Throughout 2014, the overlapped trajectories of the two main stars of Alpha Centauri prevented us from observing them. As they’ve begun to separate, scientists have been able to begin searching our nearest stellar neighbor for any sign of a potentially superhabitable planet (a planet more habitable than Earth itself).
Multiple star systems, like Alpha Centauri, were long neglected in the search for potentially habitable planets. It was believed that the formation of two or more stars would hardly leave enough mass remaining to cohere into planets, and that even if those planets were created, the gravitational pull of a close second star would expel them from their orbits — either by shoving them out of the system or pulling them to fall into one of the stars. Since Kepler launched, we’ve found multiple binary stars with planets orbiting them. That means we now know binary or even tertiary star systems can form planets, even if the orbital mechanics are trickier than single-star configurations.
In 2008 a team of researchers from the Universities of California, San Francisco State and the SETI Institute published a study in The Astrophysical Journal showing their computer simulations of planets formation around Alpha Centauri B. For all the possible initial conditions studied, between 1 and 4 planets were created orbiting that star, of which almost half of them would reside in the habitable zone. These high odds of finding planets at Alpha Centauri seemed proven in 2012 when a European team led by Xavier Dumusque announced the detection of the planet Alpha Centauri Bb. Unfortunately, we haven’t been able to confirm Alpha Centauri Bb’s existence, since it’s right on the threshold of what our equipment can detect.
The confirmation of planets in the habitable zone of Alpha Centauri would be remarkable for two reasons. First, such a planet is potentially close enough, at just over four light years, for us to reach and colonize it if we used nuclear rockets (NASA has proposed a plan for accomplishing this, dubbed Project Longshot). Second, if the predictions are accurate, we should find a superhabitable planet within the Alpha Centauri system.
Let’s take a look at the simulations that predict a superhabitable planet in Alpha Centauri and the reasons scientists think they might find one there.
Formation of planets in Alpha Centauri B
The team from the University of California ran nine different computer simulations varying them within the different possible initial conditions expected for the Alpha Centauri system. The next figure shows one of those simulations – specifically the late evolutionary stage of a protoplanetary disk initially containing 600 moon-mass embryos. As you can see, due to the gravitational pull of its twin star the only stable orbits are within 2 A.U., and after 50 million years, any mass beyond 2 A.U. has been launched into highly eccentric orbits and either migrated inward to be accreted by inner bodies, collided with the central star, or was ejected from the system.
The figure below shows the results of the various simulations performed, showing the resulting planets for each initial condition, from a protoplanetary disk with only 400 moon-mass embryos, up to 900 embryos.
In all cases, the model predicts the formation of 1-4 planets, with 42% of them statistically likely to form within the habitable zone.
Earth is the only inhabited planet we know, so we tend to use it as the ideal reference in our search for habitable exoplanets. This has led to the Rare Earth hypothesis, which details the high number of unlikely circumstances that were necessary for life to appear on Earth, and concluding that complex life must be a rare phenomenon throughout the universe. But what if Earth is not a common habitable planet, in the same way that our solar system has proven not to be a common planetary system? Specifically, the solar system lacks a so-called super-Earth. These types of planets, 1-3x larger than our own, have proven to be common across the cosmos.
The intriguing question “what could make an exoplanet even more habitable than Earth?” was launched in a live chat by John Armstrong of the Weber State University. It inspired more research from René Heller of McMaster University, cataloging the list of properties that helps to make a world habitable, and studying what types of planets and moons fits better to those criteria. This study refutes the Rare Earth hypothesis, concluding that the Earth is only a marginally habitable world, as it needed so many unlikely circumstances for the emergence of life. A variety of processes exists that can make environmental conditions on a planet or moon more benign to life than is the case on Earth.
The habitable zone of a star is only a frame of reference, but we should not limit our searches to it. Tidal forces and the greenhouse effect can turn a habitable planet uninhabitable, or create a habitable world beyond the typical Goldilocks zone. Mars, for example, lies within the habitable zone of Sol, despite the fact that we have yet to detect life on it.
The moon Europa is a good example of a body far from the habitable zone made habitable by its tidal forces. Throughout the study, these habitable moons outside the Goldilocks zone are referred as super-Europas.
Next, the study examines the different factors affecting the evolution of life on a planet, identifying which conditions offer more likelihood to develop life than Earth itself. Factors such as the sizes of the planet and the star, continental distribution, ocean depth, the amount of water present, tectonic activity, variability of the surface temperature, atmospheric composition, the magnetic shield, speed of rotation, axial tilt, eccentricity of the orbit, the type and amount of radiation received, the age of the solar system, and the possibility of panspermia within the system are all considered.
All in all, the researchers concluded superhabitable worlds will tend to orbit orange dwarfs, and be slightly older and two to three times more massive than Earth. This matches perfectly with Alpha Centauri B and the planets expected there by our simulations — making it the ideal target to search for a superhabitable world.
Detecting planets in Alpha Centauri B
It may seem odd that we are able to detect thousands of exoplanets on systems hundreds of light years away, and yet we cannot confirm the existence of planets in our nearest star, just four light years from us.
Exoplanets can be detected directly or indirectly. So far the direct detection has only been able to show giant exoplanets, several times larger than Jupiter and orbiting at great distances from their stars. We will have to wait for the James Webb Space Telescope in October 2018 to be able to directly visualize smaller planets closer to their stars.
Indirect detection of exoplanets has two main methods: detection by mass transit or by radial velocity. The majority of confirmed exoplanets have been detected by the method of mass transit, which detects the decrease in brightness of a star every time the orbit of a planet passes in front of its star, partially covering it. This means that only exoplanets with orbits that moves them to pass directly between its star and us can be detected with this method. In the case of Alpha Centauri, it has been estimated that there is only a 30% chance that its planets would be visibly aligned and detectable through mass transit.
A recent survey by the Hubble telescope didn’t detect the planet Alpha Centauri Bb by this method — though this doesn’t necessarily mean that it doesn’t exist, but that its orbit would not transit between the star and us. Instead, another transit was detected that seems to pertain to an Earth-size planet on a farther orbit than Alpha Centauri Bb. More observations are needed to confirm the planet’s existence.
Finally, there is the exoplanet detection by the method of radial velocity. This method seeks to detect the slight wobble of the star motivated by the gravitational pull of a planet as it orbits around the star. Again the difficulty lies in detecting small exoplanets at far orbits, since giant exoplanets and close orbits exert much larger gravitational pulls over the star and create easily visible oscillations.
Surveys applying this method have ruled out the existence of gas giants in Alpha Centauri. But they have been unable to provide definitive answers about the existence of Earth-size planets, because their gravitational pull are on the threshold of what our instruments can detect. More precise instruments are being installed at the moment, such as the SPRESSO that would be operational in 2016 at the Very Large Telescope in Chile with an accuracy of 10 centimeters per second (for comparison, the gravitational pull of the Earth over the Sun causes an attraction of 9 centimeters per second). These new instruments will be 10 times more accurate than the instruments currently used.
We are quickly closing in on the detection of rocky planets in Alpha Centauri. According to Debra Fisher, who leads a team at Yale working on detecting rocky planets in the system, both her team and one located in Geneva will use the next few years to develop innovative instruments with the goal of reaching 10 centimeter-per-second precision – a factor of ten gain over current precision. “The Geneva team is designing a high-resolution instrument, ESPRESSO, for the 8-meter telescopes at Paranal in Chile,” she said. “My team is designing EXPRES for the Discovery Channel Telescope. As the acronyms imply, we are both aiming for the extreme precision needed to robustly detect Earth-mass planets orbiting at habitable-zone distances.”
Once we have that capability, who knows what we’ll find?