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Orbital Dynamics and Habitability

In Stephen Baxter’s novel Ark (Gollancz, 2009), a starship launched by an Earth in crisis reaches a planet in the 82 Eridani system, an ‘Earth II’ that turns out to have major problems. Whereas Earth has an obliquity, or tilt relative to its orbital axis, of about 23.5 degrees, the second ‘Earth’ offers up a whopping 90 degree obliquity. Would a planet like this, given what must be extreme seasonality, be remotely habitable?

The crew discusses the problem as they watch a computerized display showing Earth II and its star. The planet’s rotation axis is depicted as a splinter pushed through its bulk, one that points almost directly at the star. But as the planet rotates, the axis keeps pointing at the same direction in space. After half a year, the planet’s north pole is in darkness, its south pole in light.

One of Baxter’s characters explains the consequences:

“Every part of the planet except an equatorial strip will suffer months of perpetual darkness, months of perpetual light. Away from the equator you’ll suffer extreme heat, aridity, followed by months of Arctic cold — we estimate the surface temperature will drop to a hundred degrees below across much of the space-facing hemisphere, and there’ll be one hell of a blanket of snow and ice. Even the equator would be a challenge to inhabit, for even at the height of summer in either hemisphere the sun would be low, the heat budget minimal, the climate wintry.”

A small group of crewmembers decides nonetheless to colonize this Earth II, with the rest leaving for other destinations. I suppose there are other science fictional treatments of the problems of extreme obliquity, but Baxter’s fired my imagination as I tried to picture what life would be like on a planet this extreme. The problems of obliquity and orbital dynamics have likewise interested a team of researchers led by Russell Dietrick (now at the University of Bern), working with Rory Barnes, Victoria Meadows and several other colleagues at the University of Washington. How do such factors affect our outlook on planets in the habitable zone?

It’s a significant question, and just the kind of thing I would expect to be considered at the University’s Virtual Planet Laboratory. Because observing time on Earth- and space-based instruments is precious, and the number of targets of interest is sure to grow, we need ways to narrow down our choices to a manageable level as we begin to home in on planets that are both in the habitable zone and truly habitable. Deitrick and team have used computer modeling to produce some answers, constructing a detailed treatment of the growth and retreat of ice sheets. The model comes to different conclusions than earlier attempts, as Barnes explains:

“While past investigations found that high obliquity and obliquity variations tended to warm planets, using this new approach, the team finds that large obliquity variations are more likely to freeze the planetary surface. Only a fraction of the time can the obliquity cycles increase habitable planet temperatures.”

Image: A NASA artist’s impression of Earth as a frigid “‘snowball” planet. New research from the University of Washington indicates that aspects of an otherwise habitable-seeming exoplanet planet’s axial tilt or orbit could trigger such a snowball state, where oceans freeze and surface life is impossible. Credit: NASA.

The upshot: Think of Earth’s ice ages and magnify them. The new work shows that while Earth may be relatively calm in terms of climate, a planet with higher obliquity could be a ‘snowball’ world. Similar effects may be produced by extremes in orbital eccentricity. We are thus applying to exoplanets the factors that Serbian astronomer Milutin Milanković originally applied to Earth when he examined how eccentricity, axial tilt and precession of our planet’s orbit could produce variations in the solar radiation reaching the surface, and hence marked climatic patterns.

Modeling an Earth-like planet whose climate is responding to extreme orbital forcing, the researchers find that these changes in obliquity and eccentricity drive the growth and retreat of ice caps that can extend from the poles to roughly 30 degrees in latitude. A snowball instability can result, producing oceans covered with ice. But such interactions are complex, as the paper takes pains to note, and can be affected by factors like planetary moons. From the paper:

It is particularly important to understand the eccentricity and obliquity evolution in combination, because the stability of ice sheets is intimately coupled to the obliquity and the eccentricity affects the amount of intercepted stellar energy. At a single stellar flux, a planet can be either clement and habitable or completely ice-covered, depending on the orbital parameters and the planet’s recent climate history. This further complicates the concept of a static habitable zone based on the stellar flux. We have shown that orbital and obliquity evolution, and the long time scales of ice evolution, should be considered when assessing a planet’s potential habitability.

Once again we’re reminded that the concept of a habitable zone is insufficient as the sole judge of planetary habitability, something we’ve also discussed in relation to ‘habitable zone’ planets around red dwarf stars. In this case we’re in the realm of G-class stars, where extremes in obliquity can freeze a planet’s surface. Too bad Baxter’s starship crew hadn’t discovered their Earth II’s obliquity problem before they went there, but maybe we’ll do better. Says Deitrick:

“If we have a planet that looks like it might be Earth-like, for example, but modeling shows that its orbit and obliquity oscillate like crazy, another planet might be better for follow-up with telescopes of the future.”

The paper is Deitrick et al., “Exo-Milankovitch Cycles II: Climates of G-dwarf Planets in Dynamically Hot Systems,” accepted at the Astronomical Journal (preprint).


{ 30 comments… add one }
  • Spaceman May 17, 2018, 13:39

    Interesting article! One question have somewhat related to this topic is as follows: can a planet remain habitable over billions of years even it never spawned life at any point in its history? Is life of some sort necessary to maintain the habitability of a planet over long, eon spanning time scales?

    • Nicky May 18, 2018, 1:18

      This is an extremely interesting question “Is life of some sort necessary to maintain the habitability of a planet”. This is what we call Gaia Hypothesis: Earth is a self­-regulating system in which the biota play an INTEGRAL role (Earth System, 3rd edition). This definition defines the weak form of Gaia, which most geologists, biologists and ecologists find it acceptable.

      When Gaia Hypothesis was first proposed by Lovelock, it was in strong form: Homeostasis at an optimum BY and FOR the biosphere. This form requires life to possess some intelligence or foresight, so it was criticized and said to be untestable.

      One example that is quite Gaian in nature would be vascular plants. They accelerate rock weathering rate to drawdown atmospheric pCO2 as sun becomes brighter, and they also weaken the direct dependence of weathering rate on pCO2 preventing the recurrence of snowball Earth (Principles of Planetary Climate).
      Early methanogens provide additional greenhouse gas CH4 to warm surface when the sun was 20%-30% dimmer than present-day.
      Early photosynthesis also produces the oxygen molecules to form ozone which acts as a shield to block out UV radiation.

      At same time, Earth system also runs other abiotic negative feedback loops stabilizing the climate.

  • Thomas Goodey May 17, 2018, 14:53

    “I suppose there are other science fictional treatments of the problems of extreme obliquity…”

    Yes, in ‘Uller Uprising’ by H. Beam Piper, for a start.

    • ljk May 17, 2018, 15:52

      And don’t forget planets 16 times the mass of Jupiter spinning on their axis once every 17.75 minutes….


      • Thomas Goodey May 17, 2018, 17:12

        Yes, but Mesklin didn’t have any great obliquity, as I remember. Barlennan’s expedition traveled down to near the pole, where the gravity was strongest, but did not come into a region of midnight sun or noon darkness. That would have damaged the flow of the story.

        • Charley May 17, 2018, 21:49

          OK, I’m totally intrigued! When you said the following:
          “Barlennan’s expedition traveled down to near the pole, where the gravity was strongest, but did not come into a region of midnight sun or noon darkness. ”
          What in the world did the traveled down to the poll where the gravity was strongest have anything to do with them. Finding a place to survive at this newfound world ?!?
          I’m completely baffled as to how this would ensure survival in this particular context (i.e., I never read the story), so please help me out by way of explanation…

    • David Hart May 17, 2018, 17:09

      In Larry Nivens Known Space series he has a planet (We-Made-It) orbiting Procyon A with a near 90 degree tilt, causing destructive winds around the planets soltices. The colonists landed in spring when the conditions were benign. However I don’t recall any stories using this apart from background.

      • Paul Gilster May 17, 2018, 17:39

        I can’t believe I forgot about We Made It! I read the Known Space stories when they came out. Niven’s early work was glorious.

    • andy May 20, 2018, 12:15

      The planet in Harry Harrison’s Deathworld had a fairly extreme axial tilt, among its many other appealing characteristics.

      • Paul Gilster May 20, 2018, 13:06

        Thanks, andy. I’ve never read Deathworld, but it sounds like it needs to be on my list.

  • Geoffrey Hillend May 17, 2018, 16:14

    It’s not a good idea to choose a planet with a large variation in it’s axial tilt (obliquity)for a home. Mars has a 10 to 70 percent axial tilt because it does not have a Moon. Venus does not have a Moon and a slow rotation caused by a lack of angular momentum provided by a collision like the giant impact hypothesis of Earth with Theia. With a fast rotation there is no magnetic field to stop solar wind stripping and cosmic radiation.
    Earth has a Moon so it only has a small variation in axial tilt: the between 22.1° and 24.5°, over a cycle of about 41,000 years. The Milankovitch cycles, Wiki. When the axial tilt is smallest, there is less sunlight on both poles so that an ice age is more likely on Earth. The same is true about Mars with more ice in the polar caps with a small tilt of angle of the poles. The eccentricity also plays a role with the ice age on Earth.

    • Charley May 17, 2018, 21:53

      I’m not at all certain that I agree with your following statements:
      “Mars has a 10 to 70 percent axial tilt because it does not have a Moon. Venus does not have a Moon and a slow rotation caused by a lack of angular momentum provided by a collision like the giant impact hypothesis of Earth with Theia”
      I’m not at all sure that angular momentum imparted initially to the bodies in question has anything to do with having a Moon or not having a Moon. I think the question as to whether or not life on earth developed, is somewhat independent of the question as to whether or not a particular body possesses a satellite or doesn’t.
      In our particular case, the moon is actually causing the Earth’s rotation to slow and within probably 100,000,000 years. It’s conceivable that a day on earth might be 48 hours long. Whether that’s a good thing or bad thing is up to debate. Just my opinions here.

      • Alex Tolley May 18, 2018, 15:22

        I believe Mars tilt has randed quite widely as suggested, as the surface features have changed. If a high axial tilt makes life very difficult, perhaps causing a snowball condition, it is possible that this could be detrimental to life.

  • Geoffrey Hillend May 17, 2018, 16:16

    Without a fast rotation, there can’t be a magnetic field.

    • Nicky May 17, 2018, 23:07

      With slow rotation, there still can maintain a magnetic field that is almost as strong as terrestrial one lasting for billions of years.
      The occurrence of planetary magnetic field is the natural outcome of core cooling independent of rotation rate, because in any case it results in large convection of conductive liquid.
      Fast rotation can help strengthening the field (not much), but IT IS FAR FROM BEING THE DETERMINING FACTOR, which should be core-mantle thermal evolution.
      The leading theory to the divergence of Venus and Earth in terms of geophysics is not the rotation rate but the thermal evolutions of their interiors. The tectonic mode of Venus is stagnant-lid (Earth is mobile-lid). These factors impede high rate of core cooling, which is why Venus lacks an intrinsically magnetic field, and its slow rotation rate only plays a minor role.
      Mars has small mass so its core solidified in relatively short amount of time preventing the generation of long-lived dynamo. Mercury’s core cooling is maintained by tidal heating though extremely weak.

    • Michael May 18, 2018, 2:12

      Mercury has a rotation rate of 176 days and a magnetic field strength of around 1% which at the Earths distance from the Sun would give a surprisingly large amount of protection.

  • Charley May 17, 2018, 21:59

    Time if you want to see a really bizarre take on what would happen should our planet undergo a slowing of its rotation to a standstill, please take a look at this rather (in my mind) extreme prediction of the outcome:
    ‘If Earth Stops Revolving : Most Thrilling Film ‘

  • Rob Flores May 17, 2018, 23:15

    good grief even Titan as harsh as that moon is a better candidate.
    for colonization than the fictional planet.

    Speaking of which I did a back of the envelope calulation
    to answer the question. just how big an asteroid would one
    need to penetrate to sub-surface ocean on Titan and Europa.

    Titan: most estimates are that titan has some thing like a
    100 KM thick crust. Chxililub(sp) in the yucatan has been estimated areound (assuming we are talking about asteroid, not a comet) Around 10km is the estimate.
    Doing some maths, using Kinnetc energy(joules) specific heat of
    Ice, and volume calculations. I come with
    Around 6 Km is what it would take to penetrate all the way to
    the sub-surface ocean of titan, I am assuming a strike perpendicular to the surface, and that he object impact velocity was similar to the Yucatan Asteeroid. Rarity: such strikes might be more freqent thaan the earth bombardment rate, since the Saturnian gravity is dominant in such scenarios. I put it at 350MY. essentially lots of rocks this size might strike Saturn and on the rare occasion Titan is in the way, would a stricke occur.

    Onto Europa, whose icy crust is estimated at 10km.
    Some math gives, the result that a 1km asteroid coming in hot
    like the yucatan impactor would penetrate to the subsurface
    ocean. Rarity. This is probab

  • Rob Flores May 17, 2018, 23:24

    This probaby is fairly frequent owing to Jupiters enourmous gravity reach. I would estimate an impatct every re 30 MY.

    If i am correct in all of this, Europa has received outside nutrients to its ocean about 7 times as freequently.

    Now I am big pro Titan person, (for colonizing) but if life were to be found on both oceans, It would not surprise me that Europa
    has simple animals, and Titan has primitive bacteria.

  • Andrew Palfreyman May 18, 2018, 0:10

    For the conditions described, a scifi solution has already been described for a Mercury colony – the entire base is mobile, mounted on rails which encircle the planet.

    • Michael May 19, 2018, 9:50

      There is no need to move even at Mercury’s equator, going underground and reflective insulation gives ample protection against the heat.

  • Michael May 18, 2018, 2:17

    Studying Uranus, due to its extreme axial tilt, and its wind directions could give an idea how a weather system behave on such extremely tilted worlds. Although the amount of heat Uranus receives from the Sun due to the distance would need to be factored in.

  • ole burde May 18, 2018, 7:43

    Well, if one planet became too cold because of axial tilt , then another planet ,which would otherwise have been too warm , might become just fine ….also because of axial tilt …..
    Why is it that the negative implications of almost everything are most readily talked about ?
    This reminds me about a recent Centauri Dreams-survey of SF coverage of ‘Generation Ships’ where not a single positive story was found …instead we had a talk about stuff like Robinson’s ‘Aurora’ where Heavy Guilt fills the air …you can cut it with a knife ….what existential crime-sinn is it that we are supposed to have done , that automaticly MUST make our best efforts fail miserably ?
    It’s all in the head , and time is up for cleaning out the leftover garbage which is piled up in there !

  • Geoffrey Hillend May 18, 2018, 16:30

    A magnetic field has to be produced by charged particles moving in circles like the motion of electrons in a current in a wire wrapped around a nail which is an electromagnet. Earth’s liquid core has circular movements in it and these are caused by mostly by the Earths rotation. It was thought to have been caused by convention currents. This happens in stars and might happen in Earth, but the magnetic field is mostly caused by Earth rotation which causes two rotating cylinders of the liquid iron core of the Earth. Venus does not have a magnetic field and has a very slow rotation, The magnetic field of Mercury is very weak and Mars does not have one.

    It’s logical to assume that if the Earths rotation is slowing down, it had to be moving faster in the past. The proof comes from coral and the growth rings of the shells of scallops which prove that the year had more days in it because Earth rotated or spun faster in the past. Each ring takes one day to make so you have more days in the year in the distant past. P. 147, Lyle, The Abyss of Time. There was less hours in our day the farther back in time you go so there was only 21 hours in a day in the Cambrian period. Also the tidal movement was much larger, so it moved much further inland than today over a wider shore because the tidal effect from the Moon was stronger since the Moon was closer to the Earth in the distant past. Source; The Realm of the Terrestrial Planets, Kopal. I don’t have the page number handy.

    • Nicky May 19, 2018, 0:58

      “circular movements…are caused by mostly by the Earths rotation”, please give source on that.
      How did planetary rotation affecting the convection inside a core come up? Back in 1980s, when probes first found that Venus is weakly magnetized, the thermal evolution of this planet was not fully understood. The geologists attributed this weak field as the result of slow rotation because Venus has almost exactly same radius and composition as Earth, except for that 243 days of rotation rate. Therefore, a scaling law for magnetic field strength based on rotation rate was made and widely used.
      These studies can be found:
      doi.org/10.1029/GL006i003p00213 Planetary magnetic fields – 1979
      doi.org/10.1029/RG018i001p00077 Planetary magnetism – 1980

      As the understanding of thermal evolution and geophysics advance, the generation of magnetic field depends more and more on core-mantle behavior. Core cooling process releases heat to drive the convection. The lack of magnetic field in Venus is now understand as the result of insufficient heat flux from core cooling, which is impeded by stagnant-lid. See recent reviews on geophysics and magnetic field studies of Venus:

      Planetary magnetic Fields – 2003:
      “Venus is likely to have a liquid outer core (with or without an inner core) but has no dynamo at present. The predicted dynamo field is over two orders of magnitude larger than the observational upper bound……The most probable interpretation is that the liquid core of Venus does not convect. This could arise because there is no inner core or because the core is currently not cooling. The absence of an inner core is plausible if the inside of Venus is hotter than the corresponding pressure level of Earth. This can arise because Earth has plate tectonics, which eliminates heat more efficiently than a stagnant lid form of mantle convection”.

      Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites – 2010:
      “The lack of a present dynamo does not imply that Venus never had an intrinsic magnetic field……It is important to note that the slow rotation of Venus (a Venus day of ∼243 Earth days is almost equal to the length of its year of ∼224 Earth days and its sense of rotation is retrograde) does not exclude dynamo action……The models cool more efficiently than models based on the scaling laws for stagnant lid convection. Assuming that Venus was in a stagnant lid regime throughout the entire evolution, the phase of early dynamo action would have most likely been short”.

      Divergent evolution of Earth and Venus: Influence of degassing, tectonics, and magnetic fields – 2013:
      “On a stagnant lid planet, even with a convective mantle, the lithosphere is much less efficient at cooling the deep interior and driving dynamo action in the core”.

      The direct relationship of magnetic field and rotation rate is no longer considered scientific. I don’t want to go into details about how core cooling induces magnetic field specifically, but I suggest you to learn the complex physics behind it. In fact, tidal effect does not hinder the formation of field, it helps through tidal dissipation of core heat flux.

  • Geoffrey Hillend May 19, 2018, 16:35

    Your opening up a can of worms for me here Nicky. I’ll say that scientists and geologists don’t use one example like Venus to generalize a theory or make principles about planetary magnetic fields and I am not doing that also. The principles are there first as first principles such as charged particles moving in circles creates a magnetic field, and the scientist plugs into them. I don’t have reference, but I don’t think I need one since I am using physical and geological principles. There are convection currents in the Earth’s mantle, and so with Venus and also in both their cores but I don’t think they are fast enough to generate a strong magnetic field. The Earth’s rotation causes the outer liquid core of the Earth to become organized into rolls like rotating cylinders. https://en.wikipedia.org/wiki/Earth%27s_magnetic_field

    Venus can’t have those fast cylinders because it has a slow rotation. Stars have convection currents which are believed to cause their magnetic field, but those are of plasma which has a magnetic field already built into it. It’s also the differential rotation, e.g., between the equator and poles which causes the solar flares and magnetic fields inside it. Also there is a radiation layer and a convection layer in a star based on differences in gas pressure which indicate a complex machinery to make a magnetic field.

    • Nicky May 19, 2018, 22:57

      Hi Geoffrey Hillend, you might have mistaken my reply. I totally agree with your “charged particles moving in circles creates a magnetic field” (core convection), but what I am arguing about is the role of rotation rate. I want to be a little more specific on this subject.

      First, convection and columns (what you called “rotating cylinder”), they are essential in creating magnetic field, but they are separate concepts. Please don’t use the word “cylinder”, because that confuses with a different thing (tangent cylinder) in dynamo theory. Rotation does organize convections into columns (columnar pattern of convection; Taylor–Proudman theorem).

      Second, convection is caused by planet thermal evolution. Core cooling process naturally induces liquid core convection through exclusion of light elements from the cooling and solidifying inner core. If the overlying mantle temperature is too high, core cooling process would stop, so convection would also stop.

      Third, columnar pattern of convection is caused by planetary rotation (Coriolis force) as you said. Although rotation rate was once thought to be an important parameter, as dynamo modeling advances it was found that Coriolis force has a very large effect on the flow. Even extremely slow rotation rate (Venus) can easily satisfy the required fluid motion. Various parameters of dynamo modeling found that magnetic field strength is almost independent of rotation rate (because inertial force compared to Coriolis force is almost negligible; Rossby number). Dyanomo modelings of Earth with 243 days and Earth with 1 day as rotation rate can produce magnetic fields on the same magnitude. That is why geologists attribute Venus’s case to lack of convection instead of lack of columnar motion. There are just too many sources supporting this point, please don’t ignore them (The first and second references given in the previous reply explicitly state that Venus lacks convection and slow rotation does not exclude dynamo action).
      One would be Scaling properties of convection‐driven dynamos in rotating spherical shells and application to planetary magnetic fields

      Fourth, geologists have turned their attention to Venus’s tectonic mode after realizing that slow rotation is not the cause of weak magnetic field. Without subduction of cold slabs (stagnant-lid), mantle temperature remains very high which prevents the core cooling. This in turn stops core convection and shuts off dynamo.
      Quite a lot of studies have done on this subject, see the recent reviews of dynamo modelings I have given in the previous reply.

  • stephen May 21, 2018, 13:30

    I have a few more questions about axial precession. Earth’s is about 25,000 years. Is a period of 25 years unlikely? What if it was one year?!

    Our one large moon keeps our rotation and axial tilt stable. Would a bunch of small moons do just as well? How much would planetary rings stabilize rotation and axial tilt?

    A planet could be rotating along 3 axes, at different rates along each axis.

  • Geoffrey Hillend May 21, 2018, 16:17

    I think you know the answers to these questions already. A short rotation time would have to go with a very small wobble which wouldn’t impact climate. Small moons might keep an axial tilt stable but how common is such a situation? These usually collide to make bigger moons like in a early bombardment period. I don’t think a planet can have three axis.

  • Geoffrey Hillend May 21, 2018, 16:18

    The small moons would have to be pretty big.

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