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A New Look at High Obliquity Exoplanets

Looking forward from winter into spring in North America — unfortunately still a few months out — I can thank Earth’s obliquity for a seasonal change I enjoy more every year. Obliquity is the angle that our planet’s rotational axis makes as it intersects the orbital plane, which in the case of Earth is 23.5°, so that when we reach the summer solstice, the north pole of the planet tilts toward the Sun by this angle. At winter solstice, the pole is tilted away by the same amount.

Our Solar System’s most extreme case of obliquity is Uranus, where the angle is a whopping 97.8°. Imagine a planet where the north pole points all but directly at the Sun, cycling through a year where the southern pole will eventually do the same. I’m reminded of Stephen Baxter’s novel Ark (Roc, 2011). Here, interstellar travelers come to a planet they hopefully designate Earth II (82 Eridani is its primary). Alas, the obliquity turns out to be 90 degrees, kicking off extreme seasonality. In this passage, one of the characters explains the problem to the rest of the crew:

“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.”

Is a planet with 90 degree obliquity in any sense habitable? It’s a question the crew debate and I won’t spoil what they find out. But I will point to a new study out of MIT that looks at planetary obliquity and finds that, depending on the world, habitable conditions may emerge. The work of David Ferreira (University of Reading, UK), Sara Seager (MIT) and colleagues, the paper in Icarus uses a three-dimensional computer model to simulate interactions between atmosphere, ocean and sea ice over a 3000-meter deep ocean. Shallower, more simplified oceans at 200, 50 and 10 meters in depth were also plugged into the mix for comparison.

The model assumes an Earth-sized planet at a similar distance from its star, one that is completely covered in water. Simulating three planetary obliquities — 23 degrees, 54 degrees and 90 degrees — the researchers found that a global ocean as shallow as 50 meters would absorb so much solar energy throughout the polar summer, releasing it back into the atmosphere during the winter, that the climate would remain relatively mild, with temperatures comfortably spring-like year round. It’s a result that no one on the team had anticipated.

“We were expecting that if you put an ocean on the planet, it might be a bit more habitable, but not to this point,” Ferreira says. “It’s really surprising that the temperatures at the poles are still habitable.”

MIT-AquaPlanet-01

Image: A water world at Earth’s distance from the Sun, assuming a deep enough ocean, could maintain habitable temperatures year-round even if its obliquity is high. Credit: Christine Daniloff/MIT.

It’s also a fragile situation, to judge from these results. A ten-meter deep global ocean would not be deep enough. The world would experience a runaway effect in which the first ice that formed would spread quickly onto the dark side. The eventual emergence of the dark side into light would not help, for by this point, the massive ice sheets that had formed would reflect sunlight efficiently enough to allow the ice to continue to spread, until the world became completely encased in ice. With this kind of high obliquity world in an Earth-like orbit — Ferreira and Seager call it an ‘aquaplanet’ — you’re either facing a warm ocean and benign temperatures or a global snowball.

But the potential is there for an all-water planet at high obliquity, one with a sufficiently deep ocean, to offer conditions in which life might develop. Adds Ferreira:

“The expectation was that such a planet would not be habitable: It would basically boil, and freeze, which would be really tough for life. We found that the ocean stores heat during summer and gives it back in winter, so the climate is still pretty mild, even in the heart of the cold polar night. So in the search for habitable exoplanets, we’re saying, don’t discount high-obliquity ones as unsuitable for life.”

The paper is Ferreira et al., “Climate at high-obliquity,” Icarus Vol. 243 (15 November 2014), pp. 236-248 (abstract). An MIT news release is available.

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Comments on this entry are closed.

  • Mike December 18, 2014, 14:06

    I haven’t read the full paper but the question that immediately pops into mind is if a high obliquity planet had a deep ocean and some surface land how would that affect the modeling. Say the planet had 10% or 20% land surface area or even 30% similar to Earth but at 54 or 90 degrees of obliquity?

    At what amount of land surface combined with what angle of obliquity do we start losing too much of the moderating influence of the oceans to prevent a snowball-type Earth? Maybe a follow on study to this paper might factor in these addional features.

  • Alex Tolley December 18, 2014, 16:40

    One factor to be considered is ocean current circulation that will depend of the geography. Currents distribute heat from the tropics to the poles on Earth as currently we have oceans that allow this circulation. A different geography could result in very different current patterns. Currents on a pure waterworld may be very different (it isn’t something we studied when I did my MSc in Oceanography)

    The other issue regarding life is that Earth’s oceans are deep enough to ensure that even snowball conditions do not result in complete freezing. Most likely Earth had life surviving in cold, but above freezing temperatures on the ocean floor. Dense water ensures 4C conditions, and the ice layers above would reduce exposure to radiative losses. Hydrothermal vents would also provide warm refuges. And let’s not forget life in the lithosphere as a global refuge.

    So my sense is that if life emerges, or is transported to, such a world, it will continue despite any local surface conditions, and that the factors above will help mitigate any severe freezing.

    We should always bear in mind that such models only inform us of whether life could survive on such worlds, but says nothing about whether life would exist. That is likely much more dependent of how life emerges for which we know very little and we really need some data from astronomical observations about possible probabilities.

    I haven’t read Baxter’s novel, but I have to wonder at the assumptions of our star travelers expecting to colonize the surface of such a world. They’ve been living in a star ship which has protected them from much worse conditions on their flight. Why should they not build planetary habitats modeled on the ship to survive anywhere and use the planet’s resources to expand them, rather like planet bound space colonies?

  • Brett December 18, 2014, 21:17

    When Ark says that it would plunge all but the equatorial belt into darkness for months at a time, is that literally the case? I’m trying to figure out the pattern of day-night lengths for somewhere located in the middle of what we would consider the “temperate zone” – like a location at 45 degrees north.

    In any case, a thick atmosphere might buffer the planet’s climate as well. Imagine a planet with about three or more times the atmosphere of Earth – it would be much more effective at moving heat around the globe to cope with the seasonality changes.

  • Brett December 18, 2014, 21:23

    Sorry, to add-

    Just thought about this. If the poles fell over water on a 90-degree planet, the constant heat in summer would probably generate some enormous storm systems.

  • Eniac December 22, 2014, 14:08

    I agree with Brett. Baxter exaggerates the effects. At 45 degrees latitude, the “perpetual darkness” and “perpetual night” would last 3 months each, the remaining 6 months would see normal day/night cycles. For a few weeks early in spring the days would be really short, and late in fall the nights, but the rest of the time would not be too much different from what we have in most places on Earth.

    In comparison, the conditions at the Earth’s poles are worse: “perpetual darkness” and “perpetual night” last 6 months each, with no normal day/night cycles in between. And we do have people living and working there.

  • Eniac December 22, 2014, 14:14

    Ah, not quite right: “For a few weeks early in spring the days would be really short, and late in fall the nights”. The short days would be at the beginning of spring and end of fall, the short nights at the end of spring and the beginning of fall.

    To recap: At 45 degrees latitude winter would be all dark, summer all light, and most of spring and fall would be Earth-like.

  • Rob Henry December 23, 2014, 3:32

    Sorry, but I have lost the reasoning of this article and thread a little. It starts to let a tittle hard when it says
    “so the climate is still pretty mild, even in the heart of the cold polar night”

    My expectation is that the cold tropical solstice would be far more sever, even in a planet with less ocean than Earth. At 90 deg, the poles get two and a half times more annular insolation than the tropics. Let me put it another way… for half the year the poles receive five times the average insolation of the equator, and so have only a short few months for their oceans to release 80% their stored heat and cool to tropical temperatures before that sun comes up again.

  • Michael December 23, 2014, 16:50

    I was just crunching some numbers here for the amount of hydrogen and water one of these worlds would collect from their star via magnetic field funnelling. It is a fair amount!

    At 1 proton/cm^2
    400km/s solar wind
    at ten radii collection (averaged by 2)

    I get around ~117 000 tons of hydrogen per year or 1 million tons of water per year if it reacts with oxygen in the atmosphere. Over 4 billion years that is a lot of water ~about 4 million km^3. Enough to cover an Earth sized world to a depth of 13 cm.

  • ljk December 31, 2014, 12:21

    Planets with Odd, Mercury-Like Orbits Could Host Life

    by Charles Q. Choi, Space.com Contributor | December 31, 2014 07:00 am ET

    Mercury has an oddball orbit — it takes longer for it to rotate on its axis and complete a day than it takes to orbit the sun and complete a year. Now, researchers suggest photosynthesis could take place on an alien planet with a similarly bizarre orbit, potentially helping support complex life.

    However, the scientists noted that the threat of prolonged periods of darkness and cold on these planets would present significant challenges to alien life, and could even potentially freeze their atmospheres. They detailed their findings in the International Journal of Astrobiology.

    Astronomers have discovered more than 1,700 alien planets in the past two decades, raising the hope that at least some might be home to extraterrestrial life. Scientists mostly focus the search for alien life on exoplanets in the habitable zones of stars. These are regions where worlds would be warm enough to have liquid water on their surfaces, a potential boon to life

    Full article here:

    http://www.space.com/28134-exoplanets-oddball-orbits-alien-life.html

    To quote:

    “If the sun were less intense, Mercury would be within the habitable zone, and therefore life would have to adapt to strange light cycles,” said lead study author Sarah Brown, an astrobiologist at the United Kingdom Center for Astrobiology in Edinburgh, Scotland.