This Intriguing Signal From Alpha Centauri May (or May Not) Be a Planet

Could the star system closest to us host a habitable super-Earth?

The Centauri system: Alpha Centauri A is the bright star to the left, Alpha Centauri B is the bright star to the right, and Proxima Centauri is circled in red.

A team led by Kevin Wagner from the University of Arizona has detected a signal hinting at the presence of an exoplanet orbiting Alpha Centauri A—part of the triple-star system nearest our sun, along with Alpha Centauri B and Proxima Centauri.

A planet already was known to exist in the habitable zone around Proxima Centauri, but that world, circling an M dwarf star, does not seem amenable to life. Alpha Centauri A is a G dwarf similar to our own Sun. The newly detected signal originates from within the star’s habitable zone, and implies the presence of a planet roughly five to seven times the size of Earth, orbiting Alpha Centauri at a distance of about one to two times Earth’s distance from the Sun.

The discovery was announced by the NEAR (New Earths in the Alpha Centauri Region) program, which is led by the European Southern Observatory (ESO) and conducted in collaboration with Breakthrough Initiatives. NEAR uses adaptive optics on a large ground-based telescope to compensate for distortions due to the atmosphere, along with other methods to reduce noise in the data. It also requires a coronagraph to block out most of the host star’s light in (thermal) infrared wavelengths. That way, any heat signature from an orbiting exoplanet can be more easily detected. The program has logged more than 100 hours observing the Centauri system since it began in May 2019.

Wagner emphasizes that it’s not certain the new signal is coming from an exoplanet. It could also be a huge dust cloud, and more observations are needed. Even if verified, the signal most likely comes from a gaseous Neptune-type planet rather than a large, rocky planet much bigger than Earth.

However, if there is indeed a super-Earth in the habitable zone of Alpha Centauri, the implications are tantalizing. A planet of this type and size in a Mars-like orbit (roughly one and a half Sun-Earth distances out) could be even more hospitable to life than Earth. Today Mars is only borderline habitable—too small, and with too little internal energy to have sustained its original magnetosphere and atmosphere. If Mars had been the size of Earth, or preferably larger, it most likely would have held on to its early oceans and thick atmosphere, making it much more suitable for life.

In their concluding paragraph, the authors suggest that we may not need to wait long for an answer, as we’re approaching the technical capability to directly image a habitable zone super-Earth orbiting Alpha Centauri. All we need is a factor of two improvement in resolution, or a four-fold improvement in detecting dim objects. I would bet we achieve that very soon.

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