Tilt-A-World: Another Constraint on Habitability
Did you ever ride a Tilt-A-Whirl, one of those cheap carnival rides that makes you dizzy and sick? Our planet would be like that (in slow motion) if its inclination were out of control. Without tilt stability, a new study reveals, we wouldn’t be sick, we’d be dead, or never alive in the first place. It’s not enough to be in the Habitable Zone. Would-be inhabited planets need to avoid a new problem, called “tilt erosion.”
The new constraint on habitability is described in an article by Adam Hadhazy on NASA’s Astrobiology Magazine, “Loss of Planetary Tilt Could Doom Alien Life.” Astronomers considering the factors needed to sustain life already knew that inclination was important. They knew that it provides for alternating seasons, distributing the temperate zones so that the equator is not eternally hot and the high latitudes eternally frozen. They also knew that red dwarf stars (the majority of stars), with their narrower habitable zones closer in, tend to tidally lock one face of a planet toward its star, dramatically reducing its habitable real estate (2/09/2006).
The new study by René Heller, a postdoctoral research associate at the Leibniz Institute for Astrophysics in Potsdam, Germany, shows that stable inclination is far more important for life than previously assumed. Cheerful astrobiologists envisioning life everywhere are going to have to worry about this new constraint; “The findings do not bode well for planets residing in the habitable, or ‘Goldilocks’ zones around red stars smaller than the Sun,” the article said. Here’s a summary of Heller’s findings and the implications:
- Time limit: “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.”
- Far out: Habitable planets around sun-like stars suffer far less tilt erosion. So far, so good – provided they have at least 5 degrees of tilt. If not, watch out:
- Gasping for air: “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.”
- Zero tolerance: All planets suffer tilt erosion, including Earth. “Over time, this mechanism forces the planet into a zero-obliquity equilibrium.” This limits the time available for life to originate and persist: “The length of a window of significant obliquity could be critical for the development of life.”
- Narrowing and harrowing: Most stars have too short an obliquity window: “For relatively cool, dim stars with less than half the Sun’s mass, the obliquity window becomes quite narrow.”
- Life stopwatch: Assuming that evolution needs more than a billion years for life to originate and evolve up to sentience, it’s never going to happen on worlds around stars less than 90% the sun’s mass. Red dwarfs have an obliquity window of about 100 million years; stars with 90% the sun’s mass might have a billion years before it’s too late, and the planet is forced to zero obliquity (see points 3 and 4). Heller said, “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.”
- Size doesn’t matter: Won’t larger planets fare better? Sorry; “The obliquities for ‘super-Earths’ – worlds several to 10 times the mass of the Earth – would also rapidly vanish around red dwarfs.”
- Lockout: “To make matters worse,” the article continued, tidal locking gets added to tilt erosion. One side becomes locked to face the star forever. “That side can become superheated and sterilized while the dark half of the planet enters a permanent, frozen night.”
- Jupiter’s wrath: A habitable planet too close to a gas giant suffers its gravitational blows. Thank God Earth is farther from Jupiter than 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.”
- Lucky moon? Our moon helps keep Earth’s obliquity stable for long periods, but that’s partly because we orbit a rare star “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.”
Overall, the article on NASA’s prime astrobiology website sounds pretty pessimistic for astrobiology (a science without a subject). Obliquity is coming into view as a far more critical factor for habitability than previously realized. Caleb Scharf, director of Columbia University’s multidisciplinary Astrobiology Center, who was not involved in Heller’s research, got off the Tilt-A-World onto solid ground when he said, “obliquity can certainly end up being the critical lynchpin of determining whether or not any part of a planet could be considered habitable.” All we know is that conditions are pretty darn good on the spinning globe on which we stand.
If you haven’t experienced a Tilt-A-Whirl, somebody recorded a stomach-eye view on YouTube. Enjoy. Ponder Earth on that kind of trajectory.
In The Privileged Planet documentary, Dr. Bijan Nemati (JPL) explained the combined probability of some 20 different factors needed to support life, using conservative values of 1 in 10 for each, at a thousandth of a trillionth. That makes another habitable planet unlikely in the Milky Way or local group of galaxies. This new factor might just lower the probability to make Earth unique in the local supercluster of galaxies, if not the whole universe. Let’s tally up the habitable zones astrobiologists have told us about:
- Galactic Habitable Zone, where a star must be located (09/29/2009);
- Circumstellar Habitable Zone, the right radius from the star (10/08/2010);
- Continuously Habitable Zone, because too much variety can be lethal (07/21/2007);
- Temporal Habitable Zone, because habitable zones do not last forever (10/27/2008);
- Chemical and Thermodynamic Habitable Zone, where water can be liquid (12/30/2003);
- Ultraviolet Habitable Zone, free from deadly radiation (08/15/2006);
- Tidal Habitable Zone, which rules out most stars that are small (02/26/2011).
- Stable Obliquity Habitable Zone (this entry)
Some of these probably overlap: for instance, a red dwarf will fail on the last two points due to the same forces eroding obliquity and tidally locking the planet’s rotation; a red dwarf will also subject its planet to flares and UV. It is still very clear, though, that getting all these factors just right requires winning a cosmic lottery. You can have an earth-like planet with a good moon orbiting a sun-like star, but if it is outside the Galactic Habitable Zone, no dice. Or you can have all the right combinations except a star that is less than 90% the sun’s mass, and tilt erosion will kick in, given enough time, to erase life before (using evolutionary assumptions), sentient beings could evolve. It’s like a combination lock; close enough is not good enough.
Note that the list of Habitable Zone factors ignores other requirements, like water, the right atmosphere, the right crustal composition, a global magnetic field, plate tectonics, the right moon at the right distance, and more. A planet could win the Habitable Zone lottery and still be lifeless if too volcanic (like Io), or shrouded in carbon dioxide and sulfuric acid (like Venus), or lacking surface carbon (see 05/21/2011).
Let’s face it: either we are pretty darn lucky, or Earth was designed for life, as the Creator spoke through the prophets (Isaiah 45:18). If you take the former view (luck), you have said good-bye to science. Why? Because sheer dumb luck is none other than the Stuff Happens Law (9/15/2008, 9/22/2009), the law of nature for science quitters (“Why is there life on Earth? Stuff happens”). It’s a hand-waving answer that provides no useful information or understanding.
When you combine extremely low probability (see our online book) with functional information (a planet that supports sentient life), you get intelligent design science. When you combine that with consciousness and intelligence, you get philosophy. When you combine that with revelation from the Creator, you get all the necessary and sufficient evidence you need to understand yourself, the world, and the universe. What’s the problem? Why would anyone wish to resist the only answer that makes sense of the world? Stop riding the Tilt-A-World theory of evolution, and stand on the solid rock (Matthew 7:13-29).