The world of science news was recently abuzz with an incredibly exciting new discovery—a roughly Earth-mass planet orbiting the star Proxima B a mere 4.2 lightyears away. The discovery has been described as an “astronomy dream come true.”
As shocking as it sounds, we find exoplanets, or planets orbiting stars other than our Sun, all the time now. We know of nearly 3,000 such planets plus another 2,500 planet candidates. So what makes this recent discovery, dubbed Proxima b, so particularly exciting? Not only is this newfound planet very similar to the Earth in mass at 1.3 times the mass of the Earth, but it also orbits its host star in that star’s habitable zone.
As we’ve previously discussed, the habitable zone describes the range of distances from a star with the right temperature for liquid water to exist. You can think of the habitable zone as a band or torus around the host star. Planets too close to the star will have temperatures that are too high, causing any liquid water to boil while planets too far from the star will be too cold leading to frozen worlds. Planets within the habitable zone—also called the “Goldilocks zone”—have temperatures that are “just right” and thus at least have the potential to host liquid water.
One of the main drivers of the search for planets outside of our solar system is the search for life similar to what we see on Earth. Finding planets similar to our Earth is the first, very important step.
There have been a few previous discoveries of Earth-sized planets in their host star’s habitable zone, including the planet dubbed Earth’s bigger, older cousin Kepler 452b. However, Kepler 452b is over 1400 lightyears from Earth. That means that even traveling at 5% of the speed of light—which, by the way, is far slower than the fastest spacecraft—the trip there would take over 28,000 years to complete. And that’s just one way!
So another reason that the discovery of Proxima b is so special is that it is not only much closer than previously known Earth-mass planets, but it actually resides in the closest star system beyond our own. The host star Proxima Centauri resides in the triple star system Alpha Centauri at a distance of 4.2 lightyears away.
Thus the odds of us being able to visit or send probes to Proxima b are greatly improved over our chances of exploring any other planet outside of our solar system. Loyal listeners may also remember that the program Breakthrough Starshot recently announced plans to send nanocraft propelled by ground-based lasers at 20% the speed of light toward the Alpha Centauri system.
So should we start packing our bags now? Will future generations have the possibility of visiting and eventually settling Proxima b? Here are five reasons why Proxima b might not be ready for visitors just yet.
1. We don’t yet know what kind of atmosphere Proxima b hosts.
Astronomers used a technique known as the Doppler method to find Proxima b. Even though planets are typically of much lower mass than their host stars, they still exert a gravitational pull on their star as they orbit, and that pull causes a tiny shift in the star’s position.
This wobble can be detected by monitoring the spectral emissions from a star over a long enough period. In the case of Proxima b, the planet completes an orbit around its star every 11.2 days, and so astronomers monitored Proxima Centauri for 2.5 months. They then combined that data taken in 2016 with previous data from 2000 for an even more robust result.
Since the Doppler method relies on detecting the gravitational pull exerted on the host star by its planet, astronomers can deduce the parameters of the planet that are responsible for that the strength of that tug, namely the planet’s mass and distance from the host star. Thus, we can know that Proxima b is 1.3 Earth masses and within the habitable zone of Proxima Centauri.
However, there is another very important ingredient in the recipe for life: a planet’s atmosphere. Mars and Venus are both in our Sun’s habitable zone and we definitely would not want to live on either planet! We would find it impossible to breathe on Mars which may have been just a bit too small to hold on to its atmosphere. And while our atmosphere traps in heat keeping us nice and warm on Earth, the atmosphere on Venus has proven too thick for life like us by creating a runaway greenhouse effect.
The Doppler method alone does not tell us anything about what kind of an atmosphere Proxima b might host. The next steps in the study of the planet will include attempts to observe its atmosphere to look for gases like methane, water vapor or oxygen that are out of equilibrium, a strong indicator of the presence of complex life.
2. Proxima Centauri is a low luminosity, red dwarf star.
The host star for Proxima b is a type of star known as a red dwarf. These stars are much cooler and smaller than our Sun. Thus, the habitable zone for Proxima Centauri is much closer than our distance to the Sun (~5% of the Earth-Sun distance) creating a very different global environment. With a year as long as 11.2 Earth days and thus seasons of maybe only a few days long, life on Proxmia Centauri would require a considerable amount of adaptation.
3. Red dwarfs are known for their violent outbursts.
Another hazard of being so close to Proxima Centauri is the possibility of violent outbursts of ultraviolet radiation that such red dwarf stars are known to exhibit. This radiation could prove harmful to life and/or a planetary atmosphere. Proxima Centauri appears calm now, but could have had extreme outbursts in the past or could experience them in the future.
4. Proxima b might have a “dark side.”
Pink Floyd fans already know that the Moon has a “dark side,” or a side that never faces the Earth (despite not actually being dark). Proxima b may also be tidally locked so that only one side faces its star. This could create hostile conditions for life if the so-called “dark side” is too cold while the opposite side is constantly exposed to stellar flares. However, if not too cold, the “dark side” could offer a safe haven from those UV outbursts.
5. Interstellar travel is still really, really difficult.
Perhaps the biggest reason we shouldn’t go packing our bags just yet is the incredible difficulty of interstellar travel. Even at 4.2 lightyears away, at 5% the speed of light, the trip to Proxima b would take ~80 years.
Despite the challenges of actually visiting Proxima b, its discovery carries huge implications for the search for life in our galaxy. Low luminosity red dwarf stars like Proxima Centauri actually far outnumber stars like our Sun. So if we can find a habitable planet around Proxima Centauri, this greatly increases the odds of life existing beyond our Earth. We just have to find a way to get there!