Earth currently takes about 24 hours to complete its day — that is, a rotation on the planet's axis. However, Earth once spun far more quickly, perhaps taking only 2 to 3 hours to complete a day. The gravitational pulls of the sun and moon helped slow Earth's spinning over billions of years to its current speed, an effect known as tidal braking. Earth's spin continues to slow, with the planet's day increasing by about 1.8 milliseconds per century.
Previous research has found that a variety of different factors can also speed up and slow down how fast Earth whirls. For example, prior work found that rising sea levels from melting glaciers can shift Earth's axis, increasing the rate at which the planet spins.
Earth's atmosphere may also affect the length of its day. "It is surprising, but Earth's atmosphere is about 50 trillion metric tons in mass, and so over long enough timescales — hundreds, thousands, even millions of years — all of that mass, and its drag across the surface of the planet, can have an effect," said study author Caleb Scharf, director of astrobiology at Columbia University in New York.
"Imagine, for example, if you could magically spin up the entire atmosphere so there were hurricane-force winds everywhere for centuries, all blowing in the same direction," Scharf told Space.com. "It would gradually, through drag and friction, have an effect on the rotating solid sphere of the planet."
Of course, the actual effects of atmospheres on planets "are much, much less dramatic, but again, over geological timescales, they can matter and they can counter the effects of things like lunar and solar gravitational tides," Scharf said. [Earth Quiz! Do You Know Our Planet?]
As the world turns
The amount of warming or cooling a planet's atmosphere experiences might also influence the length of the planet's day.
"As a star heats a planet like Earth, the atmosphere responds by altering its pressure — hot air expands, cool contracts, so in a nutshell, you end up moving the mass in the atmosphere around on a daily basis on really large scales," Scharf said. "That means that the mass in the atmosphere is no longer uniformly spread around the planet, and that provides a handle, if you will, like a big wrench, for gravitational forces from the star or moons to pull on the atmosphere."
The pull from the stars or moons on the atmospheres of planets is usually a tiny effect. However, sometimes the rate at which a star heats up a planet's atmosphere can "resonate" or reinforce the rate at which the atmosphere vibrates, just like an opera singer can hit the right note to make a champagne glass resonate and shatter, Scharf said. "When that happens, the mass of the atmosphere bunches up much more, and the wrench for gravity gets a lot bigger," he said. [10 Exoplanets That Could Host Alien Life]
For Earth, "we think this may have happened when the day length was about 21 hours," Scharf said. "The wrench effect, or torque, could have stalled Earth's rotation from slowing down from the moon's pull, perhaps for hundreds of millions of years."
Scharf noted that life can influence atmospheric chemistry by emitting gases such as oxygen. These gases can in turn affect how atmospheres warm and cool, and Scharf calculated that this can have an impact on a planet's rate of spin.
"The possibility that biology, or a biosphere, could conceivably influence the rotation of a planet by altering the atmospheric composition — that's crazy! But it seems that it's not impossible," Scharf said.
Life finds a way?
Life could influence the speed of planetary spin through various mechanisms, Scharf said. For example, ultraviolet light can generate ozone from oxygen gas. Ozone "is really good at absorbing sunlight and heating the atmosphere," Scharf said. "Imagine a planet where oxygen-producing life has started up, and it's still spinning fast. As ozone forms, it might 'tune' the atmosphere so that the resonance kicks in sooner, and that will act against the normal slowing down of a planet's spin."
Scharf admitted that there was a great deal of uncertainty regarding how great an effect light might have on planetary rotation. "What I've done is just lay out a plausible 'what if' scenario, with some educated guesses for the numbers," he said.
"Almost every discovery in exoplanet science during the last few decades has challenged or expanded our view of planets," said Brian Jackson, a planetary scientist at Boise State University, who did not take part in this research. Scharf's proposal "introduces yet another new and exciting idea — that by changing a planet's atmosphere, biology itself can influence a planet's rotation," Jackson said. "Although there are an enormous number of uncertainties involved, the hypothesis is very interesting and worth exploring more."
Future work should focus on using 3D computer models to simulate planetary climates and see if life might have these conjectured effects, Scharf said. Then, researchers might attempt to look at the rotation rates of real planets and see if any of these match those of simulated inhabited worlds.
However, measuring exoplanet rotation rates is a big challenge, Scharf said. If there are persistent features on planets such as large cloud-filled storms or ice fields, these might show up as variations in light reflected off these worlds, "and those variations could give us clues to rotation rates," he said. Although these variations are likely difficult to detect, "we've also surprised ourselves before by coming up with clever new techniques to study exoplanets," Scharf said.
Scharf submitted his findings online Nov. 27 to the journal Astrobiology.