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Gas Giants on Eccentric Orbits: ‘Wrecking Balls’ for the Inner System?

We often think of Jupiter as a mitigating influence on asteroid or comet strikes in the inner system, its gravity changing the trajectories of potential impactors. That would make gas giants a powerful determinant of the survivability of Earth analogues, at least in terms of habitability. While we continue to investigate the question, it’s interesting to consider the damage a gas giant on an elliptical orbit might do to habitable zone planets. Stephen Kane (UC-Riverside), working with Caltech astronomer Sarah Blunt, decided to find out what would happen if, in their modeling, they introduced an elliptical gas giant into the system of an Earth twin.

You may remember Kane’s work earlier this year combining radial velocity with direct imaging methods to find three gas giants that had been previously unobserved (citation below). The monitoring of ten target stars continues even as this new work is published. We’re beginning to find more planets at ever larger distances from their stars as radial velocity and direct imaging methods improve, allowing us to better understand how the architecture of our own Solar System measures up to systems around other stars. Kane and Blunt’s paper implies that a gas giant on an elliptical orbit does not necessarily preclude a habitable planet’s survival.

The planetary system at HR 5183 is a little over 100 light years away in Virgo, home to an eccentric gas giant in a 75 year orbit that the researchers used in their modeling. The primary here is a G-class star. Its planet has one of the longest orbital periods currently known among exoplanets. The eccentricity of this world is e = 0.84, where e = 0 would be perfectly circular, and e = 1 would be a line segment. To find out whether such a world really would be a ‘wrecking ball’ for its neighbors, the researchers introduced a habitable zone terrestrial world as a test case to study extreme system architectures and their effects on habitability.

Image: Comparison of HR 5183b’s eccentric orbit to the more circular orbits of the planets in our own solar system. Credit: W. M. Keck Observatory/Adam Makarenko.

The dynamical simulations here involved the exploration of 200 evenly spaced semi-major axes between 1.0 and 3.0 AU, intended to encompass the range of the optimistic habitable zone around such a star. Kane and Blunt then placed an Earth-mass planet at randomized starting positions and propagated the effects of the eccentric gas giant over time. Says Kane:

“In these simulations, the giant planet often had a catastrophic effect on the Earth twin, in many cases throwing it out of the solar system entirely. But in certain parts of the planetary system, the gravitational effect of the giant planet is remarkably small enough to allow the Earth-like planet to remain in a stable orbit.”

This being the case, we’re called upon to imagine the view from the surface of a habitable zone planet in this system. The gas giant is on a 75 year orbit, something akin to Halley’s Comet in our own system. Kane says that when the gas giant makes its closest approach to the terrestrial planet during that orbit, it would appear 15 times brighter than Venus, a spectacular object that would dominate the night sky before receding once again into the outer reaches.

Here’s a clip from the paper talking about the significance of these findings. Note that the Milankovitch cycles discussed below are cyclical movements — eccentricity, axial tilt, and precession — related to a planet’s orbit around a star. From the paper:

The importance of such systems from a planetary habitability perspective arises from a thorough investigation of the dynamical stability of terrestrial planetary orbits, such as the one presented here. The careful analysis of the dynamical integrations demonstrates that planets can survive within a narrow range of locations in the HZ of such systems, even in the presence of a wrecking ball whose orbital origin is likely a chaotic event involving vast exchanges of angular momentum.

So we have planetary survival in certain locations, but habitability is severely challenged:

…the case of the HR 5183 system also shows that the presence of an eccentric planet will often have a profound effect on the Milankovitch cycles of the HZ terrestrial planetary orbits, causing significant orbital oscillatory behavior. The implications for the climate effects on such worlds may rule out temperate surface conditions, although the stabilizing effects of surface liquid water oceans can also potentially prevent a climate catastrophe.

In other words, our terrestrial world in its habitable zone orbit in a system with a highly eccentric gas giant is in a dangerous position indeed, though not one that completely rules out life. This seems to represent a slight widening of habitable zone possibilities as we examine exoplanetary systems, though the ‘wrecking ball’ hypothesis still seems the most likely outcome.

The paper is Kane & Blunt, “In the Presence of a Wrecking Ball: Orbital Stability in the HR 5183 System,” Astronomical Journal Vol. 158, No. 5 (31 October 2019). Abstract / Preprint. The paper on gas giant detection is Kane et al., “Detection of Planetary and Stellar Companions to Neighboring Stars via a Combination of Radial Velocity and Direct Imaging Techniques,” accepted at the Astronomical Journal. Preprint.

{ 16 comments… add one }
  • Ron S. November 6, 2019, 13:56

    I think you put the wrong pic in this article.

    • Paul Gilster November 6, 2019, 22:12

      Yes, just caught this. Sorry!

  • Neil November 6, 2019, 16:14

    The figure here is from the previous article on microlensing.

  • Catherine Berry November 6, 2019, 16:17

    e = 1 is a parabola, not a line segment.

  • andy November 6, 2019, 16:46

    Looks like the wrong image ended up here, it’s the same as the microlensing one.

    The extreme Milanković cycles in such systems might make them an interesting setting for a Helliconia-style planetary romance.

  • Alex Tolley November 6, 2019, 17:54

    Looks like the image is from the last post on micro-lensing.

  • Alex Tolley November 6, 2019, 18:06

    I would think that any really major climate instability would tend to keep organisms simple, e.g. unicellular, to survive the extremes. Any rapid change in the local climate will cause ecosystem collapse and extinctions, much as we are doing on Earth with rapid global heating. Life can exist, but I doubt that it will allow diverse ecosystems to develop. [It is the stable, warm climate and weather of the tropics that makes them the most biologically rich areas on this planet. Diversity tends to reduce towards the poles].

    I was just reading a paper speculating on life on Venus [h/t to ljk] which suggested one locale might be below the surface where it is cooler, and there might still be liquid water. Life on a planet with changing climates might well find more stability in the deep oceans and below the land surface. [Note that in the great Permian extinction, marine life in the deeper, more stable places in the oceans and seas had a lower extinction rate than organisms in shallower waters.]

    • ljk November 7, 2019, 14:08

      Here is that Venus paper online:

      Astrophysics and Space Science

      November 2019, 364:191|

      Life on Venus and the interplanetary transfer of biota from Earth

      Rhawn Gabriel Joseph


      Evidence and observations favoring the hypothesis that Venus is habitable, and the celestial mechanisms promoting the interplanetary transfer of life, are reviewed. Venus may have been contaminated with Earthly life early in its history via interplanetary transfer of microbe-laden bolide ejecta; and this seeding with life may have continued into the present via spacecraft and due to radiation pressure and galactic winds blowing microbial-laden dust ejected from the stratosphere via powerful solar winds, into the orbit and atmosphere of Venus.

      Venus may have had oceans and rivers early in its history until 750 mya, and, hypothetically, some of those species which, theoretically, colonized the planet during that time, may have adapted and evolved when those oceans evaporated and temperatures rose.

      Venus may be inhabited by a variety of extremophiles which could flourish within the lower cloud layers, whereas others may dwell 10 m below the surface where temperature may be as low as 200 ∘C—which is within the tolerance level of some hyperthermophiles. Speculation as to the identity of mushroom-shaped specimens photographed on the surface of Venus by the Russian probe, Venera 13 support these hypotheses.


      • Gary Wilson November 7, 2019, 17:43

        Those photographs of mushroom shaped “specimens” are stretching credibility quite a bit aren’t they? Many appear fuzzy and out of focus and could surely be some type of rock as with the pea shaped inorganic compounds on Mars. I think we’ll have to do a lot better than that.

  • Geoffrey Hillend November 6, 2019, 21:40

    Sounds logical. An eccentric orbit of a gas giant could attract and repel more asteroids.

  • wdk November 7, 2019, 16:07

    Back in the late 90s, culminating in a conference paper (American Astronautical Society ) in 1997, I had presented on some early planet detections by Marcy and Butler, looking at the so-called restricted elliptic 3 body problem, the 3rd body being an analog to Earth. This was along with some other well known binary systems with large eccentricities – such as Alpha Cen, Sirius, Procyon. But additionally, there were two new examples that resembled the current case. 47 Ursae Majoris and 70 Virginis. Primaries were somewhat brighter than the sun and the substantial jovian planets had observable eccentricities, the first being the better established. Actual mass subject to the
    doppler and inclination. In one case the eccentric jovian’s orbit was
    internal to the path of the “habitable” terrestrial planet.

    There were, of course, significant pertubations of the orbits of the habitable planets, but they were cyclic. Whether the cycles could be maintained for geological periods, I don’t know. But beside the tendency toward chaos, evidently there is a tendency as well to settle into fragile stable patterns such as the Trappist system.

    For anyone that’s interested in that sort of thing, I can reply with a PDF.

  • Christian G November 7, 2019, 18:31

    The key question, of course, is whether or not there are dragons on the habitable zone planet.

  • Geoffrey Hillend November 11, 2019, 17:28

    An exoplanet Earth twin with a wrecking ball gas giant is a four body problem. It’s Star, it’s Earth twin and it’s Moon and elliptical gas giant. An exact Earth twin must have a moon, otherwise the Earth twin would have a large wobble on it’s axes without a moon. There would be a wide change in axes tilt and changes of climate over short periods like Mars.

    There is a possibility that the wrecking ball gas giant might change the orbit rather than eject the giant impactor before any moon could from and we would not have an exact Earth twin, but this might be restricted to a certain probability which is not one hundred percent so in theory there still could be an Earth twin. I would not be so much worried about the wobble on it’s axes, but whether or not the gas giant would change the eccentricity of orbit of the exoplanet and move it closer to it’s Sun which would not be good for a climate. On the other hand if the exoplanet was not in the life belt or on the outer edge it could be moved into the life belt by the elliptical gas giant.

    If you have an Earth twin with a moon and it is early in it’s evolution, there be dragons there, or giant carnivorous dinosaurs with a very close if not exact resemblance to T Rex.

  • Geoffrey Hillend November 11, 2019, 18:26

    Excuse me for the mistake here. It is a three body problem since masses have to have equivalent masses or at least close to each other in mass and the Star is too big. In order for it to be a three body problem an exoplanet moon would have to be included because an Earth twin exoplanet without a moon and a gas giant is only a two body problem and the star is not included.

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