A New Earth Awaits

Proxima Centauri b is the nearest exoplanet to us outside the Solar System, and it may be more habitable than previously thought.

Proxima Centauri b orbiting Proxima Centauri. © Ricardo Ramirez & James Jenkins (Department of Astronomy, Universidad de Chile)

4.244 light years away, the triple star system known as Alpha Centauri — our nearest stellar neighbor — a small exoplanet orbits the red dwarf star Proxima Centauri. This is Proxima Centauri b, and it may be the most habitable exoplanet we have evidence of.

At nearly 25 trillion miles distant, Proxima Centauri b is unfathomably far away from us at our current ability to travel in space. The current top speed reached by any spacecraft we’ve launched so far is about 38,000 miles per hour by Voyager 1, which was launched in 1977, left the Solar System in August of 2012, and is currently over 13 billion miles away from Earth.

The Breakthrough Starshot team, led by Yuri Milner, aims to use a “ 100-gigawatt light beam and light sails to propel some 1,000 ultra-lightweight nanocraft to 20% of light speed” (http://earthsky.org/space/alpha-centauri-travel-time). Some of those crafts could potentially reach Proxima Centauri b in a little over 20 years at that velocity.

The Breakthrough Initiative’s nanocraft would only be able to send a little data back to us, and couldn’t carry anything that might be able to actually explore the planet’s surface. And their ability to achieve such speeds via light sails relies solely on their miniscule mass. In order to truly explore our closest and most promising exoplanetary find, however, we must be able to deliver much larger masses to the Alpha Centauri system.

Artist’s conceptualization of Proxima Centauri B’s surface. Credit: NASA

Dr. Anthony Del Genio of the NASA Goddard Institute for Space Studies in New York and his team have used simulation software similar to that used by climatologists to show that Proxima Centauri b might well hold enough surface water to support life.

Proxima Centauri b is about 30% more massive than the Earth and experiences a “year” every 11 Earth days. It orbits its parent star at only about 4.6 million miles, a far closer distance than Earth’s 93 million miles. However, Proxima Centauri is a red dwarf , which means that Proxima Centauri b is subjected to an amount of heat and light that keeps it in the “habitable zone”.

The Del Genio team’s research simulations assume that Proxima Centauri b holds an atmosphere and liquid surface water in similar proportions to Earth.

Unfortunately, there are still some potential problems that might prevent Proxima Centauri b from being such a great habitability candidate:

The planet is faced with stellar wind pressures nearly 2,000 times that of Earth, bombarded by charged particles that the red dwarf is constantly ejecting from its corona. In addition, Proxima Centauri b orbits so near its parent star that it is likely tidally locked because of the gravitational forces involved — the planet might not rotate around its axis and instead has one side always facing its sun while the other exists in perpetual shadow.

https://arxiv.org/ftp/arxiv/papers/1709/1709.02051.pdf

An older simulation published shortly after the exoplanet’s discovery in 2016 based its results on a liquid sea occurring only on the warm side of the world (the “eyeball Earth”). Del Genio’s team came up with their results by considering a scenario where a planet wide ocean could exist, with warmer waters circulating around the globe and moderating the frozen temperatures of the “night” side. According to the team:

We find that an ocean-covered Proxima b could have a much broader area of surface liquid water but at much colder temperatures than previously suggested, due to ocean heat transport and/or depression of the freezing point by salinity.

The team ran more than a dozen simulations factoring in many varied conditions: size of continents vs. seas, atmospheric pressure variation and composition, ocean salinity, and more. Most simulations resulted in a Proxima Centauri b that supported the existence of a global ocean despite the extreme stellar wind and tidally-locked rotation.

The larger the fraction of the planet with liquid water, the better the odds that if there’s life there, we can find evidence of that life with future telescopes.
-Anthony Del Genio

Chile’s ELT (Extremely Large Telescope) that is currently being built may one day be able to differentiate the planet’s surface just enough that we can make educated guesses about heat being both reflected and generated on Proxima Centauri b’s surface. This will offer us even better clues as to the existence and quantity of surface water as well as some insight into whether plant, or even animal, life might exist there.

So far, there have been over fifty rocky planets found to be within their stars’ habitable “Goldilocks” zones. Proxima Centauri b may be the closest, but it is by far not the only world that holds potential as a future planet some portion of humanity could one day call home.

While we have so far discovered over 4,000 exoplanets, most using NASA’s Kepler spacecraft and Hubble telescope, even more will be found by NASA’s newest star-watching tool: the TESS (Transiting Exoplanet Survey Satellite). Our list of promising habitable zone worlds will continue to grow.

Our first steps certainly must be to “decentralize” and extinction-proof humanity by building large space stations and ultimately settling Mars. But while we take on the very long process of terraforming the Red Planet, we should probably also consider sending ships full of robotic, AI-controlled mining equipment and 3D printers to Proxima Centauri b, where our technology can begin the work of creating a city that some future pilgrims from our Solar System might one day be able to inhabit.

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