As we move farther away from the sun, our journey gets colder and colder. If we want to find warmth and light again, it will be around distant suns.
Every star has a habitable zone — a range of orbits in which a planet could have liquid water on its surface. Our system has one planet in the habitable zone. But how common is this? How many stars must we pass in order to find one with a planet in the right orbit for liquid water?
None. Because the closest star with a planet in its habitable zone is the next closest star.
At 4.25 light-years from the sun, Proxima Centauri is closer than any other star. It’s small, red, and dim. Its diameter is about 1/7th that of the sun and only one and a half times the size of Jupiter. But orbiting very close to this cool star is a planet: Proxima Centauri B. It’s about 1.3 times the size of Earth, meaning it likely has a similar composition (i.e. rock instead of gas).
Proxima Centauri B’s distance from its host star puts it square in the habitable zone. Of course, that doesn’t automatically mean it’s room temperature on the surface. There are some significant challenges to livability. Its close proximity to the host star means it gets 2,000 times as much stellar wind pressure. In addition, Proxima Centauri is a “flare star” — a star that goes through frequent and unpredictable increases in brightness, showering any surrounding planets in radiation.
Proxima Centauri B is so close to its star that an orbit is just eleven days. Because the planet is so close, it might be tidally locked, just like how the moon always shows the same side toward us. In that case, one side of Proxima Centauri B would be blazing hot, the other side cold and dark.
But even in that situation, liquid water could still exist on the surface. Right at the border between day and night the temperature might be just right. Any atmosphere that did exist would be turbulent. Powerful winds would constantly blow in from the hot side.
Imagine a narrow, windy ocean. A great red star is locked on the horizon in permanent sunset/sunrise. The light is never brighter than twilight on Earth, as the dim star casts two percent of the light that Earth gets. Even with the red sun visible, the night sky would always be there, including the binary pair of nearby Alpha Centauri A and B shining brighter than any stars in our sky.
Or perhaps Proxima Centauri B does spin, albeit slowly. Not all close orbiting planets are tidally locked. It could be in a 3:2 orbit, much like Mercury around the Sun. In this case, a day on Proxima Centauri would last about 7.5 Earth days, but that’s still enough of a spin to have liquid oceans and livable temperatures. Relatively livable, anyway.
Or course, living there is not an immediate concern. Using conventional propulsion, even sending a probe to the system would take 30,000 years. However, Project Starshot seeks to change that. It’s a $100 million program to develop a proof of concept for solar sails that could accelerate a fleet of tiny probes to 20 percent of the speed of light, arriving in the Proxima Centauri system in only 20 years.
In 2010, JAXA (the Japan Aerospace Exploration Agency) deployed the first craft to accelerate itself using a sail to catch solar wind. That was on a much, much smaller scale and Project Starshot plans to use ground-based lasers rather than just solar wind, but though there are plenty of hurdles, the early research is promising.
Regardless, we know Proxima Centauri B is out there. That knowledge matters.
In 1961 Frank Drake, an astrophysicist and one of the founders of the SETI program — the search for extraterrestrial intelligence — created an equation for estimating the number of extraterrestrial civilizations with which we could someday receive communications. This is the Drake Equation:
N = R* · fp · ne · fl · fi · fc · L
“N” is the number of alien civilizations in our galaxy with which communication might be possible. The numbers that are multiplied together are all of the factors (according to Drake) that are necessary to find “N,” such as the average rate of star formation, what fraction of those stars have planets, and how many of those planets could support life, all the way down to the average length of time which civilizations could release signals into space.
We don’t have the answers to all of the factors, but we do have answers to a few. Current estimates put the total number of Earth-sized planets in the habitable zones of sun-like stars and red dwarfs in the Milky Way at about 40 billion.
Perhaps that number would seem staggeringly impossible without the knowledge that one of those planets is around the next closest star. And if we continue to find high numbers for each individual factor within the Drake Equation, it increases the likelihood that “N” is a lot bigger than you or I might think.
One Star Over, a Planet That Might Be Another Earth – The New York Times
A Family Portrait of the Alpha Centauri System – European Southern Observatory
Starshot – Breakthrough Initiatives
Alpha Centauri: Nearest Star System to the Sun – Space.com
Prevalence of Earth-size planets orbiting Sun-like stars – Proceedings of the National Academy of Sciences
Ikaros: First Successful Solar Sail – Space.com