Today may go down as one of the most important days in the history of science, and possibly in the history of mankind. Today it was announced that a planet was discovered orbiting Proxima Centauri, the nearest star to our solar system. Not only is this planet close to us by astronomical standards, but it is also earth-sized and orbiting in the habitable zone of its star, raising the possibility that this planet may harbor life! Many other websites have news stories about this discovery, so I’m going to try to provide a little more perspective and analysis on this based on my background in astronomy.
How Do We Know About This Planet?
Astronomers have known that Proxima Centauri is the nearest star to us since the early 20th Century. It is part of a triple star system that includes Alpha Centauri A and B (two yellow sun-like stars that orbit each other) with Proxima Centauri being a very small, faint, red star in a wide orbit that goes around both Alpha Centauri A and B. Proxima Centauri is 4.25 light years away while the Alpha Centauri double stars are slightly further away at 4.37 light years. Being the nearest stars to us they have been the subject of lots of astronomical scrutiny, but despite this, no planets have previously been confirmed to orbit any of the stars in the system (though there have been a number of false alarms in the past). Planets are very small in relation to their parent stars, and they are also very faint, so detecting any planet, even around the nearest stars is very difficult. The planet around Proxima Centauri was not seen directly but instead was inferred from the motion of Proxima Centauri. By observing Proxima with a very sensitive spectrograph (an instrument that splits up light into a rainbow of colors) the velocity of Proxima either towards or away from the Earth was able to be measured. A planet, although small, has gravity, and pulls on its parent star. This pull shows up as a back and forth shift in the velocity of the parent star as the planet orbits around it. Without a planet you’d see no velocity shift. With a planet you see an incredibly small velocity shift that, nonetheless, a sensitive spectrograph can measure. These measurements are at the very limits of what our technology can detect, but technology keeps getting better. By assuming that physics works the same at Proxima Centauri as it does on the Earth we can use our knowledge of physics to infer what kind of planet it would take to create the observed velocity shifts we see. That’s how we know that this planet is at least 1.3 times the mass of the Earth and orbits Proxima every 11.2 days. We also know how far away that means the planet is from its star. And then given what we know about the brightness of Proxima we know that the planet is in the so called Habitable Zone, the distance from a star where a planet would be neither too hot nor too cold for liquid water to persist. So this all seems really promising and it is possible there are aliens right now on this planet looking up at us.
Why is this Discovery Significant?
This is big news for a number of reasons. First it’s a new planet discovery which increases the number and types of planets we’re aware of and teaches us more about what types of planets are possible. But the real excitement lies in that this is the CLOSEST star to the Earth and the star that we’re most likely to visit first, either with a probe or with actual human astronauts. So we may have just discovered where the first footfalls on another star system will take place in the future. But it’s also significant in that this planet could be habitable and host life as we know it. This is particularly remarkable because Proxima is what’s known as a Red Dwarf star. Red Dwarfs are THE MOST COMMON stars in the entire universe, outnumbering all other types of stars combined. They have lifetimes of TRILLIONS of years whereas stars like our sun only last for a few BILLION. So there is plenty of time for life to develop. But the downsides are numerous including that Red Dwarfs are incredibly faint (Proxima is only 1/20000 as bright as the sun). Even being the closest star to the Earth, it is still too faint to be seen with the naked eye. If the sun is like a raging bonfire, big enough for dozens of people to be around and receive its warmth, then Proxima is like the dying embers of campfire where you have to press in close to get any warmth at all. So the fact that a planet was found close enough in to Proxima to potentially be warm enough for liquid water to exist is remarkable. There’s much we don’t know about Proxima’s new planet, including what it’s atmosphere is made of. But even in the best case, this planet might not be a great vacation spot. The planet is likely tidally locked, meaning it keeps one side pointed towards its star (similar to how the moon only points one side to us and we can’t see its “dark side” from Earth. So Proxima’s planet probably has a bright side where the sun always shines and a dark side which is bathed in perpetual darkness. What the temperature and weather is like on such a planet is unknown. It may be freezing cold on the night side and blazing hot on the light side, with a narrow zone in between where the temperatures may be just right for life. But we really don’t know. Also, Red Dwarfs are known to produce solar flares aplenty. A tidally locked planet probably doesn’t generate a magnetic field which helps to shield the atmosphere from solar flares. Over time, the atmosphere of Proxima’s planet may have gotten blasted away by continual solar flares, leaving behind an airless rock in space. But, again, we really don’t know. There could also be other planets orbiting Proxima (it would be strange if there was just a single planet around a star) and those planets could have even more unusual properties and interactions. And we really don’t know that much about the habitability of planets either. We now think there is an ocean of liquid water under the ice of some of Jupiter’s moons, even though Jupiter is way too far from the sun to get enough heat to expect there to be liquid water. Ultimately, this discovery may be most important because it’ll catalyze more research into discovering other planets around both Alpha and Proxima Centauri and push the technology needed to discover even more planets around other stars.
How Soon Can We Go There?
This discovery may also be important in galvanizing efforts to figure out how to send a probe or humans to another star system. In the near term we can learn more by developing better telescopes and instruments. The James Webb Space Telescope will be launched in a few years and will be able to study planets around nearby stars. And there is a new generation of very large ground based telescopes that will be built in the next few years. Discoveries such as the planet around Proxima Centauri will give justification for even more sensitive planet hunting instruments for these telescopes. In the near term we should be able to learn a lot more about these planets and might even be able to detect the chemical signatures of life on them. If we exhaust what we can learn from the Earth and actually want to send a space probe to Proxima Centauri, we have to confront the fact that even the closest star is an extremely long distance away. The fastest space probe we’ve made is New Horizons, which took 9 years to travel to Pluto. At that same speed, it would take 62,000 years to get to Proxima Centauri. In fact, most space is empty and the region around a star that constitutes its solar system is miniscule. If our sun is like a porch light and the planets are like a swarm of moths hovering within a foot of the light, then the next nearest star is like another porch light a mile and a half away, with nothing but dark space in between. To get to the nearest stars in anything like a human lifetime, we’d need to figure out how to travel faster. Science fiction frequently invokes warp drive and other faster than light travel. But the reality is our understanding of physics is insufficient to conceive of any credible faster than light transportation schemes. Traveling at a significant fraction of the speed of light is physically possible, but would require energies and masses that are far beyond what our species has been able to tap into thus far (even getting a probe the size of the Hubble Space Telescope moving at half the speed of light would take the accumulated power output of the entire human civilization for a period of several years). There is only one technology that we currently have access to, that we can reasonably say would work, and that could power an interstellar spaceship capable of making a trip to the nearest star in something close to a human lifetime. That technology is a Project Orion style nuclear pulse propulsion spaceship. It would be powered by the impulse provided by hundreds of thousands of exploding thermonuclear bombs. Such an interstellar spaceship would be mind-blowingly huge, wider than a sports stadium, and weighing hundreds of thousands of tons (for comparison, an aircraft carrier weighs about 100,000 tons). It would be the largest moving object ever created by man, let alone placed into space. The cost would be staggering, probably trillions of dollars. But technologically, it COULD work and could send people to the Alpha Centauri system in a journey lasting somewhere between 50 - 100 years. I’m not saying it’d be worth it, but who knows how we’d feel if we discover evidence of life around a nearby star. It might be the greatest inspiration for us as a species to invest in technology and science and could usher in an age of rapid progress and have ramifications we can’t currently imagine. Ultimately the appeal of space is that it forces us to expand our imagination into bigger and more wondrous possibilities.
I have been excited about space since I was young, and have been involved in astronomy for over 20 years. I have built 4 of my own telescopes and continue to design and experiment with new instruments and accessories.