Looking at how the axis of planets affect their climate could help us better understand which exoplanets may be inhabited by alien life.
Researchers at the Georgia Institute of Technology recently developed computer models simulating how the climate of Earth might behave if our planet were placed in an alien solar system containing a pair of stars. By carrying out this study, researchers hope to learn more about which planets around other stars (exoplanets) are most likely to develop, and sustain, life.
Binary stars are more common throughout the galaxy than solar systems like our own, containing just a single star like our sun. Therefore, the team wanted to examine how planets in such a planetary system might behave, given the complex gravitational forces at action.
“Most Sun-like stars have at least one stellar companion and the habitability of circumstellar exoplanets is shaped by their stellar companion,” researchers explain in an article published in The Astrophysical Journal, detailing their study.
One of the vital characteristics of a planet is the tilt of its axis, called obliquity. Earth’s tilt changes slowly over time. On Mars, this rotation is much greater, and happens more quickly, than what our own world experiences.
Billions of years ago, Mars had a significant atmosphere, allowing the formation of vast seas of liquid water on its surface. Although the reasons for Mars turning into a vast frozen desert remain unclear, this extreme variation of the planet’s axis could play a significant role in ending the temperate era of Martian history, as wild climatic swings killed off any lifeforms on the Red Planet.
Over a cycle of 41,000 years, the axial tilt of Earth varies between 22.1 and 24.5 degrees relative to the Sun (we are currently around 23.5 degrees tilt). This precession is moderated by the gravitational influence of the Moon, without which the axial tilt of Earth would be rocked by resonances from nearby planets.
Despite ice ages and other short-term changes around the globe, the shallow variations in the axial tilt of Earth have moderated our climate, encouraging the development of life on our own world.
“If we didn’t have the moon, Earth’s tilt could vary by about 60 degrees. We’d look maybe like Mars, and the precession of its axis appears to have contributed to a loss of atmosphere,” Billy Quarles, research scientist in the Center for Relativistic Astrophysics at Georgia Institute of Technology, explained.
The axial tilt of Mars can oscillate between 10 and 60 degrees over the course of a two-million year cycle. At its lower limit, ice forms near the poles, as it does today. But at a 60-degree tilt, ice can form in a belt around the Martian equator.
What if Earth Lived Next Door?
The research team began their investigation by modeling what conditions would exist on Earth if our planet were orbiting around one of the stars of Alpha Centauri, the binary star system closest to Earth (there is also a third member, Proxima Centauri, but its orbit is too far-flung to affect the study).
“We chose the alpha Centauri system because the main star is similar to the Sun, the orbit of the stars are fairly well known, and the system has public appeal by being close to us,” Quarles tells The Cosmic Companion.
One of two main stars of the Alpha Centauri system consists of a larger star, A, and a smaller partner B (roughly the size of the Sun). This study concentrated on the orbital dynamics of a virtual Earth circling B, while A orbited with the system, influencing gravitational interactions between the bodies.
While a stable orbit may develop on a twin of Earth orbiting Alpha Centauri A, the same conditions are not likely to form for worlds unfortunate enough to accompany its companion, Alpha Centauri B.
The orbit of A is highly elliptical, producing a wildly-erratic gravitational effect on the system. Adding a Moon to the virtual Earth made the eccentricity of the planet even worse, just the opposite effect of what we see in our own planetary system.
“Around Alpha Centauri B, if you don’t have a moon, you have a more stable axis than if you do have a moon. If you have a moon, it’s pretty much bad news,” Quarles said.
This finding was the biggest surprise of the study, the planetary dynamicist recalls.
Even without such a Moon, fluctuations of the axis of such a planet may be too extreme for life to flourish on a world orbiting Alpha Centauri B.
“The sun, with all those planets revolving around it and dependent on it, can still ripen a bunch of grapes as if it had nothing else in the universe to do.”
— Galileo Galilei
“The biggest effect you would see is differences in the climate cycles related to how elongated the orbit is. Instead of having ice ages every 100,000 years like on Earth, they may come every one million years, be worse, and last much longer,” Quarles described.
There is ALWAYS an Evil Twin
Following simulations of an Earth-like planet in the Alpha Centauri system, the team expanded their investigation to include other worlds, in other systems.
“We simulated what it would be like around other binaries with multiple variations of the stars’ masses, orbital qualities, and so on,” Quarles stated.
The study concluded that roughly 87 percent of the virtual Earths studied would develop an axial tilt similar to that seen on Earth, raising the hopes that life may be sparked and evolve on distant exoplanets.
The distance between Alpha Centauri A and B is surprisingly close — approximately the distance between our Sun and the planet Uranus. In other systems, distances between stars are usually greater, meaning gravitational influences between stars are lessened, allowing planets to maintain their axial tilt. This bodes well for the search for life around the Cosmos.
Astronomers currently know of more than 4,000 planets orbiting other worlds, and thousands more await confirmation. So far, no exoplanets have been confirmed in the Alpha Centauri A or B. One world is known to orbit Proxima Centauri, but that planet is not likely to be habitable.
This study is an example of science at its purest — as Quarles remembers, “We simply wondered what would happen if we were in a binary star system.”
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