In the past, the number of known exoplanets has exploded, with 4093 confirmed detections so far (and another 4,727 candidates awaiting confirmation). With the discovery of so many planets that are dozens, hundreds, or even thousands of light years away, a great deal of attention has understandably been directed to our nearest stellar neighbors. Could planets be right next door, with the possibility of life being there as well?

While a potentially-habitable planet was recently discovered around Proxima Centauri (Proxima b), Alpha Centauri remains something of a question mark. But thanks to a recent study from the Georgia Institute of Technology (GIT), we might be getting closer to determining if this neighboring system supports life. In a twist, the study revealed that one of the stars in the binary system is more likely to be habitable than the other.

The study, “Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries“, recently appeared in the Astrophysical Journal and was funded through the NASA Exobiology Program. The study was led by Billy Quarles, a research scientist with the Center for Relativistic Astrophysics, and included Prof. Gongjie Li of GIT’s Center for Relativistic Astrophysics and Jack Lissauer from NASA’s Ames Research Center.

Artist’s impression shows the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser

When it comes right down to it, individual stars that have a system of multiple planets (like our Solar System) are quite rare. Binary systems like Alpha Centauri, on the other hand, are quite common. This system is made up of Alpha Centauri A, a G-type yellow star that is slightly larger than our Sun, and Alpha Centauri B – a K-type orange star that is closer in size to our Sun.

In 2012, astronomers thought they had detected a candidate exoplanet orbiting around Alpha Centauri B (designated Alpha Centauri Bb). Unfortunately, subsequent analysis led astronomers to announce by 2015 that this was a false positive that was likely just a spurious “ghost” in the data analysis. A possible planetary transit was noted in 2013, but it was reportedly too close to its primary to support life.

To determine if Alpha Centauri could have any habitable planets orbiting them, the team of astrophysicists modeled a theoretical twin of Earth into a binary system. This consisted of contrasting how Earth’s axial tilt (aka. obliquity) varies over time with the variation of Mars’ axial tilt. They then modeled Earth into Alpha Centauri A and Bs circumsolar habitable zones (aka. Goldilocks zones).

While both planets are similarly inclined – 23.4° vs. 25.19° to their orbital plane – Mars’ obliquity has been subject to more change over time. And whereas the stability of Earth’s variations in obliquity over time has ensured a stable climate, Mars’ more pronounced variations have been a major factor in its transition from a warmer, wetter world to the cold and inhospitable place it is today.

The two brightest stars of the Centaurus constellation – (left) Alpha Centauri and (right) Beta Centauri. The faint red star in the center of the red circle is Proxima Centauri. Credit: Wikipedia Commons/Skatebiker

Basically, changes in Earth’s obliquity are what is responsible for Earth experiencing ice ages and warm epochs (aka. glacial and interglacial periods). However, the precession of Earth’s tilt is gentle and slow, varying between 22.1° and 24.5° over the course of 41,000 years. These types of long-term transitions have provided lifeforms with enough time to adapt and evolve, and have also prevented any period from being too long or extreme.

Mars’ axis, on the other hand, precesses between 10° and 60° every 2 million years. When tilted to 10°, the atmosphere condenses at the poles and causes both water vapor and carbon dioxide to solidify, making the ice expand. At a tilt of 60°, Mars would be more likely to grow an ice belt around its equator, where it is otherwise much warmer and experiences surface temperatures of up to 35 °C (95 °F) at midday during summer.

The presence of the Moon is also a factor since its gravitational pull helps to stabilize our axis. Were it not for the Moon, Earth’s gravitational interactions with Mercury, Venus, Mars, and Jupiter would cause wilder changes in our tilt. “If we didn’t have the moon, Earth’s tilt could vary by about 60 degrees,” Quarles said in a recent GIT news story. “We’d look maybe like Mars, and the precession of its axis appears to have contributed to a loss of atmosphere.”

While the study modeled variations of an exo-Earth orbiting either star, it mainly focused on an Earth-like planet orbiting in the habitable zone around B, with A being the orbiting star. While Alpha Centauri A did relatively well in this simulation, the results were not encouraging for that Alpha Centauri B – showing that an Earth-like exoplanet would unlikely be able to support life.

A comparison of Alpha Centauri A and B’s projected habitable zone. Credit: PHL/UPR Arecibo

In short, Alpha Centauri A and B orbit each other at about the same distance as Uranus and our sun, which is very close in a binary system. A’s highly-elliptical orbit with B causes it to pass close to B before moving far away, which generates a powerful gravitational sling. When modeled, this effect overpowered the exo-Earth’s own dynamics, causing its tilt and orbit to vary widely.

Even the presence of a large satellite such as our own Moon did not improve the situation for the exo-Earth. In fact, it actually made it worse since it contributed to axial instability. As Quarles explained:

“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.”

With these results in hand, the team then expanded their study to encompass more in the way of star systems. When it comes right down to it, single-star systems with multiple planets (like the Solar System) are actually quite rare. Meanwhile, multiple-star systems are common, with roughly 50% of stars in the known Universe appearing to have binary companions.

This artist’s impression shows an eclipsing binary star system. Credit: ESO/L. Calçada.

From this, the team determined that 87% of “Earth-like” exoplanets located in systems were likely to have axial tilts similar to Earth’s – which is stably inclined at 23.4°. Moreover, they found that with binary systems in the more general sense, the probability that plants would experience gentle precessions in their obliquity increased considerably. Said Prof. Li:

“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. So, this is quite optimistic.”

Still, bad news for Alpha Centauri, especially any planets that could be orbiting B. It’s also bad news for those hoping to send a mission there in the not-too-distant future to search for signs of life – such as Breakthrough Starshot. However, there was a sliver of hope to be found in the study since the model showed that a planet with the right kind of orbital mechanics could support life

“Planetary orbit and spin need to precess just right relative to the binary orbit. There is this tiny sweet spot,” said Quarles. “We simulated what it would be like around other binaries with multiple variations of the stars’ masses, orbital qualities, and so on. The overall message was positive but not for our nearest neighbor.”

It’s a sort of good news/bad news situation. While it is a little discouraging to think that Alpha Centauri may not have any habitable planets (which appears to be the case for Proxima b as well), it is good to know that 50% of stars in the known Universe have a shot at supporting life. In the end, finding extraterrestrial life (not to mention extraterrestrial intelligence) is all about numbers!

Further Reading: Georgia Tech