How Planetary Tilt and Water Content Affects Exoplanet Climates

How Planetary Tilt and Water Content Affects Exoplanet Climates

A science fiction novel recently published by Dodecahedron Books (Helena Puumala’s “Ingrid on Paradiso: The Rescue of the Green Girls”) features a planet with a much lower axis tilt (or obliquity) than Earth.  This is imagined to create a rather strange and interesting climate:

The planet had massive icecaps at the poles, caps that apparently never melted much.  

Because the planet's tilt in relation to its sun was noticeably smaller than what had been true of Ingrid's home world, the seasonal variations were such that they seemed strange to her.  For example, the water that did melt from the icecaps was what kept the ecosystem functioning, although in a manner quite different from what she was used to.

For half the year the icecap around the North Pole thawed at its edges, sending by mid-summer, torrents of water down south, along the riverbeds which the waters had gouged out over eons past.  Most of this water soaked into the earth, or was absorbed by the air, creating conditions in which life could flourish, in the mid-zones of the hemisphere.  There was tundra in the melt zone, replaced by forests further south.  The northernmost trees were small, growing larger in the more temperate areas where conifers eventually were replaced by deciduous trees.  These grew tall and lush in a broad band which circled the planet.  South of that band, the trees became smaller and sparser until, eventually, they were replaced by scrub and brush.  These, in their turn, gave way to vast tracts of grassland.  Yet farther south, the grass grew shorter and drier, until, the equatorial district was a dry desert where almost no water remained in the biosphere.

During the second half of the year, the melt happened around the southern icecap while in the north the cap increased in size.  The winter months were dry, Ingrid was told, and the weeks around the equinoxes were marked by violent storms as the climatic balance shifted from melting mode to freezing mode, and vice versa.

… the atmospheric turbulence caused by the Equatorial heat which was not modified by ocean waters, as was the case on almost all the other inhabited planets of the galaxy.

….  Those deserts…are so hot, and create so much atmospheric turbulence that human beings have to get up pretty damn high to make it across them.  Like in a space vessel.”

It is always interesting to see how fictional and scientific speculations line up on such matters.  Are planetary obliquities stable?  How does planetary tilt affect climate, according to recent exoplanet modelling?  Also, how does the amount of water on an exoplanet likely affect the planet?  I had a look at some recent research on the matter, in some relevant scientific journals (Astrobiology, The International Journal of Astrobiology and Icarus), with my findings summarized below.

1 - The Range of Planetary Tilts (Obliquity)

The first matter to consider is just what is the range of possible obliquities for likely exoplanets.  Within our solar system, there is a wide range of planetary tilts, with Mercury and Venus having low obliquities (under 3 degrees), Earth and Mars having moderate obliquities (in the mid-twenties) and the outer planets varying from 3 degrees (Jupiter) to 82 degrees (Uranus).  So, there is plenty of variation.

There is some reason to believe that close-in planets will have small tilts, due to tidal gravitational influences from the star that they orbit (that’s true for Mercury and Venus).  But, beyond that, the “tilting process” seems to be stochastic (i.e. produced by various random events, such as collisions and gravitational interactions among planets, particularly with large Jupiter size planets).

It seems likely that a planet’s obliquity can also change over time due to shifts in the planets inclination with the orbital plane (i.e. plane of the ecliptic, in the case of our solar system).  Again, gravitational Interactions with other (generally large) planets can be the cause of this, as the illustration shows.

It is still an open question, as to whether Earth’s orbital tilt has changed by very much over its history.  On the one hand, a varying tilt might explain some very large scale glaciation during Precambrian times.  On the other hand, modelling shows that the Earth’s large moon may have kept its tilt very stable during the history of the solar system.

So, it seems possible for a planet to start off with just about any obliquity, and to then evolve to almost any other obliquity.  The paper by Armstrong et al delves into that in detail, modelling a number of possible planetary systems.

The idea is to see how an Earth-like planet’s tilt would evolve over time, assuming different configurations of companion planets, varying the size and location of these, relative to the Earth-like planet (which is assumed to initially be the same mass as Earth, be the same distance from its sun, and have the Earth’s present tilt and orbital eccentricity).

Basically, they set up these model solar systems in a computer, and allowed them to gravitationally interact with each other (and with their sun, of course), to see how the obliquity of the Earth-like planet evolves over long time scales (100 million years).  Standard equations related to orbital mechanics were applied within the model, and various checks were made to ensure that numerical inaccuracies didn’t creep into the model.  They also observed how the eccentricity of the orbit evolved (how circular it is), along with some other orbital parameters.

The baseline model uses actual values for Earth, Jupiter and Saturn.  In this case, the model shows relatively small periodic or quasi-periodic variations in these orbital parameters.  In particular, the planet’s tilt only varies within a small range, between about 20.5 degrees and 24.5 degrees.  Interestingly, this model uses a moonless Earth, which supports the notion that the Earth might not need a large moon to keep its planetary tilt stable, contrary to some earlier theorizing on the subject.

A second model, where the Earth-like planet is bracketed between two super-Earths, with 10 Earth masses each.  All planets have fairly eccentric orbits and are somewhat highly inclined to the ecliptic.  The produced very different results, with the tilt of the Earth-like planet eventually evolving to 90 degrees.  Such a planet would have the polar regions pointed directly at the sun in mid-summer for each hemisphere (the sun would be high in the sky at the north pole for example) and the equatorial zones would be dark at that same time.  The effect on the climate, which we will look at later, would be extreme.

A third model shows the Earth-like planet exhibiting a periodic planetary tilt, varying from about 60 degrees to 110 degrees (somewhat overturned), over time scales of about 18 million years or so.  In this case, the system has two very large planets far from the Earth-like planet.  All planets are fairly far from the plane of the ecliptic (between 10 and 20 degrees).

The paper shows a few more cases of planetary initial conditions that show other patterns in the Earth-like planets obliquity, including very rapid shifts between 10 and 60 degrees, on the order of a million or so years per period.  Another shows periodic orbital tilt shifts between 20 and 90 degrees, with the period of oscillation being about 30 million years.

So, this modelling shows that planetary tilts can vary drastically, from straight up and down to overturned.  Furthermore, they can remain stable for hundreds of millions of years, or swing wildly back and forth over short or long periods of time.  Basically, all kinds of scenarios are possible, depending on initial conditions such as mass of planets within the system, their distances from the star, the inclination of their orbits, and how circular the orbits are.

Now that we know that exoplanets might well have very low obliquities, as well as very high obliquities, we can go on to see how that factor affects the expected climate of a planet, and whether that climate is conducive to life as we know it (i.e. in the habitable zone of the star, where water can be found in liquid form for reasonably long periods of time).  The next blog will look at those issues.   

Sources:

Extraordinary climates of Earth-like planets: three-dimensional climate simulations at extreme obliquity.  Darren M. Williams and David Pollard, International Journal of Astrobiology 2 (1) : 1–19 (2003)

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets. J.C. Armstrong, R. Barnes, S. Domagal-Goldman, J. Breiner,2 T.R. Quinn, and V.S. Meadows, ASTROBIOLOGY Volume 14, Number 4, 2014

Four climate regimes on a land planet with wet surface: Effects of obliquity change and implications for ancient Mars.  Yutaka Abe , Atsushi Numaguti, Goro Komatsu, Yoshihide Kobayashi, Icarus 178 (2005) 27–39

Habitable Zone Limits for Dry Planets.  Yutaka Abe,1 Ayako Abe-Ouchi,2 Norman H. Sleep,3 and Kevin J. Zahnle4, ASTROBIOLOGY Volume 11, Number 5, 2011

The Chaotic Obliquity of the Planets.  J. Laskar & P. Robutel, Nature Vol 361 Feb 1993

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Now that you have read about the scientific prospects for exoplanet climates, you should consider reading some interplanetary Science Fiction.  How about a short story, that features one possible scenario to explain why we haven’t met ET yet (as far as we know, anyway).  Only 99 cents on Amazon.

The Zoo Hypothesis or The News of the World: A Science Fiction Story

Summary

In the field known as Astrobiology, there is a research program called SETI, The Search for Extraterrestrial Intelligence.  At the heart of SETI, there is a mystery known as The Great Silence, or The Fermi Paradox, named after the famous physicist Enrico Fermi.  Essentially, he asked “If they exist, where are they?”.

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The Magnetic Anomaly

Summary

An attractive woman in a blue suit handed a dossier to an older man in a blue uniform.

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