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.
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?”.
Some quite cogent arguments maintain that if there was
extraterrestrial intelligence, they should have visited the Earth by now. This
story, a bit tongue in cheek, gives a fictional account of one explanation for
The Great Silence, known as The Zoo Hypothesis.
Are we a protected species, in a Cosmic Zoo? If so, how did this come about? Read on, for one possible solution to The
Fermi Paradox.
The short story is about 6300 words, or about half an hour
at typical reading speeds.
Alternatively,
consider another short story, this one an alien invasion story set in the
Arctic. Also 99 cents.
The Magnetic Anomaly
Summary
An attractive woman in a blue suit handed a dossier to an
older man in a blue uniform.
“Give me a quick recap”, he said.
“A geophysical crew went into the Canadian north. There were some regrettable
accidents among a few ex-military who had become geophysical contractors after
their service in the forces. A young man and young woman went temporarily mad
from the stress of seeing that. They imagined things, terrible things. But both
are known to have vivid imaginations; we have childhood records to verify that.
It was all very sad. That’s the official story.”
He raised an eyebrow. “And unofficially?”
“Unofficially,” she responded, “I think we just woke something up that had been
asleep for a very long time.”
