The apparent discovery of a new planet around Proxima Centauri moves what would have been today’s post (on laser-thermal interstellar propulsion concepts) to early next week. Although not yet confirmed, the data analysis on what will be called Proxima Centauri d seems strong, in the hands of João Faria (Instituto de Astrofísica e Ciências do Espaço, Portugal) and colleagues. The work has just been published in Astronomy & Astrophysics.
It’s good to hear that Faria describes Proxima Centauri as being “within reach of further study and future exploration.” That last bit, of course, is a nod to the fact that this is the nearest star to the Sun, and while 4.2 light years is its own kind of immensity, any future interstellar probe will naturally focus either here or on Centauri A and B.
Years are short on Proxima d – the putative planet circles Proxima every five days. That’s a tenth of Mercury’s distance from the Sun, closer to the star than to the inner edge of the habitable zone. Despite some press reports, this is not a habitable zone world.
Where this work is significant isn’t simply in providing us with a third world at Proxima, but in the method of detection. When Guillem Anglada-Escudé and team found Proxima Centauri b back in 2016, they were working with data taken with the HARPS spectrograph mounted on the European Southern Observatory’s 3.6-meter telescope at La Silla. Confirming Proxima b demanded the newer ESPRESSO spectrograph, fed by the four Unit Telescopes (UTs) of the Very Large Telescope at Cerro Paranal. That work was accomplished in 2020 by researchers from the University of Geneva.
For the work on Proxima Centauri d, Faria and team used more than 100 observations of Proxima Centauri’s spectrum over two years, using ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) to turn up the radial velocity signature of a planet in a five day orbit. The weak signal went, with further observations, from just a hint of a new world to a viable planet candidate as the team made sure they weren’t picking up an obscuring asteroseismological signal from the star itself.
The fact that Proxima Centauri d is so small – probably smaller than Earth, and no less than 26% of its mass – emphasizes the magnitude of the achievement. This is the lightest exoplanet ever measured using radial velocity techniques, surpassing the recent find at L 98-50. The planet’s RV signature demands that ESPRESSO pick up a stellar motion of no more than 40 centimeters per second. As the paper notes:
Even in the presence of stellar activity signals causing RV variations of the order of m s?1, it is now possible to detect and measure precise masses for very low-mass planets that induce RV signals of only a few tens of cm s?1.
Pedro Figueira, ESPRESSO instrument scientist at ESO in Chile, notes the significance of the find:
“This achievement is extremely important. It shows that the radial velocity technique has the potential to unveil a population of light planets, like our own, that are expected to be the most abundant in our galaxy and that can potentially host life as we know it.”
Image: A team of astronomers using the European Southern Observatory’s Very Large Telescope (ESO’s VLT) in Chile have found evidence of another planet orbiting Proxima Centauri, the closest star to our Solar System. This candidate planet is the third detected in the system and the lightest yet discovered orbiting this star. At just a quarter of Earth’s mass, the planet is also one of the lightest exoplanets ever found. Credit: ESO.
In an email this morning, Guillem Anglada-Escudé told me that Proxima Centauri d was not necessarily a surprising discovery given how many exoplanets we are now finding, but it was nonetheless ‘a very beautiful one.’ He finds the work solid:
“It is just because ESPRESSO is a new machine and Proxima has been used to benchmark it that there might be some caveats, but I find the signal strong and very convincing. Also, the only reason to be cautious here is because it is Proxima and it has scientific and cultural relevance. In summary, I would have claimed the same as the authors did. A high cadence, regularly sampled campaign should be able to confirm it with a little more effort. Unsure the ESPRESSO folks will want to invest more time on that or not.”
Anglada-Escudé went on to make another important point: The paper shows that the scientists were able to precisely measure the signal against the stellar activity background, thus separating the planetary find from the noise. The surface of the star may be marked by dark spots and convective activity. The process of ‘detrending’ cleans up the signal to eliminate spurious artifacts so that the planetary signature can be measured. Spurious Doppler shifts affect the line width and the symmetry of a signal. Detrending, said the scientist, only works well when such changes can be measured more accurately than the Doppler shifts themselves.
Thus the power of ESPRESSO. The implications for future studies are heartening:
“…this anticipates very exciting discoveries, as that should enable the detrending of RVs, especially on more sun-like stars. Whether or not ESPRESSO will be the key to solve the ‘stellar activity’ noise floor remains to be seen, but to me, it now seems to have the tools and sensitivity to achieve that.”
The paper is Faria, et al., “A Short-Period Sub-Earth Orbiting Proxima Centauri,” Astronomy & Astrophysics 658 (4 January 2022), A115 (full text).
ESPRESSO Science Team video on Proxima d.
https://www.youtube.com/watch?v=1d8QmdFuxQ4
There should be many planets in the mass size of 1/4 earth mass to 3/4 earth mass. We have no example either of this size in our solar system. 1/4 mass of the earth is two times the mass of Mars… and what would the dark side of Proxima d look and feel like?
Interesting that Proxima d may be in the extended habitable zone.
https://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Proxima_planetary_system.svg/2074px-Proxima_planetary_system.svg.png
Figure 2. Evolution of stellar luminosity for F – M stars (F1, F5, Sun, K6, M1, M5, and M8) using Barrafe et al. [184] stellar evolutionary models. When the star reaches the main-sequence (red points) the luminosity curve flattens.
https://centauri-dreams.org/2018/09/07/extending-the-habitable-zone/
https://centauri-dreams.org/wp-content/uploads/2018/09/Screenshot-from-2018-09-05-08-59-30.jpg
Could Proxima Centauri be a polar sunspot star???
Impact of Superflares on Planet Habitability around Small Stars May Be Weaker than Thought.
http://www.sci-news.com/astronomy/red-dwarf-superflares-09942.html
With Proxima d close fast racetrack around Proxima Centauri of a 123 hours how many comets are going to impact on this small world to replace the water destroyed by the much brighter first 1 billion years.
Maybe small planets are very common. Perhaps they are just hard to detect with our present technology.
Small planets are most likely very common, this detection was helped by the fact that the planet is very close to a lightweight star. While they still had to push the limit down to 40 centimetres per second, which is one accomplishment. It still require many datapoints and statistical analysis to see a signal. And that’s the reason there’s a small lingering risk that make the researchers involved add a small note of caution that a confirmation is needed.
As for Proxima as a target for colonisation, I strongly doubt it ever will happen. If and when we have the capability send out one colonisation ship, we will have acquired a long list of more promising destinations at only slightly more distant stars provide a much more benign environment.
But what about all those ‘Blue Meanies’ on the rocky world of Pandora and all that precious, precious ‘Unobtainium’ nuggets just waiting to be picked up and hauled away by Earthly miners yearning to get rich on ??
It’s not in the “habitable zone”, but I would assume Proxima d could be reasonably colonized, and maybe even habitable. So close to a small star, the planet is almost surely tidally locked, so it should be ringed with regions of comfortable temperatures, possibly adorned with deposits of ice like Mercury’s poles. The strong solar radiation, strong heat sink, and wide choice of planetary resources should make it at least a highly desirable base for exploration, industry, colonization, or conquest, ahem, civilization. Builders of interstellar spacecraft will not chafe at the delta vee requirements! And there’s always the chance for some absurdly deep canyon like everyone dreams of finding on Mars, stocked with water, life forms, alien monuments, and cold beer.
“Another one of them new worlds. No beer, no women, no pool parlors, nothing. Nothing to do but throw rocks at tin cans, and we gotta bring our own cans!”
– Cook in Forbidden Planet (1956), commenting on the prospect of landing on exoworld Altair 4.
Even amid the pandemic, some people line up to throw their money into jackpots with many millions of dollars, and an even lower chance of winning. The odds coming out of an extended Drake equation – one which includes not only the chance of an alien intelligence, but whether it has noticed us, wants to help, is cautious about disrupting our development, has decided that leaving an Ellis Island style reception center on Proxima d is the best compromise, and has stocked that location with a wide range of creature comforts for refugees (including beer) based on observation of Earth, and will not merely accept astronauts who fly there but will recreate anyone who sends their ashes that direction into space or even a 140-character message through a METI program … well, it’s *unlikely*, but surely it isn’t as unlikely as winning the lottery. I never understood why the consumers of lottery products haven’t found other ways to entertain themselves.
Though I would like to be wrong in this regard, I am pretty sure there are no native intelligent life forms on Proxima Centauri d, at least of the higher variety. If there are, they are probably visiting for whatever reason, and that reason probably does not include us. If it did, they would be here in our Sol system, not “hiding” 4.2 light years away.
Possibly one of the very, very best movies of science fiction that have ever been made or thought up by the mind of man. A classic in every sense !
I am inclined to agree with you:
https://centauri-dreams.org/2017/09/11/creating-our-own-final-frontier-forbidden-planet/
and…
https://centauri-dreams.org/2017/09/12/creating-our-own-final-frontier-part-two/
I hope it’s tidally locked – Mercury shows that planets can fall into something like a 3:2 resonance instead, which would actually make it harder to build on this planet since you couldn’t just stay on the dark side.
Good point, Brett! Centauri Dreams ran a quite nice article on the complexities of tidal locking ( https://centauri-dreams.org/2020/01/20/a-deep-dive-into-tidal-lock/ ) but even that doesn’t cover everything about 3:2 resonances. (It does describe cases like Venus with a thick atmosphere changing the heating pattern, or simply worlds that start with very rapid rotation…) I found a somewhat recent preprint ( https://arxiv.org/pdf/2002.08600.pdf ) which referenced an open access paper from 2017 ( https://link.springer.com/article/10.1007/s10569-017-9783-7 ) which says “Tidally locked planets on circular orbits will rotate synchronously, but those on eccentric orbits will either librate or rotate super-synchronously… The orbits of potentially habitable planets of very late M dwarfs () are very likely to be circularized within 1 Gyr, and hence, those planets will be synchronous rotators. Proxima b is almost assuredly tidally locked, but its orbit may not have circularized yet, so the planet could be rotating super-synchronously today.” I don’t have the expertise to say that Proxima d should be in the same situation with higher odds of circularization, but I certainly can make a naive assumption. In any case, the paper here said the data is compatible with a circular orbit. Now it’s up to the astronomers to give us some data that doesn’t fit the model. :)
It’s probably not possible even with the existing proposed Starshade proposal, but I’d love to be able to direct image these planets and get a feel for what their atmospheres and actual ambient temperatures are.
At just 0.029 AU from its host star even with Proxima’s close distance , that’s going to take a seriously large aperture telescope with a seriously high performance ultra high performance ( with all the extra technological baggage and cost that brings ) to see it. Don’t rule out transit spectroscopy though. Using the transit probability equation
– R*/a
Where R* is the radius of Proxima expressed as a fraction of the Sun – 0.1221 and ‘a’ – it’s semi major axis in AUs , 0.029 .
This works out to a small but not insignificant 4.21 % transit chance.
Hi
Yes this is an impressive find, I’ll be looking forward to images of Proxima’s solar system 3 interesting planets here. There could also be a few mars sized world too further out?
Mike you make some good points too I was thinking the same things, Now to read the paper.
Thanks Edwin
For a tidally locked planet, the dark side – the cold side – should be survivable by metabolically active (=heat producinng) organisms with adequate insulation to hold the heat in. Such insulation could be of biologic origin through evolution, or perhaps technologic origin hrough cultural change. To gain the initial foothold, a reasonably stable temperate zone at the terminator weuld be needed to serve as a early nidus, regardless of whether panspermia or abiogenesis is invoked.
We HAVE a starshade Trojan 1 km across that was just detected, though the other Earth Trojan is of more use here perhaps.
Now, with OSTP’s bully Eric Lander out-we space advocates have an obligation to put beam-energy advocate Garritson in Lerner’s place. He can reach across the aisle and be a Space Solar Power advocate-which is a beam enabling tech.
This could salvage some of the Green New Deal, as Gene Meyers of Space Island Group detailed on The Space Show.
Like SLS/Shelby or no-it might be a way to do an end run around the two troublesome Senators.
Throw a bone to SLS states…and if SuperHeavy winds up orbiting MOST beam-energy payloads and SLS dies later-it would be worth its cost as a foot in the door for the rest of us.
Unlike most of you space advocates, I dare look over the space-fence to see political openings. We have one here. Better hurry before the window closes!
This provides some interesting reading ( http://petergarretson.com/ ). I think you may be underestimating Lander, who has been a very prominent molecular biologist for many decades – I would expect a new science advisor to have a similarly deep background. I’ve only glanced at Garretson’s work, but it seems mostly strategic, and includes publications like https://thehill.com/opinion/national-security/456214-space-force-defenses-must-stretch-to-the-moon , which describes a Chinese plan to collect 24 MW of energy in space and beam 1 MW to Earth.
His strident nationalistic tone bothers me. He cited evidence of dual-use technologies by other countries, notably a Chinese satellite with a robot arm that might be used to interfere with another satellite, to back his headline that “Space Force defenses must stretch to the moon”. Remember Operation West Ford ( https://www.wired.com/2013/08/project-west-ford/ ) in which half a billion pieces of copper wire were sprayed into (low) orbit in 1963. I think it would be easy for many nations, even North Korea, to put a lasting end to most aspects of space development tomorrow if space starts to become dominated by someone else.
If power beaming were to have a high-level advocate, I hope it would be a seasoned scientist who can acknowledge that astronomical bodies were made by no one, and their benefits should be apportioned peacefully by world leadership agreeing together to benefit all people on Earth, and especially the most needy and ecologically vulnerable. Someone who can contrive a mechanism, which I cannot imagine, that would ensure that laser launching arrays and microwave satellites are actually used for peaceful purposes rather than to set continents on fire.
Like I said… he’s a foot in a door, and some Red State support would come. Gene Meyers has really done a lot of work behind the scenes. Remember—even folks in the military are focusing on climate. I see bipartisanship coming on space solar power…Dyson Harrop cables might drain a future Carrington event and SPSS act as sunshades. No ground based energy source can do that.
I think I see what you mean by the SPSS ( https://en.wikipedia.org/wiki/Space_sunshade ), but to me it sounds infeasible and polluting with present resources – unless manufacturing facilities on the Moon are in full swing. Surely it would be more effective to avoid pollution and evade a Carrington event by roofing the majority of buildings in solar panels. Power will never become cheap if it always comes from some vast infrastructure with one or a few owners at the far end of a wire. Besides, I would hope to see some science advisor found who can plot the demise of flu and Covid, rather than leaving us staring at certain destruction in the first bio-war that comes along (if not sooner due to one of the animal reservoirs).
Given the size of the new planet candidate, this is truly an impressive achievement!
There seems to be an open question in exoplanet science that is partially addressed by the discovery of this candidate planet.
Are super-Earth and mini-Neptune exoplanets the most common type of planet in the Universe, or, are Earth mass and sub-Earth mass exoplanets actually more common, but our instrumentation is not yet capable of detecting these smaller worlds in large numbers? The way I think about it is in terms of selection effects similar to when we first started finding exoplanets around solar type stars in the mid-to-late 1990s. In other words, the so-called hot Jupiter exoplanets were detected first NOT because they are the most common type of exoplanet. Instead, they were the easiest to detect with the instrumentation available to planet hunters at the time. It would stand to reason that Earth mass and sub-Earth mass planets are actually the most common type of planet, as Nature tends to form more smaller objects than larger objects whether we are talking about asteroids or pebbles.
I have two hypotheses to address the aforementioned question.
Hypothesis 1:
Earth and sub-Earth mass exoplanets are actually more common than super-Earth and mini-Neptune exoplanets. The larger worlds seem more common because they are the easiest to detect with current instrumentation.
Prediction of hypothesis 1:
Once better instrumentation is available, then we will find Earth mass and smaller exoplanets in even larger numbers than the super-Earth and mini-Neptune mass exoplanets.
Hypothesis 2:
Earth mass and sub-Earth mass exoplanets are truly less common than super-Earth and mini-Neptune mass exoplanets. For some reason, there’s a cutoff mechanism in the planet formation process whereby these smaller planets just do not form as efficiently. We know they exist, but they do not exist in comparable numbers to super-Earth and mini-Neptune exoplanets do to this cutoff in the planet initial mass function.
Prediction of hypothesis 2:
Better instrumentation will not reveal a substantial population of Earth mass and sub-Earth mass exoplanets.
Caveat to hypothesis 2:
A failure to detect larger numbers of Earth mass and sub-Earth mass exoplanets does not mean that they formed in smaller numbers than super-Earth and mini-Neptune exoplanets because these smaller planets were cast into interstellar space by early planet migration.
Test of caveat to hypothesis 2:
Microlensing surveys could help us determine if there is a large population of free-floating Earth mass and sub-Earth mass exoplanets, but they just do not reside in orbit around stars because they got kicked out by their bigger relatives :-(
Is it possible that mini-Neptunes and rocky worlds are the 2 sides on an unstable equilibrium? Slightly below the equilibrium mass and the low molecular mass atmospheres either quickly disappear or cannot stably form at all. On the slightly higher mass side, the gravity is sufficient to retain such atmospheres and the mass continues to build making this status ever more stable.
So mini-Neptunes in the inner system are basically rocky worlds with dense, low molecular mass atmospheres. This might contrast with Neptunes in the outer system beyond the snow line with a mantle overlying the rocky core of frozen volatiles like methane, ammonia, and water.
Perhaps these min-earths end up colliding with other proto-planets much like the proto-earth and the putative Thea collided giving us the moon among other things.
Mini-earth formations are a given as any planet that is larger must have been a mini-earth at some point in its evolution. Perhaps a better area of study is to determine the likelihood of a mini-earth surviving the chaos of planetary formation.
Questions:
Given ESPRESSO’s amazingly low detection threshold, does anyone know if the instrument is being used to search for low mass exoplanets around other red dwarf stars in the solar neighborhood??
Has anyone heard of any updates on the radial velocity search for planets around Alpha Centauri A and/or B?
Hi, I’m Alejandro Suárez, one of the authors of the article.
We (ESPRESSO consortium) and other teams are in fact searching for low-mass exoplanets around other nearby red dwarfs. Most of what’s reasonably bright and reasonably well behaved is being studied right now.
Alpha Cen A and B are not ideal for ESPRESSO, as they are too bright to be studied with an 8 meter telescope (Vmags 0 and 1.3). We are studying Tau Cet (Vmag 3.5) and it already requires a fairly extreme configuration to avoid saturating the detectors.
Alpha Cen A and B are also too difficult to observe right now because their orbit makes them too close to each other in the sky. Most spectrographs can’t properly separate their lights, and won’t be able to do it for a while. In a few years they will move away from each other and it will be possible to study them again.
Thank you for your insights, Dr.Suárez. I appreciate your joining the discussion!
How about useing some DYNAMITE on Proxima Centauri’s exoplanets!
ALIEN EARTHS; DYNAMITE
https://eos-nexus.org/dynamite/
Hidden Worlds: Dynamical Architecture Predictions of Undetected Planets in Multi-planet Systems and Applications to TESS Systems.
https://arxiv.org/abs/2007.06745
The question is will this work on RV data? It has been used extensivly with transit data but the parameters should be the same from RV data. Anyone know if this is the case?
Interesting info from wikipedia, they show Proxima d having a radius of 0.81 earth!
https://en.wikipedia.org/wiki/Proxima_Centauri_d
Either it’s made of Chiffon cake or Swiss cheese??? ;-}
In order to have the same density as Earth, I calculated that Proxima d should have a 0.64 Earth radius.
So, with a 0,81 Earth radius, it has a density lower than Mars.
Well the theory I’ve been promoting is that close in planets around M Dwarfs would be hit by a large numbers of comets.
“With Proxima d close fast racetrack around Proxima Centauri of a 123 hours how many comets are going to impact on this small world to replace the water destroyed by the much brighter first 1 billion years.”
Could we be seeing a water world or more likely a planet with large amounts if water in its interior? This could make for a very fluid plate tectonics and some very interesting dynamics, like a large magnetic field. Maybe they need to search for signs of radio signals.
“It’s not, to be clear, a technosignature hinting at an alien civilization; rather, it seems to be the result of an interaction between the exoplanet and the star’s magnetic field, generating intensely strong auroras that can be detected using the Low Frequency Array (LOFAR).”
https://www.msn.com/en-us/news/technology/mysterious-radio-signals-from-distant-stars-suggest-the-presence-of-hidden-planets/ar-AAPnMwU
Wikipedia cited the ESO preprint of the same article, which uses the Ulmer-Moll random-forest method ( https://arxiv.org/pdf/1909.07392.pdf ). I don’t necessarily *understand* that, but I think the radius is entirely guesswork, so this radius implies it is made out of the most common variety of Swiss cheese used in exoplanet construction. (As opposed to the hollowed-out ironclad secret reptilian base one might find elsewhere: https://phys.org/news/2022-02-psyche-iron-giant-asteroids-thought.html )
ESPRESSO has a hypothetical low threshold of around 10 cm/s – adequate to pick up Earth mass planets . There is a big difference between hypothetical and real life detection though. Historically the lower RV threshold has been around 1m/s for some time NOT because of instrument factors but because of the stellar photospheric – surface noise, which makes picking out legitimate signals below this threshold extremely hard . Not just for active M dwarf stars either but all stars. This remains the case, though Debra Fischer and the team behind the similarly sensitive ESPRES spectrograph at the Discovery telescope ( amongst others ) have been working hard on developing photospheric modelling algorithms to help compensate. These will hopefully help reduce the RV threshold well below the current reliable standard of 1m/s.
Proxima is an outlying case ironically thanks to Guillem Anglada-Escude and the Red dot team ,who measured the photospheric activity of Proxima in parallel with their extended RV measurements and so confirmed a previously suggestive signal into Proxima b by enabling the exclusion photospheric activity. Presumably those background measurements helped with Proxima d too.
Hi, one of the authors of the Proxima d paper here.
One of the things we are seeing with ESPRESSO is that a significant fraction of that noise floor attributed to stellar activity was in fact instrumental noise. Once the quality of the data increased (and when the observational cadence is good enough) what we see is that a lot of stars are better behaved than what we thought. Proxima is a prime example. If you look at the RV figures in the paper, the activity variations (while somewhat large) are very slow and smooth. With the right tools and data they can be modelled. There was an expected stochastic component that we simply don’t see with ESPRESSO.
Hi Alejandro
So what does the lower intrinsic noise from Proxima say about its flare potential? Super-flares would have rather exciting effects on planetary atmospheres situated as close as Proxima’s habitable zone.
I just wonder if thie new technique exploiting the Talbot effect could reduce the noise by 100 times?
New technique amplifies weak optical signals while reducing noise.
“A technique that can amplify weak optical signals, while simultaneously reducing noise has been developed by researchers in Canada. By exploiting the Talbot effect, the team showed how arbitrarily-shaped signals can be reliably detected, even when buried in background noise. The research was led by José Azaña and Benjamin Crockett at Quebec’s National Institute of Scientific Research.”
“Building on previous work, Azaña and colleagues used a more advanced form of the Talbot setup to passively redistribute the energy of a weak signal into a high-intensity pulse. Crucially, this could be done without distorting the waveforms of the signals. The technique also reduced the overall signal-to-noise ratio. This enabled the researchers to amplify non-periodic signals by a factor of more than 100, while reducing relative noise in the measured signal.”
“Azaña’s team envisage a wide array of potential applications for their approach. They hope that it could be adapted to convey 2D and 3D images – with promising potential uses in fields including astronomy, photography, and holography. With further improvements, their Talbot effect concept could also be adapted to amplify noisy signals carried by other wave types. These may include additional electromagnetic wavelengths, sound, and potentially even quantum matter waves.”
https://physicsworld.com/a/new-technique-amplifies-weak-optical-signals-while-reducing-noise/?
Optical signal denoising through temporal passive amplification.
https://opg.optica.org/optica/fulltext.cfm?uri=optica-9-1-130&id=468599
“Temporal Passive Amplification” Boosts Weak Optical Indicators a Hundredfold .
https://desiserialonline.com/temporal-passive-amplification-boosts-weak-optical-indicators-a-hundredfold-however-leaves-the-noise/
With at least 3 worlds around Proxima, this must make the star an increasingly attractive target for the first interstellar probe missions, even limited fast flybys. It is also making this an attractive target for a telescope that can image these planets too, however low the resolution. A FOCAL mission or similar allows the possibility of imaging at least these 3 planets (and probably more) without significantly having to move away from the gravitational focal line.
This is a wonderful news. The system is nearby in astronomical terms, nearby enough to see a probe reach it within a human lifetime. With multiple planetary bodies it bodes well for both research and in far future exploration and perhaps even resource utilization. Habitable or not, we have been given a great opportunity as a species when it comes to this stellar neighbour.
Hi Paul
Proxima d is orbiting much too quick for a dense sub-stellar cloud-mass to form and keep the heat off – that requires a rotation period longer than ~16 days. With an instellation level as high as Venus, it’s hard to imagine it would be a pleasant place to live. Yet the Imagination stirs… Super-Mars Proxima d seems more interesting an abode for Life than Super-Earth Proxima b. The Terrestrial Planet Mass-Radius relation of R = M^b, b=0.27, gives a radius close to 0.7 Earths and a surface gravity of 0.54 gee. Orbiting so close to Proxima, a belt of desert and temperate polar caps seems probable, though deep enough global oceans will probably circulate heat away from Sunside and deep into Nightside. Perhaps a Ring-River on the Terminator provides the only habitable area? In his speculative historical mystery book “Children from the Sky”, Duncan Lunan speculated on a partially terraformed tide-locked planet with an artifical Ring-River. Perhaps Proxima d is such a world?
Proxima d canals?