“They’re out there,” goes the common quip about extraterrestrials. It would seem more likely to be true in light of a 2019 study by astrophysicists at the Georgia Institute of Technology who modeled the evolution of axial tilts of Earth-like analogs in different star systems, including binary stars. Earth’s axial tilt varies only slightly, an important ingredient for climate stability that favors the evolution of complex life. Of all their simulated Earth analogs with axial tilts similarly steady to Earth’s, the astrophysicists found that 87% of them were in binary star systems.
“Multiple-star systems are common, and about 50% of stars have binary companion stars. So, this study can be applied to a large number of solar systems,” said Gongjie Li, the study’s co-investigator at Georgia Tech’s School of Physics.
Earth’s Axis Tilt vs Mars
The researchers started out contrasting how the Earth’s axis tilt, also called obliquity, varies over time with the variation of Mars’ axis tilt. Whereas our planet’s mild obliquity variations have been great for a livable climate and for evolution, the wild variations of Mars’ axis tilt may have helped wreck its atmosphere, as explained in the section below.
Our Nearest Neighbor Nixed
Then the researchers modeled Earth into habitable, or Goldilocks, zones in Alpha Centauri AB – our solar system’s nearest neighbor, a binary system with one star called “A” and the other “B.” After that, they expanded the model to a more universal scope. “We simulated what it would be like around other binaries with multiple variations of the stars’ masses, orbital qualities, and so on,” said Billy Quarles, the study’s principal investigator and a research scientist in Li’s lab. “The overall message was positive but not for our nearest neighbor.”
Alpha Centauri A actually didn’t look bad, but the outlook for mild axis dynamics on an exo-Earth modeled around star B was wretched. This may douse some hopes because Alpha Centauri AB is four lightyears away, and a mission named Starshot with big-name backers plans to launch a space probe to look for signs of advanced life there.
No exoplanets have been confirmed around A or B; an exoplanet has been confirmed around the nearby red dwarf star Proxima Centauri, but it is very likely to be uninhabitable.
Even with its ice ages and hot phases, Earth’s climatological framework has been calm for hundreds of millions of years – in part because of its mild orbital and axis-tilt dynamics – allowing evolution to take big strides. Wildly varying dynamics, and thus climate, like on Mars would stand to regularly kill off advanced life, stunting evolution.
Earth’s orbit around the sun is on a slight incline that seesaws gently and very slowly through a slight precession, a kind of oscillation. As Earth revolves, it shifts position relative to the sun, circling it a little like a spirograph drawing. The orbit also precesses in shape between slightly more and slightly less oblong over 100,000-year periods.
Earth –The Exo Model?
Earth’s axis tilt precesses between 22.1 and 24.5 degrees over the course of 41,000 years. Our large moon stabilizes our tilt through its gravitational relationship with Earth, otherwise, bouncy gravitational interconnections with Mercury, Venus, Mars, and Jupiter would jolt our tilt with resonances.
“If we didn’t have the moon, Earth’s tilt could vary by about 60 degrees,” Quarles said. “We’d look maybe like Mars, and the precession of its axis appears to have helped deplete its atmosphere.” Mars’ axis precesses between 10 degrees and 60 degrees every 2 million years. At the 10-degree tilt, the atmosphere condenses at the poles, creating caps that lock up a lot of the atmosphere in ice. At 60 degrees, Mars could grow an ice belt around its equator.
Hope at the Cosmic Scale
In Alpha Centauri AB, star B, about the size of our sun, and the larger star, A, orbit one another at about the distance between Uranus and our sun, which is very close for two stars in a binary system. The study modeled variations of an exo-Earth orbiting either star but concentrated on a modeled Earth orbit in the habitable zone centered around B, with A being the orbiting star.
A’s orbit is very elliptical, passing close by and then moving very far away from B and slinging powerful gravity, which, in the model, overpowered exo-Earth’s own dynamics. Its tilt and orbit varied widely; adding our moon to the model didn’t help. “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.
Even without a moon and with mild axis variability, complex, Earthlike evolution would seem to have a hard time on the modeled exo-Earth around B. “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 1 million years, be worse, and last much longer,” Quarles said.
“Tiny Sweet Spot”
But a sliver of hope for Earth-like conditions turned up in the model: “Planetary orbit and spin need to precess just right relative to the binary orbit. There is this tiny sweet spot,” Quarles said.
When the researchers expanded the model to binary systems in the universe, the probability of gentle obliquity variations ballooned. “In general, the separation between the stars is larger in binary systems, and then the second star has less of an effect on the model of Earth. The planet’s own motion dynamics dominate other influences, and obliquity usually has a smaller variation,” Li said. “So, this is quite optimistic.”
In an email to the Daily Galaxy, Li wrote, “Stellar binary companions influence the orbits of the Earth-like planets, which leads to spin-orbit resonances and obliquity variations. We found that an Earth-like planet would experience lower (in comparison with Earth) obliquity variations for around ~70% of solar-type binaries. This could imply more severe climate variations due to obliquity variations for some of the alien worlds, but the constraints on the development of advanced civilization are complicated and are beyond the scope of our study.”
Since 2019, Quarles, Li, and Lissauer have continued their research of precession and dynamical effects on Earth-like analogs in Alpha Centauri-like binaries. In an email to the Daily Galaxy, Quarles wrote, “We turned to simple climate models that use energy balance, which is the subject of our most recent work. A preprint is available but the manuscript is currently under review at MNRAS and some details may change after it is accepted for publication.”
Impact of Ubliquity Variations
When asked about how obliquity variations –such as the current Holocene epoch on Earth–might affect the development of advanced civilizations, Quarles responded in an email to The Daily Galaxy, “Questions regarding the Holocene or development of an advanced civilization make a series of assumptions on the longevity of civilization despite the environmental influences. During the 70s and 80s, it seemed uncertain whether humankind would destroy itself and that civilization might be highly transient. Disregarding that issue, our new work shows that very long transitions from low to high obliquity can lead to snowball Earths (complete ice coverage), where milder variations (but still large compared to Earth’s) can lead to oscillating states for the distribution of ice on the surface (polar caps or an ice belt).”
The researchers published their study, which was co-led by Jack Lissauer from NASA Ames Research Center, in Astrophysical Journal on November 19, 2019, under the title: “Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries.” The research was funded by the NASA Exobiology Program.
Image credit: NASA/ESA Hubble Space Telescope