like me, when Kepler data starts to put new questions and hopefully constraints on old models.

    As for earlier commenters, the state of flux in the topic raises some points.

    – That planetary systems would be individual samples out of distributions is a given. And if we look at enough variables we will increase the likelihood that those individuals are outliers in some parameters.

    But I think the earlier problems to recreate something like the solar system pointed to spread.
    And the early Kepler data confirms that while systems with planetary and putatively orbit sizes like ours are to be expected the systems are weird.

    Sometimes they are tightly packed so you can’t plunk another planet in between without disturbances, such as ours.

    Sometimes they are tighter packed so they are massively disturbing each other, yet persist for Gy! (Kepler 11 is ~ 2 times older than Sun; thats ~ 10 Gy already there.)

    In fact, I’m surprised this paper derives so many solar system analogs.

    – This proposed result is, as the paper notes, dependent on the model, which I take it is “classical”.

    And even within that model they assume planetary distributions that are not predicted by the naive Kepler data, for example one or more Jupiter sized planets, that terrestrials drop after some 1/10’s of AU et cetera.

    But the initial Kepler results is under criticism from other groups, that derives slightly different results from the same raw data.

    – I’m interested in habitability at large (as I suspect many others may be), and uniqueness doesn’t affect that. [/shrugs] Number of habitable planets and stability would.

    In fact, if as the paper describes LHB “punctual event” as fairly finetuned: “strong enough to remove most of the outer planetesimal disk and give the inner Solar System a small kick but did not destabilize the inner Solar System or impart a large eccentricity to Jupiter. … [LHB instability] is much weaker than the instabilities inferred from the exoplanet eccentricity distribution,”

    So most systems doesn’t have to endure LHB. Yet it seems as if life existed at the time it could have survived, according to several results. We were unlucky, but beat the “system”.

    – This result may be expensive for TPF and other missions, if zodiacal light scatter is expected to be much more extensive in most systems. [I see now it is mentioned in the paper; I’m browsing as I write and post. Apparently they will try to quantify this later, good!]

    1. Come to think of it, LHB may go both ways. Systems without an early clearing would have to endure a more intense low grade bombardment during later times than we, right? When you start to get to eventual multicellulars, the (again!) exceptional hit the Chixculub impact made in biogenic massive calcium and sulfurous mineral deposits (IIRC) would be a more frequent problem for animal & plant types than for bacteria and fungi type life. They would be tough bastards.

      Of course, the worst beating the biosphere has to survive during its history is the conversion from Gy of environmental pollution from a benign atmosphere to the outrageously reactive and poisonous stuff that we call “an oxygenated atmosphere”! A few rocks here and there is no problem in comparison.

  5. Another interesting model, but it is just a model. I would say that in general not enough is known about the planet formation process to make many “reasonable assumptions.” Studies of existing proto-planetary discs by Spitzer I think in time will help clarify the planet formation mess quicker than looking at existing systems since the problem is that we don’t have a large enough sample size and we are potentially missing a lot of small planets since the equipment is not sensitive enough. Then not unexpectedly out comes a new theory explaining how the apparent lack of small terrestrial planets means in fact that they are not even there to be seen.
    I think that the Kepler data was enlightening in that it showed just how woefully incomplete the standard models were. But this was already evident from the exoplanet discoveries made to that point. There are alot more variable end results than was thought possible and occuring to a far greater degree of frequency than expected. This model does not even take into account the strong posibility that some gas giants may be forming by gravatational instability which would not be dependent upon the frost line. Such gas giants would form much earlier than any via core accretion and potentially have a dramatic impact on how subsequent planet formation turned out in such a system. I also have a strong suspicion that the type of star has a large influence on planet formation.
    I think in the end we will find that there are aso many possible factors effecting planet formation that the end results will turn out to quite simply anything is possbile. Nevertheless it is important to determine what the general rules are and how they play out by working such models and by that get some idea what the most likely outcomes are. This kind of modelling work is helpful in that regard, but as always I would take it with a grain of salt or two. Personally using the word assume/assumption is a pet peeve of mine.

    1. Camelot! It’s only a model…

      Models are of interest in so far as they fit existing data and also generate predictions – and as soon as a model’s predictions fail to deliver, the model is quickly forgotten.

      I think this is a nice example of scientific method in action. If the ‘debris disk indicates terrestrial planet likelihood’ idea holds out, these folks can feel smug. If it doesn’t, I guess they will move onto something else.

      Science is necessarily full of assumptions – and I am not sure we could get anywhere without a bit of that.

      I agree the exoplanet database isn’t sufficiently representative for anyone to start making definitive calls about ‘general rules’. We are still in ‘guesstimate’ territory.

  6. “Astronomy without a telescope – our unlikely solar system”
    Not liking the headline Steve. It is quite misleading for some people.

    Models have there place, but the real discoveries in astronomy are being made with telescopes. These discoveries, whether made with Kepler, Swift, Hubble, or by Mike Brown or Anthony Wesley (with his home made reflector!), or other astronomers, help constrain the models. Models can nail down theory (just look at the NASA web-site today regarding a finding re: GRBs.) But we wouldn’t even know about the existence of gamma-ray bursts without telescopes (and orbiting satellites). Gotta have eyes on the universe first, and computers to help us sort out all the data.

  7. Wow. All this ultimately assumes is the general composition of the starting point from the original nebula from which the sun once resided, and the process in which the debris disk was formed, then, in turn, how the planets formed.
    Surely the limitation on rocky planets or gas giants proportions relies on the composition of the original nebula. It depends also on the initial star formation rate (SFR), the cause of the nebula collapse (spiral density wave, supernovae, HII region expansion, nebula size etc. Hell even the magnetic fields!) We already know of the molecular composition differs wildly from nebula to nebula, so the initial parameters of the original solar nebula would likely have some indication of the planetary outcome.
    There are an awful lot of “what ifs” to even get to these Raymond et. al. scenarios.

    Look at the comments here it is interesting no one has hit on the variances between masses of the stars, let alone the range of ten million to eight-odd billion year yellow solar massed stars. Surely correlation of the age of the star with its planetary system will be crucial. Lawrence was right on the money when he said “Most solar systems would not hold a planet such as Earth in a stable orbit.” Why? The general evolution of planetary systems is overruled by chaos theory and perturbations, whose initial parameters are predictable but the system soon collapse into unpredictability at some point. The larger the massive body in the solar system, and its placement from the sun will very likely dictate the placement of the smaller bodies throughout time. Simulations over the years have often shown the presumed stable orbits of little planets (or bodies) in the long term are tossed unceremoniously out of the system altogether, or in some circumstances hurl them directly into their suns. (Another important variant is planets orbiting either on the same plane, or with high inclinations and or several planets together with varying inclinations. Each variant has a different consequence.)
    We also see the same kind of things with comet and asteroids in our solar system. to a lesser degree. We also see some of these similar consequences of such interactions with components in multiple stars.
    Considering the number of initial variables before reaching the creation of debris disk, the 10%-15% forming planets, then followed by the chaos and perturbation effects this study finding a combined 60% probability that one or several inner planets is extraordinarily high survival rate! We can only conclude that once the planets are formed, one or a few are likely to survive. What those solar systems finally look like will range over all different kinds of combinations. In the end, I think planetary systems might prove most variable than we think, and the Earth ends up being a very very lucky survivor of the game of celestial billiards.

    Thanks for this story, Steve!

    Note: IMO, I cannot wait to they find the unlikely signature of two planets in the same orbit. They would not last long, though it is theoretically possible
    (Once the old crazy of Immanuel Velikovsky will then be salivating in his grave, with “I told you so!”)

    1. Corrections:
      1) Looking at the comments here it is interesting no one has hit on the variances between masses of the stars,

      2) In the end, I think planetary systems might prove more variable than we think, and the Earth ends up being a very very lucky survivor of the game of celestial billiards. [Pool, for you dear Americans.]

  8. Hey, guys, new person here! I actually read the abstract and some of the paper itself. It actually dovetails with his former research and simulations rather well. I’ve been following them for some time. The simulation depended on initial conditions as follows: an inner disk of planetesimals and planetary embryos, three giant planets at Jupiter-Saturn distances, and a massive outer planetesimal disk. Basically his conclusions this time were the same as before in his research, at least from what I’ve seen in the abstract. That is, that eccentric gas giants typically clear out all the dust, but also fling terrestrials into the void. Circular ones are better for terrestrials, but typically don’t clear the dust to the degree that ours has been cleared. Therefore, we’re “rare” in that sense. We’re also “rare” in the sense that our gas giants didn’t fling us out! Like the man said, in most systems only one or 2 will be left, even if one of the two is in the habitable zone! Check out Raymond’s other research papers…systems with no gas giants at all tend to grow tons of Earths, Super-Earths, and Neptunes all the way to the snow line. So, either you’re part of the 1% of systems that look a lot like ours (right down to the happy little terrestrials and big, friendly Cold Jupiters), or you’re part of the 99% that went wacky (from our point of view of course…)

    What I really want to see is whether or not the Kuiper belts are denser around other stars that have them than they are around ours. I heard somewhere (maybe someone can back me up on this) that KBOs are far less dense than was predicted all those decades ago. Maybe that research will correlate with this well. When the “billiards” started up and shook them all loose, in that whole population came!

    I think whether Earth is “rare” or not depends entirely upon whether or not you have a hyperdrive. That’s the thing about true diversity in the universe…it means that everyone is a minority!

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