Is the Proxima System Our Best Hope For Another Earth? (Script) (Patreon)

On the Mediterranean shore of Egypt some 2000 years ago, Ptolemy of Alexandria made careful note of the position of a bright star that hung just above the southern horizon. He imagined pin-pricks in the celestial sphere. He had no idea that he was gazing on the stellar hosts of a world much like the Earth, and on humanity’s first destination when we would finally venture to the stars.

Alpha Centauri is the first and brightest of the constellation of the centaur, and the nearest star in our galaxy of 200 billion. From Ptolemy’s first record in his Almagest, more than a millennium and a half passed before we started to realize its incredible significance. By the end of the 17th century, the star had long since slipped below the horizon of Alexandria in its galactic wanderings. But it shone bright from southern India, and there a Jesuit priest peered at Alpha Cen through a newly invented device - the telescope. Father Jean Richaud saw two stars instead of one, these days dubbed Rigel Kentaurus and Tolimar, a binary pair bound in an 80 year waltz.

This telescope thing improved over the next century and a half, allowing astronomers to watch alpha-cen sway relative to the background stars as Earth orbited the Sun. That parallax revealed the system to be close - closer than any other - but still more than four light years distant. The two stars were so near and so familiar - both within 10% or so of our own Sun’s mass. Perhaps they also bore planetary systems - and even planets like the Earth. Dreams of an interstellar humanity started with the twins at the centaur’s foot.

But our greatest hope of another Earth isn’t with the twins. Forward another century, and a Scotsman named Robert Innes photographed the southern stars from Johannesburg to track their galactic wanderings. The fast-moving alpha-cen system was a prime target, but Innes found something utterly unexpected - this wasn’t a binary system after all - it was a trinary. A faint red star crawled on a vast, half-million year orbit around its brighter siblings. For the entire history of humanity that orbit has placed this red dwarf closer to the earth than its companions. So it was named Proxima Centauri, or just Proxima - our solar system’s true nearest neighbour.

At first Proxima’s proximity was mostly a curiosity. Red dwarfs are the smallest, coolest, and faintest of the stellar menagerie. This one was barely a star at all, really, at only 12% of the Sun’s mass and only a little larger than Jupiter. Hardly exciting compared to its vibrant, Sun-like siblings. But as the decades passed, we began to see how special Proxima really was.

The clue to its uniqueness was in its emission lines - sharp spikes in its spectrum resulting from electron transitions in the atoms and molecules of the star’s atmosphere. Proxima’s emissions lines seemed to shift back and forth from the wavelengths dictated by the laws of physics. And it took until … well, now, to finally understand the mystery of Proxima and the true magnitude of the importance of the Alpha-Cen system. In 2016, Spanish astronomer Guillem Anglada-Escudé and the Pale Red Dot team figured it out: Proxima had a planet. Or more technically an exoplanet - for extra-solar planet.

Planets don’t really orbit stars. A planet-star pair mutually orbits its mutual center of mass - its barycenter - which is usually deep inside the star. Planets make stars wobble, and that motion induces something called Doppler shift in the star’s emission lines. Essentially, the wavelengths of all the star’s light are stretched as the star moves away from us and compressed as it moves towards us, causing emission lines to oscillate in wavelength very, very slightly - just like with Proxima. This way of finding exoplanets is called the radial velocity method. So named because the Doppler effect results from the component of the star’s velocity that’s either direction towards or away from us - in the “radial” direction. We couldn’t see a planetary system that’s “face-on” - fully in the plane of the sky. But the more edge-on the orbits, the more motion there is in the radial direction, and so the more chance of spotting the wiggling spectrum.

The most prolific method for finding exoplanets has been the transit method, in which planets are identified by their dimming of their parent star as they cross or transit the face of that star from our perspective. That restricts the transit method to planetary systems that are almost perfectly edge-on. Nonetheless the Kepler mission found 2600+ exoplanets this way, and extrapolating from that revealed that most stars host planetary systems. Now that we know this, the radial velocity method can step in and potentially catch many more systems - as long as they’re not perfectly face-on. The radial velocity method is emerging as a competitor to the transit approach due to rapid improvements in a class of highly specialized spectrograph.

OK, so, equipped with this cool new tool, our intrepid team of exoplanet hunters realized that Proxima’s shifting emission lines made sense if the star was wobbling in a tight circle, moving at about a mile per hour over a period of 11 Days. That would then be the length of the year of a hypothetical exoplanet responsible for that motion.  Such a short orbital period, combined with the star’s mass, gave them an orbital radius for the exoplanet of around 20 times smaller than the Earth’s. That sounds perilous, until you realize that Proxima’s energy output is nearly 600 times lower than the Sun’s. That places the new exoplanet exactly in the habitable zone of the star - just the right distance for the intensity of the star’s radiation to potentially allow water to exist in liquid form.

There was one more stunning calculation to come: with the distance between exoplanet and star combined with the speed of the wobble, astronomers could calculate its mass. It’s practically the same as the Earth’s. Maybe 10-50% more massive, but almost certainly a rocky world, of the type sometimes inhabited by bipedal apes. Suddenly, after centuries of pondering the alpha-centauri system, it’s full significance became clear. The nearest habitable exoplanet orbited the nearest star. Surely we’d identified the first port of call for our interstellar future.

The discovery of this exoplanet - Proxima Centauri B - or Proxima-B - was just the first of this little star’s surprises. In 2019 a new team reported the discovery of a second exoplanet, its signature also embedded in the slow dance of Proxima’s emission lines. Proxima C is much bigger than it’s earth-like neighbour, at 7 times earth’s mass. It’s also way further out, with 5-year orbit at the same distance as Mars. And then in 2020 a third exoplanet was tentatively identified. The prospective Proxima D is just a quarter of Earth mass and orbits once every 5 days, well inside the orbit of Proxima B. That’s interior to the habitable zone, making this prospective exoplanet literally boiling hot. Followup observations in January this year somewhat solidified the measurements of Proxima D.

And that brings us to today, approaching 2 millennia since Ptolemy’s observation. We have a bona fide planetary system in at least one of the stars in the Alpha-Cen system. And there are much more tentative detections of planets around its Sun-like siblings - a Neptunish body may have been imaged around Rigel Kentaurus and an Earth-sized body may have transited Toliman on an orbit that would fry it to a crisp. Those are quite speculative. But Proxima B is almost certainly real, and so weirdly similar to the Earth. But is it similar enough to have an atmosphere? Oceans? Life? Let’s take a closer look.

One very important point is that Proxima B is probably tidally locked to its star. It’s close proximity to the star means strong tidal forces, which will have forced the planet’s rotation period to be in resonance with its orbital period. The simplest case is for the length of the planet’s day to be the same as its year - both 11.2 earth days in this case. That would keep the same side of the planet facing the star at all times. One side baked and the other frozen, and perpetual twilight at the boundary. This situation doesn’t sound conducive to life for many reasons. For one thing, unless there’s a lot of atmospheric circulation, that permanent night could cause the atmosphere to collapse, freezing it to the surface. But it may also be that the large temperature differential drives powerful atmospheric convection - planet-wide gales of extraordinary strength that could distribute heat from the day-side to the night. Or if the planet has significant oceans, these could also distribute heat.

It’s also possible for a planet to be tidally locked without a perpetual day and night. Other orbital resonances can occur - for example, 2 days per year, 3 days per 2 years, etc. Proxima B may be in one of these. However it’s worth noting that any resonance besides permanent day and night would also result in massive tides, assuming the exoplanet has an ocean to be pulled and squeezed by its nearby star.

On the other hand, we might actually hope for a 1-to-1 resonance. In that case, the dark side might be the only survivable part of the planet, as long as there’s enough atmospheric cycling to keep it warm. Why? In order for life to have a chance in this system, it needs to be protected from the star itself.

Don’t let their size fool you; there’s nothing cute about red dwarfs. They can be angry, violent little monsters. Proxima is no exception. It’s what’s known as a flare star. Massive convection through the star’s body generates crazy magnetic storms, which can cause the star to have powerful outbursts - flares - that blast high energy particles and radiation through the planetary system. During flares, Proxima B is blasted with X-rays and ultraviolet light and high energy particles. A sufficiently thick atmosphere and strong planetary magnetic field could in principle protect any surface dwellers, who would then get to enjoy pretty spectacular auroras. Auroras which may even be visible from Earth by a near-future telescope.

Despite this violence, Proxima really is faint. At Proxima B’s location there’s enough light to keep water liquid but most of that is infrared. There’s far less visible light, so to our eyes the star would appear very dim. Photosynthesis in these conditions would be difficult. Native foliage would probably be black for maximum absorption, and may have to rely on alternative photosynthetic pathways that can make use of the infrared light.

By the way, unlike the Sun which grows brighter over time, red dwarfs fade. In order for Proxima B’s hypothetical oceans and atmosphere to have survived this earlier, hotter period it would have to have formed further out and then migrated inwards. That’s not crazy, because planetary formation models indicate that there wouldn’t have been enough material so close to the star for it to have formed in that position. Nonetheless, we’re focusing in on a relatively stringent set of requirements for Proxima B to be able to support life.

But even if it’s a stretch, we need to imagine all the ways that life could have formed there, because that’ll help us build the instruments needed to detect signatures of life. We’re now building and planning a new generation of giant telescope - 30 to 100 meters in diameter, which should be sensitive enough to detect emission lines from molecules in Proxima B’s atmosphere, if it has one, or even the light reflected from the planet’s surface. Both of these may bare the characteristic signatures of life, as we’ve discussed previously.

Ultimately we would want to visit our neighbour to pay our respects. In fact, the mission is already being planned. It’s the breakthrough Starshot program - something we’ve discussed previously. The plan is to use a giant laser array to accelerate a train of mylar light sails to 20% the speed of light in the direction of Proxima. They’ll each carry a tiny computer chip sporting a miniature camera, to take fly-by pics as they zip through the system 20 years after launch. There’s no update on when this might launch, but not tomorrow.

Hopefully one day we’ll be sending people instead of computer ships, to study the system and maybe even settle there. After all, Proxima, with its many-trillion-year lifespan will far outlive our own Sun. So let’s fast forward our story another few millenia. A descendent of humanity stands on the black grassy plains of Proxima B, which sway in the planet-wide gale. Four lights are visible through the thick atmosphere - the gleaming white twins Tolimar and Rigel Kentaurus, the glowering red orb of Proxima herself. And there’s another white star on the horizon, but it’s slowly slipping away on its own orbit around the Milky Way. Perhaps this post-human Proximan will wonder if the stories are true - that people began there, on a planet around Sol, Alpha-Centauri’s nearest neighbor across spacetime.