Proxima b, Twilight Worlds and Equatorial Superrotation

A few weeks ago, the scientific community announced an amazing discovery - the existence of exoplanet "Proxima b." As discussed in my post on extra-solar planets from June, we've discovered many many such planets. So why is this one different? First of all, it happens to be the closest exoplanet that could exist. It orbits Proxima Centauri, which is the closest star to our own. Secondly, most exoplanets that have been discovered are either gas giants (like Jupiter), way too close to their parent star to support life or way too far away. Proxima b shows no signs of those drawbacks. It is situated in the "habitable zone" around it's parent star and it is roughly Earth-sized (somewhere between 1.2 and 3 earth masses).

If this isn't shockingly amazing to you then let me just remind you of something: the whole point of searching for exoplanets is to search for possible new homes for humanity in the future. So just to reiterate, the closest star to us has a potentially habitable, Earth-sized planet around it. This potentially represents humanity's best chances yet of establishing permanent civilization elsewhere in the cosmos and pushing out the boundaries of our manifest destiny.

This is seriously something straight out of science fiction (I know, because the novel I'm currently writing happens to take place on a planet with Proxima b's exact characteristics. Click here to see my progress on that book), but it's not fiction. Now before you pack your bags for Proxima b, there are several important differences between Earth and Proxima b as well as some unanswered questions that will mean the difference between it being able to host life and not. 

Planet Druidia or Planet Spaceball?

First: an atmosphere. Does Proxima b have one? We don't know. Too thin, and you asphyxiate like Mars. Too thick and you suffocate like Venus. After the discovery of this planet you can bet your N*ass*A that a lot of telescopes are going to be swung in Proxima's direction to try and answer that question. The easiest way to do that though would be to witness a transit of the planet in front of Proxima Centauri and observe the frequencies of light that are absorbed as it passes around the planet. But alas, we haven't witnessed any transits of Proxima b. Since the star is so close to us, we could send a telescope into space perpendicular to the line between here and there. This telescope could then be continuously trained on Proxima Centauri until we observed a transit. But that could potentially take years depending on the orbit. Alternative, we could send a probe there directly, but that could take somewhere between 30 and 100 years. So... big unanswered question here.

You Spin Me Round

Second: the planet's rotation. Proxima Centauri (the star) is much much smaller than the Sun. In fact, it's much closer in size to Jupiter than the Sun. Because of this, the habitable zone around Proxima Centauri is very tight and close to the star, meaning that Proxima b is really really close to Proxima Centauri (it only takes Proxima b 11 days to make an orbit around the star). In fact, by my calculations, Proxima Centauri would appear about three times larger in Proxima b's sky then the Sun does in ours (it'll take up 1.7 degrees of the sky, vs 0.5 degrees to be precise) despite Proxima Centauri's extremely small size relative to our sun. Why is this important? Because that extreme closeness means that Proxima b has a high probability of being tidally locked. 

To be tidally locked means that the planet's day (rotation on its axis) and year (orbit around the star) are the same. That means that there are effectively no days and nights. One side always faces the star (eternal day side) while the other side always faces away (eternal night side). This is common when astronomical bodies are very close to a much larger astronomical body that it orbits. The moon, for instance, is tidally locked to us (one side always faces Earth and one side always faces away).

If Proxima b is tidally locked to Proxima Centauri, that would be a big problem for hosting life. Because one side would be excessively hot and one would be excessively cold, habitation would be difficult. An atmosphere would mitigate this (air currents are the great equalizer when it comes to temperature variation), but even that has its limits. If there were a decent atmosphere that circulated the air, a sizable stretch of the planet on both sides of the transition line between permanent day and permanent night would still be habitable. 

However, in October 2015 a group published a study where they used modeling software to examine the atmospheric dynamics in tidally locked Earth and Super-Earth sized rocky planets with orbital periods between 5-100 days orbiting red dwarf stars (very fortuitous, no?). This paper found that for Super-Earth sized planets with orbital periods between 5 and 12 days (which is the case for Proxima b), a lot of them exhibit a very unfortunate atmospheric pattern called "equatorial superrotation." The details are not that important, but the upshot is that in these cases, the normal circulation of air between day and night side gets disrupted, allowing the hot side to get really really hot and the cold side to get really really cold. If you would like to read more about equatorial superrotation and a discussion of this paper, then click here

This is all speculation at this point, but given Proxima b's characteristics there is a really good chance it is tidally locked to Proxima Centauri. And if it is tidally locked, then there is a good chance that it will exhibit equatorial superrotation (if it even has an atmosphere). But all is not lost! If one side is inhospitably hot and the other is inhospitably cold, then the intermediate value theorem tells us that there must be a comfortable spot in between. Algebra class finally comes to the rescue! This comfortable place would exist on a relatively narrow strip just on either side of the transition line between day and night. On this strip of eternal twilight, you could live relatively comfortably... if you're not killed by extreme weather :) 

Nuclear Wonderland

Third: radiation. With a planet that close to its parent star, the radiation from the star could be very intense. Since it is a somewhat cooler red dwarf, there will be *less* radiation, but there still may be enough to fry anyone living on the surface. So called "cosmic rays" (which is just a comic-book-sounding way of saying solar radiation) would be stronger the closer you got to being directly under Proxima Centauri (i.e. the sun being directly overhead in the sky). Luckily, that place wouldn't be very habitable anyway since it would be boiling hot (per the discussion above). Unluckily, the other place where the cosmic rays would be particularly strong would be at the magnetic poles, which would likely be located near that twilight line (which would have been otherwise habitable).

This is because a planet's inherent magnetic field deflects the cosmic rays towards the magnetic pole. That's what causes the auroras on Earth at both poles. At the poles, the magnetic field doesn't protect you nearly as much and you bare the brunt of that strong solar wind. That means some of our most promisingly temperate real estate in the "twilight zone" would actually be a nuclear wasteland if the solar wind is strong enough. That then further limits our habitable zones to a narrow strip of land straddling day and night with interruptions near the poles (envision two long, thin rectangles on the border between day and night). Plus side? The auroras on Proxima b would be phenomenal.

Light probe similar to the ones proposed by  Breakthrough Starshot

Light probe similar to the ones proposed by Breakthrough Starshot

Homeward Bound

So where does that leave us? With more questions than answers, I'm afraid. I'm positive that Proxima b will very quickly become one of the most studied objects in the sky, but even so, it will likely take about 50 years before we get our answers (as we will be able to get a probe there by then using Breakthrough Starshot). In a time where we have all gotten used to instant gratification, the speed of genuine invention remains agonizingly slow.  But while 50 years may sound like forever to us, let me remind you that flight was invented only 110 years ago and that we first landed on the moon less than 50 years ago, so I think it's reasonable to assume that traveling to another star system may take a while.

In any case, as Proxima b continues to infiltrate the public's consciousness and infect their imaginations, we will hopefully see a reinvigoration of public and private financing for these technologies. You hear me, NASA? Get on it! I want to see pictures of Proxima b in my lifetime :)