Using the James Webb Space Telescope, astronomers may have directly imaged a Saturn-mass gas giant in the habitable zone of a star in the solar system next door. While the planet itself isn’t habitable to life as we know it, moons around it could be.
Alpha Centauri is a triple-star system, made up of a pair of close-orbiting, Sun-like stars (A and B) as well as the red dwarf Proxima Centauri, the closest star to the Sun.
Astronomers have already found three planets swirling around Proxima Centauri, but now they may have spotted one circling Alpha Centauri A as well. The results are reported in two papers that will appear in The Astrophysical Journal Letters (paper 1, paper 2).
Colin Stuart
Fascinating discovery, particularly because α Centauri AB is a pretty tight binary, with the minimal distance between the stars about the same as the distance between Saturn and the Sun in our own solar system. Finding a planet in a stable orbit around one of these stars would further support the idea that planets can form in challenging stellar configurations and so are basically ubiquitous in the universe. As of now though, the papers are based on a single visual detection – and two negative follow-up observations – correlated with another 2019 sighting from the European Southern Observatory’s Very Large Telescope, so the uncertainty remains high and we can expect the results to be revised considerably based on future observations of the system.
As the closest star system with Sun-like stars, α Centauri was featured in several science fiction stories. Possibly the most famous by now is the movie Avatar, where humans land on Pandora, an Earth-like habitable extrasolar moon in this system. The proposed planetary candidate orbits in the star’s habitable zone, and it would not be that farfetched to expect it to have a moon system as well, which should be stable over astronomical timeframes at that separation from the star.
On second thought, we should probably wait to get the planet’s refined orbit. The current expectation is for a mildly eccentric and inclined orbit, which would expose it to periodic swings of the orbital elements via Kozai resonances with the outer star α Centauri B, which in turn could disrupt its moon system, possibly ejecting outer moons or causing collisions between them. This issue aside, I doubt any moon could grow as large as Pandora around a mere Saturn-sized giant planet. Saturn’s largest moon Titan is after all barely 2.5% of Earth’s mass, and it potentially had a lot more material available during its formation.
Speaking of changing orbits, the other science fiction work centered around α Centauri is Liu Cixin’s novel The Three-Body Problem, which was recently adapted into a series by Netflix. I should have probably written a review of its first season by now, but to keep it short: while the core idea is sound – a planet in the habitable zone around either of these two stars would experience more extreme orbital and thus climate extremes that Earth, causing frequent upheavals on the surface and possibly civilization-ending disasters – the way it’s portrayed on screen quickly becomes implausible. One scene depicts people floating up in the air supposedly because of gravitational swings; needless to say, if you float off the planet’s surface like that, the atmosphere and water would be gone along with you, and life would not restart after the planet settles into a more stable orbit.
The series – and I presume the book at its origin as well – grossly misunderstands orbital dynamics: if an orbit became so chaotic as to swing by one of the stars, the body would not survive in that state for much longer and it would most likely get ejected from the star system altogether. If the configuration was so unstable, it raises the question how life and civilization managed to arise under such extreme conditions over billions of years. The longer you think about it, these contradictions make less sense.
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