Is There Life On Proxima Centauri b?

On Wednesday, August 24, 2016, a paper was published in Nature announcing the discovery of Proxima Centauri b (also known as Proxima b), a new, potentially habitable planet orbiting our closest extrasolar star, Proxima Centauri. Not since Pluto was demoted to “dwarf planet” status by some grumpy astronomers has there been so much buzz in the planetary science community. While 34 potentially habitable exoplanets (planets outside our solar system) have been found so far, Proxima b’s discovery is special because Proxima Centauri is only 4.25 light years away. 4.25 light years is just a hop and a skip in cosmic terms, and the proximity of Proxima b means that it could one day be feasible to travel to the planet in search of extraterrestrial life, terraforming prospects, or just to get away from other Earthlings.

Proxima Centauri

Proxima Centauri belongs to a class of stars known as “red dwarfs.” Red dwarfs are the smallest, coolest, longest-lived, and most plentiful stars in the universe. At 1/8^th the mass and 1/7^th the diameter of the sun, Proxima Centauri is 40 times denser than the sun, and with a hypothesized lifetime of 4 trillion years, it will undergo fusion reactions in its core as a “main sequence star” for nearly 400 times longer than the sun will. Slow and steady wins the race!

Proxima Centauri is a flare star, meaning is occasionally exhibits dramatic changes in luminosity due to convection processes within the star itself. But with an average apparent magnitude of only 11.13, it is invisible to the naked eye. Because it is so faint, its “habitable zone” is very close to the star itself. The habitable zone is the range of orbits around a star where enough radiation is received to support liquid water on the surface given adequate atmospheric pressure, thus raising the potential for extraterrestrial life.

Proxima b

At only 0.05 astronomical units (AU) away, Proxima b is only 1/20^th the distance from Proxima Centauri that the Earth is from the sun, where one AU is the average distance between the Earth and the sun. Proxima b’s year is only 11.186 days, meaning that if I lived on Proxima b, I’d be celebrating my 770^th birthday today. Size estimates vary considerably, but Proxima b is at least 1.3 times the size of Earth, and possibly up to three times as large. Proxima b is able to be so close to Proxima Centauri and still remain within a “habitable zone” because Proxima Centauri is a very low-energy star.

Like the vast majority of other exoplanets, Proxima b is too faint to be observed directly with a telescope. Instead, it was discovered via a method known as doppler spectroscopy. Scientists noticed that light emanating from Proxima Centauri was periodically shifting from reddish to bluish. Just like those summer anthems blasting from your neighborhood ice cream truck increase in pitch when the truck is approaching you and decrease when the truck has passed, the apparent frequency of electromagnetic waves increases when the object emitting those waves moves toward the observer and decreases when the object moves away from the observer via the doppler effect. Along the visible spectrum, red waves have a longer wavelength than blue waves, so the red/blue shift is evidence of this effect.

By using this information, scientists at the European Space Observatory were able to deduce Proxima Centauri’s relative velocity (RV) with respect to their telescopes on Earth. You can clearly see that Proxima Centauri’s relative velocity is oscillating in a sinusoidal fashion, with a period of approximately 11.2 Earth days. This oscillation is due to the gravitational tug of Proxima b, and the period shows that Proxima b takes approximately 11.2 days to orbit Proxima Centauri. By analyzing the doppler shifts in detail, scientists deduced that Proxima b was at least 1.3 times as massive as Earth and only around 4.5 million miles away from Proxima Centauri.

Note that the oscillation has a amplitude of approximately 5 km/h. This is obviously MUCH slower than the speed of light, so the red/blue doppler shifting described above was measured with extremely sensitive instruments, not the naked eye.

Does Proxima b Support Life?

This is the million dollar question, and of course, we don’t know the answer. However, it seems pretty certain that if there is life on Proxima b, it is far more primitive than life on Earth, as conditions on Proxima b are likely not as hospitable as those on Earth. First off, because Proxima Centauri is a flare star and Proxima b is extremely close to it, Proxima b would be subject to extreme bursts of UV and X-Rays from Proxima Centauri’s flares. Unless Proxima b has a strong magnetic field like Earth or an induced magnetosphere like Venus, these flares could seriously harm surface life and even prevent an atmosphere from forming in the first place.

Speaking of atmospheres, we don’t know if Proxima b has one. Average planetary temperatures could be as high as Venus if Proxima b has an extremely thick atmosphere, or they could be closer to temperatures found on Earth. If Proxima b doesn’t have an atmosphere, surface temperatures would likely average between -30 and -40 degrees Celsius. For comparison, Earth’s average surface temperature would be -20 degrees Celsius if we had no atmosphere.

Additionally, since Proxima b is so close to Proxima Centauri, it may experience effects of tidal locking, where the year and day on a planet are the same. Our Moon has this effect; it spins on its axis every 28 days and takes 28 days to orbit the Earth, and as such, one side is always facing the Earth. If this were to occur with Proxima b and Proxima Centauri, one side would always be facing the star and one side would always be facing away. As such, life may only exist where the Proxima Centauri only shines lightly, like high latitudes or the “twilight region” between the sunny and dark sides.

One thing is for sure – you’ll hear a lot more about Proxima b in the future. There are already plans to send unmanned “nanocraft” (extremely small spacecraft) to visit Proxima Centauri via the Starshot Project. These nanocraft would use “lightsails” to capture electromagnetic radiation from a ground-based light beamer, allowing them to move at speeds of up to 100 million miles per hour, or 15% of the speed of light. At that speed, they would visit Proxima Centauri in just over 20 years. Seeing as the Voyager spacecraft took 12.5 years to reach Pluto, 20 years isn’t bad at all.

Thanks for reading! For much more information, visit the European Space Observatory’s webpage on this amazing discovery.

Charlie