Big Astronomy News: A Planet Has Been Found Orbiting Proxima Centauri

I’d originally intended to finish my much delayed overview of some of the books I’ve read this year, or perhaps comment on the US presidential race (particularly the incredible blathering of the Republican candidate), but instead I’ve decided on a very interesting piece of recent news in the field of astronomy. Last week it was announced that a planet had been found orbiting Proxima Centauri, which is the closest star other than the Sun (hence its name). Not only is this the closest exoplanet that has ever been discovered or will be discovered (unless we find a closer free floating planet or one orbiting a currently undiscovered nearby brown dwarf), it is at the right distance from Proxima Centauri to have a reasonable chance of having liquid water on its surface and thus being habitable to life as we know it (though as I will get into below, a number of factors could substantially increase or decrease its habitability). All things considered, this looks like the most important exoplanet discovery yet.

Proxima Centauri is just over 4.2 lightyears from our solar system. It is also sometimes called Alpha Centauri C, as it probably (though not certainly) is gravitationally bound to the binary star Alpha Centauri, orbiting the other two stars (known as Alpha Centauri A and Alpha Centauri B) at a distance. The Alpha Centauri binary is about a tenth of a light year further away and is one of the brightest stars in Earth’s sky. Proxima Centauri, on the other hand, is not visible to the unaided eye. This might seem surprising considering the proximity it is named for, but not quite as much so if we keep in mind that it is a red dwarf. Such stars are actually by far the most common in the galaxy (and presumably the rest of the universe), but they are much smaller and cooler, and thus much dimmer than even the Sun, which is less intrinsically bright than the majority of stars that are prominent in our skies.

Since Proxima Centauri is a red dwarf, its habitable zone – as the range of distances at which an orbiting planet might be reasonably expected to have liquid water on its surface is known – is far closer in than it is in the case of the Sun. The newly discovered planet, for now simply known as Proxima Centauri b (or just Proxima b), is much closer to its star than Mercury is to the Sun, and its year is just 11 Earth days. This means that it is probably tidally locked with one side always facing Proxima Centauri. Though this means that the planet would have one hot side and one cold side, if it has a substantial atmosphere that could moderate temperatures enough for it to remain habitable. Nevertheless, it is still unlikely to be as comfortable an environment for life as Earth. This is even more so because of a major issue with Proxima Centauri itself, namely the fact that is a flare star, prone to outbursts that would bathe the planet in intense radiation. Some astronomers think this alone may make it unlikely that life has arisen on the planet, but others note that if the star hasn’t always been as active as it is now, life on the planet may have had a chance to evolve some resistance, or other factors might help mitigate the radiation problem.

Another difference between Proxima b and Earth is that the former is more massive. The figure that has appeared in reports about the planet is 1.3 Earth masses, but this is actually the minimum mass – it could be quite a bit more massive, depending on the angle from which we are viewing its orbit. However, it is at least 90% likely to be less than 3 Earth masses, which is still closer to Earth in size than most known exoplanets, so in this respect at least it’s one of the most Earth-like planets found yet (on the negative side, the most Earth-like planet in terms of mass that I know of is Venus, which is not at all habitable now, though it might have been in the remote past). What we don’t know is what kind of atmosphere it has, and that is naturally a crucial piece of information in determining its habitability. It needs to have enough of an atmosphere to warm the planet up past the freezing point of water (the Earth itself would be frozen if it didn’t have an atmosphere that trapped heat, though of course our current problem is that we’re changing it so that it traps too much) and to spread heat around if it is tidally locked. Other factors that may affect how habitable Proxima b is include whether it has a strong magnetic field (which would help stop some of that deadly radiation from hitting the surface of the planet), whether it has plate tectonics (which keep things stirred up inside the planet, ensuring that elements essential to life as we know it get to the surface), and how much water is in the system.

Despite all the unknowns which could potentially lower or even eliminate the chance of life having developed on the planet, it is clear that based on what we know now, this is the most exciting discovery yet in terms of potential life bearing planets outside our solar system. Proxima b is, as noted above, fairly close to Earth in mass, orbits in the habitable zone, and is closer than any other potentially habitable exoplanet we have found or that we will ever find in the future. The only discovery that could beat this one would be an Earth-sized planet in the habitable zone of Alpha Centauri A or Alpha Centauri B. Why? Because these two stars, unlike Alpha Centauri C (Proxima Centauri), are much like the Sun, with A being a little larger and B being a little smaller. However, my understanding is that even if there is such a planet around either of these stars, it would be undetectable with the technology we have now. This is because an Earth-sized planet at that distance would not tug on its star enough for us to detect it. There was an announcement several years ago that a planet had been found orbiting Alpha Centauri B; I even wrote a blog post on it. But that discovery has never been confirmed, and later analysis seems to indicate it doesn't exist. In any case, it was said to be orbiting very close in (making it much easier, though not actually easy, to detect than a planet in the habitable zone). So until in the (we hope near) future advances in technology make it possible for Earth-like planets, if they exist, to be found in the right orbits around the Sun-like Alpha Centauri stars, Proxima b is our best bet for exploring a potentially life-bearing exoplanet.

One thing must be kept in mind, though, when we use words like “close” to describe Proxima Centauri and its planet, and that is they are only close in comparison to other stars and their planets. They are still extremely far away. As I’ve mentioned in several previous posts, it’s important to keep in mind that interstellar distances are enormous, so much so that even Proxima Centauri is currently far beyond our reach. It took New Horizons almost 10 years to reach Pluto, while light makes the same journey in just over five hours. By contrast, it takes light over four years to travel from the Sun to Proxima Centauri. Voyager 1, the most distant spacecraft humanity has launched, has traveled more than 100 AU since its launch in 1977, but that’s a tiny fraction of the distance to Proxima Centauri. Neither Voyager 1 nor New Horizons is aimed at th is e Alpha Centauri system, but if they were, it would take them tens of thousands of years to get there. As for exploration by humans, we haven’t gotten as far as Mars yet, so it will take some really big technological advances to send humans to Proxima b, if it ever becomes possible at all.

However, this isn’t to say that Proxima b will remain beyond our reach forever. While humans are unlikely to go there in the foreseeable future, we might send robotic spacecraft there as soon as the next half century. Unless we make a major effort, we probably won’t find a way to accelerate New Horizons-type craft to the speeds necessary to reach the Alpha Centauri region this century, but we might pull it off with something smaller, such as the miniature spacecraft of the Breakthrough Starshot project. By the (possibly optimistic) timeline that those involved have suggested, a fleet of these laser-propelled probes could be launched within a few decades, if all the difficulties can be overcome. If they are launched, are successfully accelerated to a significant fraction of the speed of light, and a few of them make it all the way without incident (a collision with even a piece of dust at that speed would likely destroy the craft), we might be getting some kind of pictures and other data (depending on how successful the project is at miniaturizing the science instruments to fit on such small probes) from Proxima b forty to fifty years from now. I hope, however, that a few craft are sent on to Alpha Centauri A and B, since whether or not a planet has been found in the habitable zone around either star by launch time, if no search has been able to rule such a planet out, it would be worthwhile to look for one, since as noted above these two stars are much more similar to the Sun than their dim companion Proxima (or for that matter any other star within ten light years). There’s even a slim chance that all three stars have habitable planets or even inhabited ones. If just one planet in the Alpha Centauri system, whether it’s Proxima b or a planet orbiting the other two stars, is indeed home to life, it will drastically change our view of the universe. Even if the system is without life, close-up views of a planet orbiting another star would be well worth the time it would take to get there.