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).