Loss of planetary tilt could doom alien life

Jan 12, 2012 By Adam Hadhazy
Earth's axial tilt of 23.5 degrees drives the seasons by varying the intensity of sunlight striking the planet's hemispheres, as seen in these pictures from the EUMETSAT Meteosat-9 satellite. Credit: NASA/EUMETSAT

Although winter now grips much of the Northern Hemisphere, those who dislike the cold weather can rest assured that warmer months shall return. This familiar pattern of spring, summer, fall and winter does more than merely provide variety, however. The fact that life can exist at all on Earth is closely tied to seasonality, which is a sign of global temperature moderation.

The driver of our seasons is the slight "lean" Earth has in its rotational axis as it revolves around the Sun, known as axial tilt or obliquity. According to René Heller, a postdoctoral research associate at the Leibniz Institute for Astrophysics in Potsdam, Germany, astrobiologists have not yet paid much attention to this variable in gauging the possibility for alien life to exist on distant planets.  

"Obliquity and seasonal aspects are an important issue in understanding exoplanet habitability that has mostly been neglected so far," said Heller. 

To address this gap, Heller and his colleagues published two papers recently looking at how the gravitational interactions of stars and planets eventually erode a planet's axial tilt. The findings do not bode well for planets residing in the habitable, or "Goldilocks" zones around red stars smaller than the Sun. These zones are the just-right temperature bands wherein water can remain liquid on the surface of a planet.

According to computer simulations, red dwarf stars quickly erase the axial tilt of habitable, Earth-like exoplanets. This temperature-moderating tilt is nullified in such a short time that life may never have a chance to get going. An exoplanet that fits this barren scenario is Gliese 581 d, usually considered one of the best candidates for life.

On the other hand, terrestrial planets around Sun-like stars fare much better. These worlds should not see their axial tilts erode to dangerously low levels until many billions of years down the road, well after life has arisen and possibly evolved into technological civilizations. A planetary abode representing this scenario is Kepler 22-b, the first near-Earth-sized world discovered in a habitable zone by NASA's planet-hunting Kepler space telescope.

A tale of the seasons

To envision obliquity, think of the tilt on a spinning desktop globe. Obliquity is measured as the angle that a planet's poles are offset from being perpendicular to the plane of the planet's orbit around a star. Earth's obliquity is presently about 23.5 degrees, although how it might have changed over geological time is a matter of debate. Planets get their obliquity from a number of factors, including impacts from objects early in a solar system's history, stars passing by and the gravitational influences of other planets.

Seasons arise from this tilt as follows: When a planet revolves around a star and spins on its axis, obliquity causes the intensity of sunlight reaching portions of the planet to cyclically vary. For instance, during the several months of the Northern Hemisphere's winter on Earth, the northern half of the planet is tilted away from the Sun. Rays of light strike the ground there at an angle and must travel through more atmosphere, thus diminishing the amount of delivered energy. The daily period of solar illumination is also shorter. Meanwhile, the Southern Hemisphere soaks up the long, warm days of summer before autumn's nippiness creeps back. 

The tidal deformation of Earth caused by the gravitational drag of the Sun, with the deformation highly exaggerated in this not-to-scale image. Credit: Heller et. al., Origins of Life and Evolution of the Biosphere, 2011

Overall, Earth's obliquity coupled with daily axial rotations bathe the world in a smooth distribution of temperatures. The peak highs and lows do not exceed about 200 degrees Fahrenheit in variance.

From pleasant to apocalyptic?

But take away the Earth's axial slant, and the place might become a lot less inviting.

With an obliquity of less than five degrees or so, an Earth-like planet's broader equatorial regions bear the full brunt of a sun's radiance. The polar regions also receive far less sunlight than they do with seasonal ebbs and flows. The result: extreme temperature gradients based on latitude. "Your equator is heated enormously while the poles freeze," said Heller. 

In theory, bands of habitability in temperate, mid-latitude zones could persist. In a worst-case scenario, however, the entire atmosphere of a zero-obliquity planet could collapse, Heller said. Gases might evaporate into space around the planet's blazing middle and freeze to the ground in the bleak north and south.

Life, had it ever emerged, would be stopped dead in its tracks.

Obliquity lost

Such a fate could have befallen Gliese 581 d, according to calculations by Heller and his colleagues.

They modeled how the gravitational dance between a star and a planet grinds away at any obliquity the planet might possess. This process, dubbed "tilt erosion," happens because a star's gravity pulls more on the side of the planet nearest it. That attraction deforms the planet into a slightly non-spherical shape, with tidal bulges pointed toward and away from the Sun. On Earth, we experience a similar effect, where the nearby Moon's gravity gives the Earth's oceans their tides. The misalignment between the two bodies' centers of gravity imparts a torque to the planet. "This torque tends to align the tidal bulge with the two centers of mass," explained Heller. Over time, this mechanism forces the planet into a zero-obliquity equilibrium. 

The length of a window of significant obliquity could be critical for the development of life. On Earth, it took in the ballpark of a billion years for bacteria to emerge; complex, sentient animals such as human beings took another nearly three and a half billion years to start drawing on cave walls.

For relatively cool, dim stars with less than half the Sun's mass, the obliquity window becomes quite narrow. That is because exoplanets must reside in tight, Mercury-like or closer orbits around red dwarfs in order to collect enough heat and sunlight to power biological beings. At these short distances, a star exerts strong tidal effects, Heller said. 

Lifeless under a red sun

As it turns out, for an Earth-like planet in the habitable zone of a star with a quarter of the Sun's mass, obliquity is eliminated in less than 100 million years. In fact, only terrestrial planets orbiting in the habitable zones of stars with about 90 percent of the Sun's mass can hang on to an appreciable obliquity for more than a billion years.

"We found that extrasolar terrestrial planets in the habitable zone of low-mass stars lose their primordial obliquities on time scales much shorter than life required to evolve on Earth," said Heller. 

Habitable planets might have too short a window for life to develop before gravitational interactions with a close, red star destroy obliquity, and therefore seasons. Credit: David A. Aguilar, CfA

The obliquities for "super-Earths" – worlds several to 10 times the mass of the Earth – would also rapidly vanish around red dwarfs.  The super-Earth Gliese 581 d orbits a red star with just 31 percent of the Sun's mass, and the system is reckoned to be perhaps twice the Sun's age at about 9 billion years. As a result, Gliese 581 d should have lost its axial tilt long ago.

To make matters worse for any beleaguered life forms, in the tilt erosion process a planet's spinning on its axis slows as well. Given enough time, besides losing its seasons, a world becomes "tidally locked" – that is, the same side of the planet constantly faces its . That side can become superheated and sterilized while the dark half of the planet enters a permanent, frozen night.

Optimally Earth-like

Yet for habitable planets around Sun-like stars, obliquity loss caused by tidal interactions with the star should not be a show-stopper. Compared to red dwarfs, the habitable zone is anywhere from two to three times farther out from a warmer, brighter star like our own, and at that distance tidal forces are much weaker, said Heller.

That means Kepler-22b, among the most Earth-like exoplanets yet found, might bask in its own version of the four classic seasons. "Our results suggest that this planet would still have its initial obliquity, and therefore could experience seasons," said Heller.

The example of our own planet would seem to bear this likelihood out, of course. But the picture is indeed a lot more complicated, Heller points out.

Quite a number of astronomical phenomena can alter the rotation of a planet on its axis, including the presence of a moon and the gravitational influences of other planets. In our solar system, the biggest bully on the block is Jupiter, whose gravity can disturb planet's axial tilts. Studies have suggested that our relatively large moon has balanced against this force, thereby limiting axial wobbling and preserving the planet's obliquity over long periods of time.

For an opposite case, consider Mars. Hulking Jupiter wreaks havoc with the Red Planet's obliquity, causing it to vary by perhaps as much as 60 degrees over the course of a million years, Heller said. Those disturbances lead to big swings in global temperatures and glacier cover, and on more habitable worlds that sort of climatic chaos could spell the end for life.

Yet big moons might not be a saving grace for habitable-zone, terrestrial worlds around red dwarfs. The habitable planet's necessary close proximity to a dim star could destabilize lunar orbits, said Caleb Scharf, director of Columbia University's multidisciplinary Astrobiology Center, who was not involved in Heller's research.

Getting a bead on tilt

Calculating the long-term gravitational interplay between astronomical bodies is a demanding process, even for fast computers. As such, Heller and his colleagues have limited their analysis to planets and stars, though three- and four-body simulations are in the works.

For now, knowing the true states of both Gliese 581 d and Kepler-22b will have to wait. Gauging an exoplanet's obliquity, especially a terrestrial-sized world's, remains a tricky feat given today's instrument technology. "The obliquity of exoplanets is essentially an unknown at this time," said Scharf.

Overall, understanding how obliquity is lost, gained and ultimately steers climates will continue to keep scientists very busy. "Obliquity is definitely a very important variable," said Scharf. Depending upon a range of other habitability factors, Scharf said that "obliquity can certainly end up being the critical lynchpin of determining whether or not any part of a planet could be considered habitable."

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antialias_physorg
5 / 5 (6) Jan 12, 2012
Seasonal variations can also occur due to elongated orbits. However, this isn't the prime factor on Earth for seasonality (and probably isn't for any other planet native to a solar system whose orbit hasn't been disturbed during its existence, since such orbits should be not too eccentric). but it does have some impact. (There are other factors, too. Average albedo is different during summer and winter because we have more landmass in the northern hemisphere. This more than counteracts the effect from being further away from the sun during northern summers so that the Earth is, on average, warmer when furthest from the sun as opposed to when it is closest in southern summer).
Mars does experience noticeable, global swings in the weather between aphel and perihel.

But a planet that gets 'caught' by a solar system on a highly elliptic orbit might well have seasonal changes even without any axial tilt.
thuber
4.4 / 5 (7) Jan 12, 2012
Evolution takes care of this. If you have constantly sunny regions or stable temperatures, you simply get differing biological systems for thermal regulation. Extrapolating life supporting conditions on earth as the only ones that can support life in the universe is at best simply sophistry.
Sean_W
3.7 / 5 (10) Jan 12, 2012
I don't think that a very good case has been made for the lethality of a non-tilted rotation. The mid section would still have night times and while it would be hotter than if tilted it would not become exponentially hot. The poles might not have the midnight suns they would have in summer due to a tilt but they would not be plunged into near total darkness for months either. There might be much stronger winds and sea currents from the temperature and pressure differences but these would serve to balance the energy difference. There seem to be a lot of assumptions required for the theory about tilt to be workable. It seems like the proposition of tilt being needed is adopted first and then they work backwards to try to justify it.
Modernmystic
1.3 / 5 (17) Jan 12, 2012
Extrapolating life supporting conditions on earth as the only ones that can support life in the universe is at best simply sophistry.


Indeed, I'm often perplexed at people who say "life should be common in the Universe" with a sample of 1.
EverythingsJustATheory
2.8 / 5 (4) Jan 12, 2012
One could argue that in a "perfectly stable environment" that seasonality (orbital tilt) could actually be detrimental to life, in that each species could more perfectly evolve to their environment if it remained constant throughout the year. However, when an event that altered climate did occur, life would be far less capable of adjusting to survive.

A loss of obliquity would mean stronger ocean currents and more violent hurricanes, as the planet attempts to reach equilibrium. There would also be issues with the hydrology cycle in most places, where snowmelt could not be replaced.

I also think it is naive to cubbyhole life based on the earth's weak anthropic conditions.
nkalanaga
5 / 5 (1) Jan 12, 2012
They claim that without obliquity, the atmosphere could be lost. Yet numerous studies over the last 15 years have shown that a sufficiently dense atmosphere, and it takes less than ours, will redistribute the heat even in a tidally locked planet with no tilt.

Earth's temperatures aren't moderated by the relatively slow change of seasons, but by winds and ocean currents.

With no tilt, there would be no "polar darkness", as the star would shine horizontally across the poles.
Myno
2 / 5 (2) Jan 12, 2012
If I understood the article correctly, it is the potential total loss of atmosphere that might be the life killer... not that I agree that an atmosphere is required for life.
Modernmystic
1.5 / 5 (13) Jan 12, 2012
Yet numerous studies over the last 15 years have shown that a sufficiently dense atmosphere, and it takes less than ours, will redistribute the heat even in a tidally locked planet with no tilt.


Interesting, can you list those studies? It would seem that wind speed would be a huge factor in distribution of heat on a tidally locked planet. Constant 300 mph winds would make for some interesting adaptations.
Davecoolman
1.4 / 5 (10) Jan 12, 2012
Great comments and informative.
Callippo
1.3 / 5 (6) Jan 12, 2012
Evolution takes care of this. If you have constantly sunny regions or stable temperatures, you simply get differing biological systems for thermal regulation.
The main problem is, fast paced evolution needs a frequent changes of life conditions. Without such a changes the organisms have no reason to mutate. Without tilt the terresterial life would suffer with strong polar atmospheric circulation and tornadoes in similar way, like the surface of Venus. The Earth surface would become very dry and inhabitable.
antialias_physorg
5 / 5 (3) Jan 12, 2012
Without such a changes the organisms have no reason to mutate

Organisms change their own environment by taking in nutrients and excreting waste (where do you think all that oxygen in our atmosphere came from? It wasn't there in the original planet's makeup). So even ecosystems which are not subject to external disturbances can change and be conductive to new mutations.

The simplest mutation would be: mutated form X learns to eat non-mutated form Y.
Predator-prey interrelations have caused numerous evolutionary cycles in felines: from speed to tougher claws/fangs (e.g. sabretooth tiger) due to heavier hides by prey animals back to speed as they prey adopted the "flee" approach again to get away from the now almost immobile predators.
HarshMistress
4.5 / 5 (2) Jan 12, 2012
From dailymail.co.uk article "The shrimp that lives in water four times hotter than boiling point":

A new species of shrimp has been discovered living deeper than any seen before in the world's most extreme deep sea volcanic vents.

British scientists made the discovery while on an expedition to explore boiling undersea springs - which may be hotter than 450C - on the Caribbean seafloor.

Some 5,000 metres down, in a rift in the seafloor, exists a volcanic spring known as a 'black smoker', which fires a jet of mineral-laden water more than a kilometre into the ocean above.

But despite the extreme conditions, the vents are teeming with thousands of a new species of shrimp that has a light-sensing organ on its back.

The pale shrimp congregate in hordes - up to 2,000 shrimp per square metre - around the six-metre tall mineral spires of the vents.

antialias_physorg
5 / 5 (2) Jan 13, 2012
The pale shrimp congregate in hordes - up to 2,000 shrimp per square metre - around the six-metre tall mineral spires of the vents.

They don't live IN the vent (where 450 degrees are reached) but AROUND the vent (where temperatures are still hot, but much lower)
Kafpauzo
not rated yet Jan 13, 2012
Even if we assume that the planet's atmosphere and oceans can't redistribute temperatures efficiently, and thus the planet has huge temperature differences between poles and equator, it seems to me that this only moves such a planet's goldilocks zone for carbon-and-water life a bit farther away from the sun.

If such a planet is farther away from its sun, the planet's poles will be far too cold for our type of carbon-and-water life. But, at the right distance from the sun, the zone around the equator could be just right.

Presumably the storms on such a planet will be far too harsh for Earth's trees, mammals etc. But I'm sure life evolving on such a planet will adapt just fine, even to very extreme storms, if that's what it takes to survive.
Modernmystic
1.3 / 5 (10) Jan 14, 2012
Even if we assume that the planet's atmosphere and oceans can't redistribute temperatures efficiently, and thus the planet has huge temperature differences between poles and equator, it seems to me that this only moves such a planet's goldilocks zone for carbon-and-water life a bit farther away from the sun.


That may not be the case though. It may be that not only can't temperatures be distributed efficiently. It may be that the inefficiency may cause the atmosphere on the "day side" to evaporate and freeze out on the "night side". Moving the planet further back means that the "day side" and "night side" have that much longer to be exposed to the sun or not...it doesn't solve the problem...it compounds it.
foolspoo
1 / 5 (2) Feb 05, 2012
"The fact that life can exist at all on Earth is closely tied to seasonality, which is a sign of global temperature moderation."

How can you write such nonsense and expect me to read this? Adam!?