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The ‘Cold Jupiter’ Factor

Gas giant planets in orbits similar to Jupiter’s are a tough catch for exoplanet hunters. They’re far enough from the star (5 AU in the case of Jupiter) that radial velocity methods are far less sensitive than they would be for star-hugging ‘hot Jupiters.’ A transit search can spot a Jupiter analogue, but the multi-year wait for the proper alignment is obviously problematic.

Still, we’d like to know more, because the gravitational influence of Jupiter may have a crucial role in deflecting asteroids and comets from the inner system, thus protecting terrestrial worlds like ours. If this is the case, then we need to ask whether we just lucked out by having Jupiter where it is, or whether there is some mechanism that makes the presence of a gas giant in a kind of ‘protective’ outer orbit likely when rocky worlds inhabit the inner system.

Enter the computer simulations run by Martin Schlecker at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. Working with scientists at the University of Bern and the University of Arizona, Schlecker has run simulations of 1000 planetary systems evolving around Sun-like stars. In this case, ‘Sun-like’ means solar-type stars with the same mass as our own Sun. The simulated planetary systems are derived through a model known as the Generation III Bern global model of planetary formation and evolution, and the team has used the results to make a statistical comparison with a sample of observed exoplanet systems.

Schlecker calls gas giants in more distant orbits ‘cold Jupiters.’ They’re beyond the snowline, so that water can exist in the form of ice. The paper homes in on ‘super-Earths’ more massive than our own planet but far less massive than gas giants, which are estimated to orbit as many as 50% of stars in class F, G and K. In question is how cold Jupiters affect the formation of these inner super-Earths, and if in fact they make it easier for super-Earths to survive. Some recent studies have found a positive correlation between super-Earths and cold Jupiters. Is it sound?

Image: Artistic impression of a planetary system with two super-Earths and one Jupiter in orbit around a Sun-like star. Simulations show that massive protoplanetary disks in addition to rocky super-Earths with small amounts of ice and gas often form a cold Jupiter in the outer regions of the planetary systems. Credit: MPIA graphics department.

The team’s goal is a “population synthesis of multi-planet systems from initial conditions representative of protoplanetary disks in star forming regions.” The computer model tracks how systems starting from random initial conditions evolve over several billion years as planets coalesce and interact with other objects in the system, with the possibility of collisions or ejections as orbits change due to the gravitational interactions factored in. You can see that with the difficulty in observing actual cold Jupiters in such settings, computer simulations are the way to proceed as the tools of observation are tuned up for future detections.

A hot Jupiter would be likely to disrupt planetary orbits, perhaps preventing the formation of some inner worlds altogether, but a cold Jupiter is far enough from the star that we can expect systems with inner super-Earths to exist, and indeed, some studies suggest that systems with a cold Jupiter often contain a super-Earth. But the simulations of Schlecker’s team indicate that only about a third of cold Jupiters occur in systems accompanied by at least one super-Earth. The correlation of inner super-Earths and cold Jupiters is there, but somewhat weaker than has been found through actual observation.

A possible explanation for the discrepancy between observation and simulation: Gas giant migration involving ‘warm Jupiters’ on intermediate orbits, causing super-Earth collisions or ejections. Here we can look to the parameters of the simulations. If the tendency of the simulated gas giants to migrate is slightly lowered, more inner super-Earths remain in numbers that are compatible with what we have recorded with our telescopes and spectrographs.

Here the limited number of actually observed systems with gas giant and super-Earths becomes an issue. Whereas the study’s simulations find a tendency to produce systems with both a cold Jupiter and at least one dry super-Earth (“with little water or ice, and a thin atmosphere at most,” as Schlecker notes), we only have a small number of systems (24) out of the 3200 planetary systems currently known that follow this configuration. Schlecker also notes that the Earth is comparatively dry — “…the Earth is, despite the enormous oceans and the polar regions, with a volume fraction for water of only 0.12% altogether a dry planet.” What we don’t find observationally are ice-rich super-Earths with a cold Jupiter in the same system.

Image: Schematic diagram of the scenarios of how according to the analyzed simulations icy super-Earths (a) or rocky (ice-poor) super-Earths form together with a cold Jupiter (b). The mass of the protoplanetary disk determines the result. Credit: Schlencker et al./ MPIA.

The mass of the protoplanetary disk may be the key. A massive enough disk can form inner rocky planets with cold gas giants beyond the snowline, with the rocky planets being poor in ice and gas. Outer super-Earths that form ice-rich would be unable to migrate inward because of the giant planet. In disks of medium mass, by contrast, material to produce warm super-Earths is limited, and gas giants beyond the snowline give way to super-Earths abundant in ice.

In the authors’ words:

We find a difference in the bulk composition of inner super-Earths with and without cold Jupiters. High-density super-Earths point to the existence of outer giant planets in the same system. Conversely, a present cold Jupiter gives rise to rocky, volatile-depleted inner super-Earths. Birth environments that produce such dry planet cores in the inner system are also favorable for the formation of outer giants, which obstruct inward migration of icy planets that form on distant orbits. This predicted correlation can be tested observationally.

and…

…low-mass solid disks tend to produce only super-Earths but no giant planets. Intermediate-mass disks may produce both super-Earths and cold Jupiters. High-mass disks lead to the destruction of super-Earths and only giants remain.

The mass of the protoplanetary disk is thus crucial, with super-Earth occurrence in combination with cold Jupiters rising with increasing disk mass, then dropping for disks massive enough to become dominated by giant planets. Interestingly, the study indicates that host stars of high-metallicity tend to have planetary systems with warm giant planets that can wreak havoc on inner planets in the system.

We’re in that region of the discovery space where we’re building theories that can be tested against future observations. Thus the next generation of space telescopes like the James Webb Space Telescope, as well as the Extremely Large Telescope from the European Southern Observatory. Do the simulations of Schlecker and team hold up? Do they show a disposition for planets of Earth size, rather than the larger super-Earths, to exist in systems with cold Jupiters?

We won’t know, the scientist acknowledges, until we have the ability to detect planets like ours in large numbers. For now, the only numbers we can begin to run are those of the larger super-Earth population, and even here, we’re still building the statistical sample. What the new instrumentation shows in coming years will help us fine-tune these simulated results.

The paper is Schlecker et al., “The New Generation Planetary Population Synthesis (NGPPS). III. Warm super-Earths and cold Jupiters: A weak occurrence correlation, but with a strong architecture-composition link,” in process at Astronomy & Astrophysics (abstract).

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{ 13 comments… add one }
  • Alex Tolley October 15, 2020, 14:33

    Here we can look to the parameters of the simulations. If the tendency of the simulated gas giants to migrate is slightly lowered

    I don’t know what this means exactly. As written, it seems to imply a free variable that can be adjusted. I thought these planetary formation models were using basic physical processes to run simulations, so that the planets formed purely by the physics of stochastic particle aggregation. Using a free variable implies tuning a model, which in turn implies fitting the model to the desired result. This seems to me to defeat the purpose of the model.

    Can someone with some knowledge of these models provide some more detail?

  • Ron S. October 15, 2020, 18:57

    It seems to be almost a tautology to say that Jovian class planets, hot or cold, don’t form from a low mass accretion disk. I hope that this was addressed in the full paper.

    “…the gravitational influence of Jupiter may have a crucial role in deflecting asteroids and comets from the inner system, thus protecting terrestrial worlds like ours.”

    Or the opposite. Asteroids in our own system that fall into orbital resonance with Jupiter are pumped into high eccentricity orbits. That puts them at risk of system ejection by means of a third body interaction, or collision with an inner planet. I have to wonder whether over the long run Jupiter is a net benefit or threat to us.

  • AlexTru October 16, 2020, 3:40

    I respect efforts of the people involved in very complicated process of Simulator development and programming, it requires multiple knowledge and skills of the team members.
    But in same time, suppose that simulation results are disconnected from real world, so suppose that discussed results have some meaning for simulator developers, but no any meaning for astronomers.
    To build more realistic simulations, we need more astronomic observations.
    As I understood from article, our own Solar system – is paradox for this simulator :-), i.e. this nice simulation program still cannot simulate even one real world case, no need to add anything else…

  • Thomas Mazanec October 16, 2020, 6:51

    Are there “cool Jupiters” between warm and cold?

  • Martin Schlecker October 16, 2020, 8:36

    It is great to see so much interest in our work! There is one thing I would like to comment on to avoid misinterpretations: The “24 out of ~3000 systems” could be taken as a mismatch between our model and observations, which is not the case.

    There are huge differences in detectability of different planet types (far out-close in; low-mass-massive/small-large) and different instrumentation or observing strategies. Most of the thousands of known exoplanets have been detected by Kepler and these are mainly transiting and (easy to detect) close-in planets. With the transit technique it is hard to detect “cold Jupiters” – we need RV measurements using large ground-based telescopes.

    Given these inconveniences, we currently have only 24 systems where we know for sure that there is a close-in rocky planet and an outer giant planet. This admittedly small sample agrees well with our prediction (see mass-radius plot in the paper).

    To unequivocally proof our theory, though, we would need the opposite sample: systems with super-Earths where we know for sure that there is no cold Jupiter. This is much harder to get and we’ll need to wait for future instrumentation and surveys.

    • Paul Gilster October 16, 2020, 9:40

      Great to have you here, Martin. Thanks for chiming in on this intriguing work!

    • AlexTru October 19, 2020, 2:59

      we currently have only 24 systems where we know for sure that there is a close-in rocky planet and an outer giant planet

      Just wandering, among those 24 candidates, have many orbiting around G dwarfs stars?
      Suppose none (excluding our own), please correct me if I am wrong.

  • Geoffrey Hillend October 16, 2020, 16:52

    I like the idea of the cold Jupiter being related to the mass of the protoplanetary disk because it supports the idea of the rare super habitable idea which needs a solar system to be exactly like ours. I recall reading somewhere online that the size and mass of the proto planetary disk is correlated with the size of the star. A larger star like our Sun , G class star might make a larger proto planetary disk than smaller stars or K and M dwarfs which would affect the kinds of planets in a solar system. The changes of distance in the life belt due to different star mass would support the above idea of ice super Earths in smaller stars like M dwarfs without gas giants and size and distance of the protoplanetary disk from the class of star.

  • Geoffrey Hillend October 16, 2020, 16:55

    I agree that more data and study of more solar systems would be needed to further support these theories.

  • Robin Datta October 17, 2020, 4:56

    The efforts to understand and predict patterns of solar/stellar system formation and evolution thus far seems to show our solar system to be an exception. Whether truly an exception or an artifact attributable to the shortcomings of present techniques and instrumentation is a question for which clarification by improvement of the instruments and methods is eagerly awaited.

    • AlexTru October 18, 2020, 2:06

      I suppose that exact model/simulation of our Solar System history is impossible task, too much hidden variables here.
      In same time I am sure we cannot (yet) tell about our system as exception. We do not have enough astronomical observation (yet) to tell about any exception, our current instruments does not allow to detect planetary composition around G dwarfs , so no data meanwhile.
      We are blind men, who try to discuss Salvador Dali paintings, fun…

  • Geoffrey Hillend October 18, 2020, 16:04

    We are not completely blind as we can theorize from what we know. Our Earth Moon in the life belt around a G class star system has more contingencies for it to form than other star systems, but due to probability it would not rule out similar such systems, but only make them more rare or not a lot of them taking in mind we have 100 billion Sun’s in our galaxy and only eight percent of them are G class stars. How many of those eight billion stars have cold Jupiters and rocky inner planets? We don’t know. We could add some outer ice giants which is another contingency.

  • Ronald October 20, 2020, 10:46

    A fascinating and admirable study.
    And equally admirable, Paul, how you, again, manage to clearly summarize the essentials of it! Your excerpts, I think, represent the most important conclusions.
    I found it a very interesting paper, but not easy to abstract the essentials from the multitude of diagrams and figures. So, finding the very clear and concise conclusions at the end was a great relief.

    In all modesty and with respect, I have a few issues with this otherwise great paper:
    – The paper uses the term ‘super-Earth’ for planets with a mass between 2 and 20 Me (or rather Me * sin(i) ). I find this term rather misleading and I would think that, in accordance with now standard (?) terminology, by far the greatest part of this range should have been called ‘ice-giants’ and/or ‘gas-dwarfs’.
    – The use of the terms for the different masses of protoplanetary disks (low-, intermediate-, high-) is not entirely clear and consistent. In particular, the very important conclusion 3 is in contradiction with Fig. 18 (and its description in 5.3), also the image in this CD post: according to conclusion 3, a) in Fig. 18 should have been low-mass disk (instead of intermediate- ) and b) should have been intermediate-mass disk (instead of high- ).

    Can we then carefully conclude that there are 3 main planetary system types, as per the conclusion (and Paul’s 2nd excerpt)?:

    1) Low-mass disks resulting in only ‘super-Earths’ all over (actually ice giants and gas dwarfs).
    2) Intermediate-mass disks resulting in cold Jupiters plus (dry) ‘super-Earths’ in the inner system; maybe also ‘wet super-Earths’ in the outer system.
    3) High-mass disks resulting in only hot (and warm?) Jupiters, that have devoured all others.

    Is this about it?

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