How extraordinary that the nearest star to Earth is actually a triple system, the tight central binary visually merged as one bright object, the third star lost in the background field but still a relatively close 13000 or so AU from the others. Humans couldn’t have a better inducement to achieve interstellar flight on the grounds of these stars alone. We get three stellar types: The G-class Centauri A, the K-class Centauri B, both of which are capable of hosting planets, perhaps habitable, of their own.
And then we have Proxima Centauri, opening up M-class red dwarf stars to close investigation, and we already know of a planet in the habitable zone there, adding to the zest of the venture. If extraterrestrial beings in a system like this would have even more inducement to travel, with another star’s planets perhaps as close to them as our own system’s worlds are to us, we humans are also spurred to undertake a journey, because 4.2 light years is a mere stone’s throw in the overall galactic distribution.
Image: The central binary at Alpha Centauri, with the two stars only resolved in the x-ray image. Credit: X-ray: NASA/CXC/University of Colorado/T.Ayres; Optical: Zdenek Bardon/ESO.
I like this image, used by Dirk Schulze-Makuch to illustrate a recent popular science article, because it includes the Chandra X-Ray imagery. That’s how we can separate the central stars, which are at times nearly as close as Saturn is to the Sun while they orbit their common barycenter. Centauri Dreams readers will recognize Schulze-Makuch (Technical University Berlin and an adjunct professor at Washington State) not only as a prolific writer but the author of a host of scientific papers including many we’ve looked at in these pages. He’s played a valuable role in presenting astrobiological matters to the general public, part of the flowering of interstellar investigation that continues as we keep finding interesting worlds to explore.
If you’re wondering about Proxima Centauri’s location, the image below flags it. Credit: ESO/B. Tafreshi (twanight.org)/Digitized Sky Survey 2; Acknowledgement: Davide De Martin/Mahdi Zamani).
I like to keep an eye on what appears in the popular press from respected scientists, because they’re bringing credibility to matters that often get distorted by mainstream media attention (not to mention what happens on social media sites). We should always give a nod to scientists willing to explain their work and the broader issues involved given that kind of competition for the public’s attention. It’s interesting in this case to get Schulze-Makuch’s take on habitability at Alpha Centauri. He’s pessimistic about Proxima but is surprisingly bullish on Centauri A and B:
The other two stars in the system are believed to have planets, although they have not been confirmed. (A possible Neptune-size planet was reported in 2021 orbiting Alpha Centauri A at roughly the same distance as Earth orbits the Sun, but this could turn out to be a dust cloud instead.) The apparent lack of any brown dwarfs or gas giants close to Alpha Centauri A and B make the likelihood of terrestrial planets greater than it would be otherwise, at least in theory. The chances of a rocky, potentially habitable planet in our neighboring solar system might therefore be as high as 75 percent.
The Proxima Centauri problem is, of course, the X-ray flux, although Schulze-Makuch also considers tidal lock a distinct negative. The Chandra data (citation below) revealed a relatively benign influx of X-rays for Centauri A and B, making them fine hosts for life if it can develop there. But Proxima is deeply problematic, receiving an average dose of X-rays some 500 times greater than Earth’s, and some 50,000 times as great during periods of flare activity, which M-dwarfs are particularly prone to in their younger days.
Here the word ‘younger’ is a bit deceptive. Recall that this kind of star can live for several trillion years. That’s a bit humbling, considering that the universe itself is thought to be 13.8 billion years old. In that sense all M-dwarfs are ‘young.’
Just as we can zoom in via X-ray to see the central stars, we can also take a look at Proxima Centauri’s movements via spectroscopic data, which we’ll examine next time, along with a fascinating speculation on the origin of Proxima b.
For more on the X-ray environment at Alpha Centauri, see Ayres, “Alpha Centauri Beyond the Crossroads,” Research Notes of the AAS Vol. 2, No. 1 (January, 2018), 17 (abstract). The possibility of a ‘warm Neptune’ at Alpha Centauri is discussed in Wagner et al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 12, Article number: 922 (2021). Full text. We’ll be talking about this one a bit more in coming days.
While we’re considering multiple systems like Alpha Centauri, it is important we also understand some basic facts about stellar evolution. The fact all stars in a multiple system are of the same age (they must have formed together, at the same time) is potentially misleading when we consider their SETI potential. The most important factor determining a star’s evolution is its initial mass. Massive stars evolve faster than low-mass stars, and the function is a very steep one. That is, they go through all their evolutionary stages earlier. Although both Alpha Centauri A and B are of similar spectral class, and probably both candidates for planetary systems, one of those stars will evolve faster than the other, and when it does it will probably threaten any life that may have evolved on the other. Unless two members of a binary system are extremely far apart, the more massive member will have a veto over the development and survival of life or civilization on the other.
From what we know of this system, all three members are old and stable stars and there is a possibility life may have gained a foothold around one or more of them. But in general, the possibility of finding a habitable or populated multiple system will depend on the masses and spacings of the members at the time of their formation. For example, if our Sol were part of a binary pair, and its companion were a nearby 10 solar-mass giant, life might very well have arisen here right on schedule, but when the companion evolved (in a relatively short time) into its supergiant or supernova stage, the life forms on our world would be doomed.
And even after the companion fully evolved and finally settled down to a quiet white dwarf state, there is no reason to believe life could arise here again. In general, the possibility of finding life on any multiple system is ultimately determined by the distances of the more massive members.
I think the limit for white dwarfs is around 8 solar masses, above that booooom !
Great article!
With respect to X-rays, I am not convinced they are at all relevant for the prospect of life or habitability. Earth’s atmosphere shields them completely, I believe.
Always good to see you here, Eniac!
The problem could be that they chip off the upper atmosphere or energise it so the solar wind takes off even more than normal.
Only if the planet has no magnetic field. While Prox b may not have a moon, it orbits close enough to its primary that the tidal influence likely churns it up enough that geothermal convection currents likely generate a field for it. Note that our own Sun’s tidal influence on Earth is 1/3 of our Moon at present, and its influence on Venus is likely what keeps its volcanic systems working.
Also a number of papers have demonstrated that significant water content on the surface, and atmosphere, would prevent tidal locking.
Finally, even if it was tidally locked, this does not preclude any rotation. It could rotate with its axis locked towards its primary, which would do wonders for organizing and maintaining a liquid outer core.
Water would flow to the dark side, freeze, and glaciers would extrude along the terminator line into the day side, melting to feed a hydrological cycle featuring an ocean on the north pole facing the primary giving the planet an eyeball appearance.
I have to ask whether the distance apart of the 2 stars, similar to that of Uranus from our sun, acts as the gas giants in this case.
The problem for habitability, as I see it, is the orbital eccentricity of the 2 stars makes any stable stellar flux on a planet in the HZ unlikely. I would have thought that this would make the periodic ice ages on Earth seem like a mild winter by comparison. If there is a living world around either star (or both) they would experience such relatively large changes in energy from the 2 stars that life would have to adapt in possibly novel ways. Plants may have to die off leaving seeds to germinate in a thaw decades later. Animals would have to find some way to match that long, dormant cycle too, possibly unable to become warm-blooded but behave more like insects with long cycles like cicadas, Or perhaps the planet[s] are more temperate with very hot “summers” also requiring dormancy to survive the heat.
If we do determine that there are rocky worlds in the HZs, and that one or more appear to have biosignatures, the scientific (and public) desire to send a probe[s] would be very strong. A biosignature would imply that life is ubiquitous in the galaxy which should stimulate research into imaging exoplanets and sending fast probes to the nearer ones for observation.
@Alex, there have been a few studies that show that planets in the current HZ for both stars will be stable for much of the system’s history.
I haven’t seen much discussion about the age implications of the system. It’s probably slightly older than the Sun, and Alpha may be well on its way off the Main Sequence. This will disrupt the original HZ. The Earth is near the inner edge of the Sun’s HZ so even a small change in the Sun’s luminosity will be bad for us.
A sub-Neptune in or near the HZ of either star will be bad news.
I think Proxima’s HZ planet is going to be an airless, waterless world, like the inner (and probably most of) the Trappist 1 planets.
I am only talking about the received energy from the stars, not their orbital stability. Earth has only one major source of stellar energy, and our low orbital eccentric ensures that this is fairly constant. Solar cycles only change solar energy a little.
But with Alpha Cen A&B, their orbital eccentricity suggests that the total energy received by a planet in the HZ, even with a perfectly circular orbit around its primary, will receive varying amounts of energy from the 2nd star. It may not be much given that star’s distance, but it will be different, and affect the energy received by the planet. For example, if the star has an average period of that of Uranus, and that eccentricity changes the distance of closet approach to 10 AU and farthest to 30 AU, then the energy received from this star varies from 1% to 0.1% which is an order of magnitude greater than the effect of the 11-year solar sunspot cycle. This would be just a minimum effect, moderated or exacerbated by other factors, such as axial tilt.
Are my numbers a great exaggeration of the situation?
“varies from 1% to 0.1%”
The annual variation of solar insolation on Earth is greater than this. It’s due to the small but not negligible eccentricity of the orbit. Presumably any HZ planet will have at least that much variation. Will an superimposed similar variation due to a stellar companion with a multi-year orbital period be significant?
For a planet in the HZ of A, it would receive the equivalent of about 0.005 (or so) of the Sun’s luminosity from B at closest approach.
For B, a planet in its HZ would get about 0.015 Suns worth at closest approach – a bit more energy than what Saturn gets from the Sun.
Both are not insignificant. For A, it may be on the order of the sky brightness due to light pollution over mega cities?
This is over a period of 80 years, so it may be slow enough for life to adapt.
@Ron , Frank
You are both saying that the change in energy a planet receives due to the eccentricity of the binary is not important enough to worry about.
If our sun’s output varies by 0.1% over its 11 year cycle and this may cause temperature changes of a 1-2C, wouldn’t a variation of 1% energy received by a planet be more impactful? Over an 80 year cycle, doesn’t that allow snowpack and ice to accumulate, or the same to disappear over that cycle, exacerbating the climate effects by changing the albedo as well as other weather effects?
Maybe all this depends on the energy flux – a hothouse or glacial condition may not be much affected by temperature changes sustained over decades, but if the climate was closer to the one we have on Earth, where changes in ice cover has important feedbacks, I have to wonder whether this eccentricity has more effect than we might think. Purely speculation on my part, but I would love to see what, if anything, a climate model would show with small insolation changes.
The Three-Body Problem.
The original cloud that formed Alpha Centauri and Proxima must of had a much larger mass then Sol’s, so the Oort cloud must be huge. Proxima Centauri long period orbit of 550,000 years with it’s periastron at 4300 AU and apastron of 13,000 AU would put it in the Alpha Centauri Oort cloud. If the system has existed for 4.5 billion years then only 8000 orbits of Proxima Centauri have been completed.
What would we have here when it comes to comets for both Alpha Centauri A and B and Proxima? The dynamics of comets being launch into A and B from Proxima’s orbit could make for some interesting orbits in the binary system.
https://centauri-dreams.org/2016/12/27/orbital-determination-for-proxima-centauri/
Imaging low-mass planets within the habitable zone of α Centauri.
https://www.nature.com/articles/s41467-021-21176-6
ALMA Discovers Cold Dust Around Nearest Star.
https://www.eso.org/public/news/eso1735/
Alex, I would put it down to an application of hysteresis: how great does the hysteresis have to be for an effect to become climate rather than weather variation?
Of course, we have ample examples here on Earth. Most are daily or annual, and in the case of the solar cycle you mention, a little over 10 years. Climate is not affected by annual cycles. However, that really relies on how we define climate. But we call these short cycles (seasons due to axial tilt, day/night, ocean warming) weather. Live is well adapted to these short cycles.
The solar cycle? 11 years can be more difficult to adapt to, if the insolation variation were large. But it isn’t and it doesn’t really matter.
Stellar companion orbital period? 80 years is longer and, yes, 1% variation is significant. Certainly the average air temperature will track that pretty closely, but not exactly due to hysteresis. For example, outside the tropics, the maximum and minimum annual temperatures typically occur 1 month after the solstices. Perihelion is nearly coincident with the northern hemisphere winter solstice (aphelion for the summer solstice) but there are greater factors at play (e.g. more ocean area in the south than in the north).
Ocean surface temperatures have a longer phase delay. That’s why the hurricane seasons lag the solstices by several months, far greater than the lag in air temperatures.
Glaciation? The hysteresis for that appears to be centuries. Ice reservoirs build and decline very slowly. 80 years won’t do it. I suspect all you are likely to see in the case of an Alpha Centauri HZ planet (near Earth analogue) is an added temperature cycle and extreme weather cycle. I don’t see why life can’t adapt to that.
In the other direction, the companion’s cycle might instead drive a high temperature/arid cycle (not snow/glaciation) that affects the tropics more than temperate latitudes, and it will hit quickly because the hysteresis will be small.
All of this is speculation until the particular of a planet’s environment and structure are known. Details matter.
Nuisance: Eclipse retinopathy is well known to occur after total or even partial eclipses because people will tolerate looking at a sliver of the Sun for a longer period than the whole Sun. Near totality the irises may constrict less. Note that the Sun seen from any distance is the same brightness per unit area on the retina. No matter what glowing reports Clarke gave in 2001, my guess is a star at the position of Jupiter, or the opposite star at Alpha Centauri AB, or the Sun seen from a terraformed Jupiter moon, might work like a welder’s arc in the sky, burning pinholes into retinas every time someone’s attention wandered.
‘Bouncing comets’ could spread the seeds of life.
https://earthsky.org/space/bouncing-comets-planets-astrobiology/
Can comets deliver prebiotic molecules to rocky exoplanets?
https://royalsocietypublishing.org/doi/10.1098/rspa.2023.0434
Orbit of Our Solar System Through the Milky Way Helped Form Earth’s First Continents.
https://cdn.sci.news/images/enlarge10/image_11130e-Early-Earth-Crust.jpg
https://cdn.mos.cms.futurecdn.net/dCkm3vhdzViA4rBiNsFbWY.jpeg
We forget we live in a very dynamic universe, just like the typhoon arms we pass through the milky way arms and have increases in comet and molecular clouds in our solar system. The higher number of rouge planets cause increased comets in the inner solar system, even the clouds could add a huge number of comets to the original solar system reserves.
The planets around the M dwarfs will lose their atmospheres to early radiation storms but will be replenished by comets when passage through the arms of our galaxy.
There needs to be a study of Orion Spur that we are in right now to see if recent impacts in the last million years are related; 12800 years ago, the tektites of 780,000 years ago, etc…
It’s not black and white but much more complicated just as the earths weather, geology and biology.
Thank you for your coverage. I don’t say it enough.
Why thank you. Glad to have you as a reader.
Thanks Paul
Another interesting read again. I wounder of Webb has looked at Proxima B yet?
I’m sure were going to have more news on this star system in the future.
Cheers Edwin
All this puts the Alpha Centauri system high on everyone’s list of places to look for extraterrestrial life. (Toward that end, the Breakthrough Foundation has provided early funding for the TOLIMAN space telescope program, which has the goal of discovering additional possibly habitable planets in the Alpha Centauri system. The current Phase 2 of the project—which has no officially announced launch date—includes the design, build, and integration of a spacecraft with the telescope.)
https://daily.jstor.org/the-hunt-for-life-in-alpha-centauri/
How about an update on TOLIMAN?
Too bad we don’t have a star at 13000 AU: It would be a fantastic incentive to develop the next (two) order of magnitude in space travel. Our furthest craft have gone ~160 AU.