Henry Cordova, whose recent critique of traditional SETI kicked off a lengthy discussion in these pages, has been mulling over issues of habitability in the galaxy’s vast population of red dwarf stars. While we’ve focused on the questions raised by stellar flare activity and the climate challenges of tidal lock, the narrow band of habitability among the fainter M-dwarfs poses its own problems. How big a factor is a narrow circumstellar habitable zone? Henry comes by his interest in these matters by way of US Navy training in both astronomy and mathematics. A retired geographer and map maker now living in southeastern Florida, he’s keeping up with exoplanetary issues as an active amateur astronomer and collector of star atlases.
by Henry Cordova
I am curious as to how the width of a star’s habitable zone varies with respect to its luminosity.
It would not be unreasonable to assume that the surface temperature of a planet is directly related to the radiant flux of its star. Furthermore, it seems reasonable that the range of surface temperatures in which water can be a liquid on at least part of a planet’s surface is directly related to the stellar flux at its orbital distance. There may be many other factors involved, such as the properties of the planet’s atmosphere, its rotational characteristics, orbital elements and the variability and spectrum of its parent star; but let us ignore them for the moment and simply consider the geometrical parameters involved.
All else being equal, the radiant flux received by the planet must then be directly proportional to the luminosity of the star, and inversely proportional to the square of the planet’s distance from it. In other words, if one star is a hundred times more luminous than another, a planet orbiting the fainter star must be ten times closer to its primary in order to receive the same flux. The same reasoning can be applied to both the inner and outer edges of the star’s habitable zone. Regardless of how we define the HZ, it will become much narrower as the luminosity of the primary decreases. And the narrower an HZ is, the less likely there is a planet there.
Consider our own Sun’s habitable zone. Although there is some controversy about its dimensions, let us for the purposes of this argument say that it is limited by the orbits of Mars and Venus.The two planets have semi-major orbital axes of roughly 1.5 and 0.7 AU, respectively. These two figures mark the limits of Sol’s HZ, and their difference gives an HZ width of 0.8 AU, plenty of room to squeeze Earth in.
If our Sun were a hundred times less luminous, the HZ boundaries would be at 0.15 and 0.07 AU, which translates to an HZ width of only 0.08 AU! Clearly, the HZs of faint stars can be very narrow. The chances of a planet forming there, or migrating in, are substantially reduced.
Astronomers have detected planets in the HZs of some red dwarfs, but I feel this is due primarily to selection effects. Many of our planet detection techniques are very sensitive to big planets orbiting small stars in close, highly elliptical orbits, circumstances which are not conducive to life and yielding statistics that may give us a distorted idea of how solar systems form. Because of these considerations, red dwarfs may not be good candidates for life, even if we disregard other problems such as flares and tidal locking. It is true that these stars are often old, stable for long periods of time and by far the most common type of star, but I think we’re better off looking at brighter main sequence stars, such as spectral classes K, G or even F.
Several recent papers have pointed out that albedo effects on the planets of red dwarf stars may significantly expand the sizes of their habitable zones. See Joshi and Haberle, “Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone,” Astrobiology Vol. 12, No. 1 (23 Jan 2012); here’s the abstract. For Centauri Dreams‘ discussion on this, see M-Dwarfs: A New and Wider Habitable Zone.
These monographs suggest that on worlds with significant snow and ice cover, the effective albedo is much lower than snow and ice on Earth because frozen water absorbs more radiant heat at the red and infrared wavelengths emitted by red dwarfs than it does in the visual part of the spectrum as in Sol’s case. This effect would certainly extend the size of the habitable zone, but that would be dwarfed (no pun intended) by the much greater inverse square law effect (several orders of magnitude) of the much lower M-dwarf stellar luminosity.
Of the 53 known systems (66 stars) within 5 parsecs (16.3 ly) of the Sun, there are 48 red dwarfs (spectral class M) ranging from absolute magnitude 8.09 to 16.20 (an enormous range of luminosities!), and only 2 of them brighter than absolute magnitude 10.0. Absolute magnitude is the intrinsic brightness, how bright the star would appear if it were exactly 10 pc distant.
Keep in mind that magnitudes are exponential: A 5th magnitude star is 100 times brighter than one of 10th magnitude. Or alternatively, each magnitude is 2.512 times brighter than the next. The Sun’s absolute magnitude is 4.84, Barnard’s star, 13.23. Proxima Centauri is absolute magnitude 15.56. Although our own Sun is considered to be in the mid-range of stellar luminosities, it is still much brighter than most other stars. Red dwarfs may be very numerous, but they are very, very faint. All together, they don’t provide much habitable space.
Henry, Henry, Red Dwarf systems scale the size of the planets distance just as the larger stars do. The most recent research shows that they have at least 3 planets in the habitable zone around these Lilliput planetary orbits. The fact is the larger K and G stars normally only have 1 or 2 planets in the habitual zone. Take a close look at the Trappist 1 system and you will see the most common place to find life in the universe. The giant suns that light our night sky have no comparison to the myriad M dwarfs that exists. We fixate on these giant stars because of our weak eyes are tuned to see them, but the most common eyes in the universe will be large infrared sensors that have no problem seeing the eighty percent of the stars that are red dwarfs. Much longer life, trillion of years, for every sun like star 14 red dwarfs and twice as many habitable planets around them. They provide a giant habitable space, compared to the over luminosities for the immature giants. Do not be taken in by the dazzling bright lights but look to the steady glow of infinitely powerful M dwarfs.
+1 Jupiter is also an example, although no real ‘habitable zone’ tidal forces also provide a chance for life.
It is not fact at all, it is pure speculation.
present astronomy instruments do not allow to find planets orbiting K and B stars, so effectively as it can be done for M stars. So we do not have enough data to compare.
There are a few K types with planets.
Repeat again – it is limited by resolution of our astronomical instrument present time…
If I could use your argumentation 35 years I could postulate that no any planet has been found in Universe outside of our Solar System…
Flawn logic .
I’m a little confused by your comment Alex. I think you’re right that a general rule about exoplanets around K type stars cannot yet be put forward but aren’t there a number of known exoplanets around K’s (and even in the habitable zones)? I would suggest the following are now known: Kepler 442b, 155c, 235e, 62f, 62e, 283c and 440b and are in the habitable zones of K type planets if the literature is correct.
Gary, compare this numbers with number of planets that has been detected around M dwarfs.
I’m sure that planets around K, G, F class stars exist with exactly same frequency as around M dwarfs, lack of detection – it is current state of our astronomical instrument’s resolution – i.e. measurement’s bias.
Meanwhile planets around K and G stars can be detected for star systems that has good orientation relatively to Earth’s observer.
Hi Alex. Sorry for the late reply. Ok, thanks. I see your point. I will love it when we have enormous amounts of data (hundreds of thousands or even millions of exoplanets) to look through and categorize. It will be tremendously interesting if we can find biosignatures on or around a significant number of those planets and try to make some general rules.
I imagine even if you did manage to fit a habitable planet into the narrow HZ of a red dwarf star, the effects of gravity from other planets in the system would make it challenging for it to stay there for billions of year (negating much of the benefit of the red dwarf star’s longevity and stability).
Although I always thought the biggest show-stopper with red dwarf habitable planets was the pre-main sequence phase. The much higher luminosity in that part of the star’s life-cycle (and for potentially billions of years) would burn off all volatiles on the surfaces of any planets that did make it into the HZ, meaning they’d have to be replenished from impacts later on.
If the planets are in stable orbits after their formation, they will actually remain stable also over very long timescales. There will naturally be quite some precession, but they will remain at the same distance from their star.
Yes one of the show stoppers as you put it is indeed the intense early years, which will strip a planet bare – even though it later will be at a perfect distance from the star. Another ‘show stopper’ which seem to be often forgotten is that the amount of visible light is very small – in some cases better compared to moonlight on Earth. Photosynthesis in one form of other is the basis for complex life and such worlds would have very slowly growing vegetation – if any. Chemosynthesis is often mentioned as one alternative, but also that would be limited from the start. In the end we might find one Earth like planet in the HZ of a red dwarf. But since that world have to get so many things just right, I do not expect such worlds would be more common than anyone that might be found in the vicinity of larger stars. And the long time it took on Earth before advanced life did evolve, which suggest that long timespan will be required for life also elsewhere to get their shoes on and obtain a ticket to the galactic zoo. Now the M-dwarf planets are long lived and easy to study, we should keep looking. Since their longevity make then good candidates as only those remain stable long enough for something more interesting to happen than prokaryotes.
If you speak to astrophysicists and geophysicists , as I have been fortunate to do over the last decade, they cite the stellar activity of M dwarfs, both pre and post entry to the main sequence , as the biggest habitability deal breaker . Rory Barnes of Washington State University has explored and published in detail on the uniquely more (UV) luminous and hostile pre main sequence period of red dwarfs . I’m sure Ramses Ramirez can expand upon this too.
It should also be noted that for smaller stars with very close in ‘habitable zone’ planets , the tidal effects of the parent star become significant in contributing to an exoplanet’s overall energy burden – adding gravitational energy on top of the stellar energy distribution, to such planets overall energy load. This independent of any complications induced by synchronised rotation .
Post ZAMS ( zero entry to main sequence ) stellar activity is generally higher in younger stars due to rapid rotation causing magnetosphere induced photospheric activity . Then a calming after this induced by magnetic braking ,’spin down’ over several
Billion years before a further activity increase induced by an enhanced magnetosphere as the star becomes increasingly convective – certainly for most K, G and smaller F class stars. M dwarfs spin down much slower as they are more convective ( indeed entirely so below 0.3Msun) maintaining higher photospheric activity throughout their entire main sequence lifespan . Although much less so for those stars earlier than M3 . Older examples of which (like nearby Lalonde 21185) can often be markedly quiescent.
The fact remains though that m dwarfs are active with frequent coronal mass ejections of FUV , X-rays and particulate radiation ( often hourly for active stars but certainly daily for stars like Proxima Centauri and Trappist-1 ) up to orders of magnitude greater than ever seen with the Sun – with aggressive background stellar winds. All engulfing any planets in their close in hab zones . As seen with Proxima Centauri b.
The question is as to whether any such terrestrial planets can weather this extended pre and post ZAMS storm without having either their primary and/or secondary atmospheres -and crucial volatile loads – stripped. Perhaps within their mantles or alternatively by delayed inwards orbital migration. Work with Spitzer and the HST has hinted that this might be the case for Trappist -1 d and e though the evidence is limited to a few precious transits spectroscopically analysed within precious telescope time. JWST will hopefully clarify this and maybe even have enough sensitivity to look for CO2 thin ,terrestrial style atmospheres . We shall see.
Meantime Kevin France ,at the university of Boulder has utilised Cube-sat and balloon sounding telescopes to begin examining the effects of FUV on M dwarf planets. He currently has a finalist 0.5m UV telescope , ESCAPE, under consideration for the next NASA small Explorer programme . This can look at UV emissions from m dwarf stars down to just 0.1 microns and should go along way – in conjunction with JWST’s transit spectroscopy work of selected M dwarfs – to resolving the effects of stellar activity on at least 200 exo-atmospheres in its primary two year mission .
The small explorer winner is announce this autumn for a 2025 launch.
Good luck Kevin !
I say forget about the M’s. I think K’s are the stars to look at for habitable planets. They are more stable than M’s and have a wider habitable zone. They are longer-lived than G’s and are more numerous, but not as numerous as the M’s.
Abelard: I agree! At least, for all good K-star candidates, I’d give them investigation priority over M’s. Unfortunately, I think the list of K’s is still rather short.
“Unfortunately, I think the list of K’s is still rather short.”
I know that. But I think we need to accept the reality that most of our lovely universe is simply hostile towards life.
I don’t really understand your scaling argument. Yes, the absolute width of the HZ is narrower with M-dwarfs, but so is the size of the disk the planets form in. For any system of n planets, they will space themselves out and the probability that any will be in the HZ should remain nearly constant. I do understand that there might only be room for 1 planet in the HZ, while wider HZs could accommodate more than 1 planet, but how important is that?
You don’t provide any data from the observations we have, although you caveat that data with observational bias issues. We should be able to empirically determine your argument based on data, perhaps what we already have. I appreciate the biases, but these can be [at least partially] accounted for. Do we have an estimate of any planet type (not just rocky worlds) in the HZ of the different star types?
The numbers of exoplanets are just too small – with the proximity detection bias alluded to – for a meaningful figure. The best guess for all planets orbiting M dwarfs – heavily based on the Kepler data- was published in June 2019 by Tuomi et al. Around 3 planets per M dwarf with a bias towards rocky planets and sub/Neptune mass types.
The difference between visual and bolometric magnitudes for red dwarf stars is much greater than for G and even K stars, since more of their energy is going to be towards the IR.
This helps a little with the HZ limits, since the actual luminosity will be higher. Still, flare activity and early luminosity bump makes them less attractive for life as we know it.
I agree with Abelard Lindsey – put greater effort into finding planets around K stars (and G stars).
IF red dwarfs are actually brighter overall, than one would think, because of IR luminosity, is that going to be of much use for photosynthesis, given the lower energy of IR photons? Of course I’m thinking of photosynthesis as done on Earth. But maybe other photo-chemistries would work.
There are naturally occurring types of chlorophyll that can photosynthesise in the NIR and synthetic variants have been created that can exploit longer IR wavelengths ( though still in the NIR). So such things are possible. Obviously even M dwarfs radiate in the visible a bit and it’s conceivable to think of photosynthesis occurring across a wide spectrum. Extending from shortwave blue visible light all the way into the NIR- as opposed to the peaks of activity seen in blue and red light with terrestrial photopigments . Thus making plant life look a baleful black on an erstwhile M dwarf hab zone planet (to the human eye anyway ) rather than the mellow green we are all used to on Terra.
I’ve not heard of pigments able to exploit even UVc but I guess it might be possible . There are photopigments operating in high atmospheric bacteria on Earth (thus exposed to far more energetic UVb ) but these tend to act in a protective rather than productive capacity.
Which would be very useful in the face of EUV and FUV stellar flares.
In terms of available energy from the star the ice albedo fraction for IR radiation is far lower than for visible light . So far less incident stellar flux would be reflected and could help warm the atmosphere instead. Thus potentially extending the habitable zone out to as far as 0.25 the solar flux received by Earth.
Thanks Ashley Baldwin, for the info. I was thinking of checking on alternative chlorophylls but hadn’t got around to it. Pigments operating in UVc would have to tread a fine line between working and being bleached into oblivion, me thinks!
I think he is right, they are very narrow HZ’s but resonances can occur getting a few tightly packed planet system. Now been near one of those dim stars has other dangers, impact velocities are much higher and therefore more devastating.
While I am sure that the UV and other flare events pose great obstacles for Earth like life biological history, I still suspect that routes to life might take unfamiliar forms. For example, save for Jupiter, all the solar system planets have surface gravity very near to Earth’s. When you take the
examples of Saturn, Neptune and Uranus, the mass is 95x, 20 or 30x times Earth’s, but the surface gravity is close to 1. Yet orbital velocities and the gravitational fields are quite extensive. Let’s take Saturn-like situation: the mass is close to 100 x and the radius close to 10. The escape velocity is square root of 10 or around more than 3x terrestrial.
Consequently, for an atmosphere to make a go of it in a flare environment as described, it would help if the object it is attached to had dimensions greater than Earth’s, more mass, but not necessarily the same density. Would it have a “surface”? Wish I knew.
Another aspect of this different environment is that for the case of the Earth, we invoke an exceptional collision which formed the Moon.
And then we come out several billion years later with an atmosphere of
14.7 psi at the surface…. I can’t believe that 14.7 psi was a geological or
atmospheric constant in the midst of bombardment episodes. Not when there were once dragonflies with 2 foot wing spans. What’s more we did not start with free oxygen, but were provided with it by
earlier generations of bacterial life. I just have a hunch that a world larger than Earth with higher escape velocity has a better chance of
having a biosphere in M dwarf conditions.
As to planetary stability, Trappist and the Galilean satellites both provide interesting cases of long time stability – but why? They
have resonances and geophysical disturbance. But it would appear that
some sort of damping feedback keeps these systems stable. Consequently, it looks like M dwarfs can pack HZs even if they might not be able to tend them.
I suspect the most common habitable planet around an M-dwarf is going to be a larger super Earth or mini Neptune thats had its atmosphere stripped down to something more suitable for life.
In terms of life on M-dwarf planets with a habitable terminator, what effect would the increased slant range have on radiation levels at the surface, by this I mean the distance through the atmosphere that EM radiation would have to traverse before hitting the surface?
One final thought could the stellar flares be a source of energy for life? I’m thinking of something like the process that produces the hydrocarbon smog on Titan.
If we go to https://www.circumstellardisks.org/ we can see a zoo of 153 pre-main sequence disks of widely varying sizes. I don’t see mass estimates – how much has been worked out? If we could estimate the mass of the future mature star, could we use the mass-luminosity relationship to see how well the potential positions for planets match up with a presumed habitable zone?
I tend to be skeptical of habitable zones anyway unless we find many biospheres to compare. If Venus and Saturn have clouds with some amount of liquid water, what planet can be assumed dead without a probe? A planet that is “too hot” could have dark side life sustained by a fairly thin atmosphere, or one that is “too cold” could have a thick atmosphere to hold in heat, or they might all be more inventive with biochemistry than some would assume. Even the geology should have surprises to throw at us … I’m not ready to imagine how the weather and heat circulation of a carbon planet would vary from our own.
An annex to the above:
Given that the escape velocity for the more massive planet provides more of a clamp on the atmosphere ( e.g., vescape x 3, the next issue to consider is the Maxwell (?) distribution of velocities for the affected gas particles, molecular and atomic. I don’t have a factor at hand, but atmospheric science discussion for nominal cases ( Earth and solar system planets) would give some clues. Doubtless, this is only background for what analysis underway. But we might have better luck if we do not focus entirely on Earth analogs or give super Earths and small Neptunes short shrift.
While conceptualizing xenobiology as systems close to our own, it may be worthwhile to explore the concepts further afield in the realms of imagination. Even if from afar the biology may be completely unrecognizable to us, some technosignatures may be less likely to elude our attention.
References to Robert Forward always get my attention. Thanks for this, Robin.
If we are going to mention Xenobiology and out-of-the-box thinking on the subject matter, one must add this reference:
http://www.xenology.info/
Even more related references here:
http://www.rfreitas.com/AstroPubls.htm
I also find this site, Orion’s Arm, to be a refreshing and science-based take on how advanced beings might exist in our galaxy and beyond:
https://www.orionsarm.com/
While the focus is usually on native beings who could (or could not) evolve in such red dwarf systems, perhaps we should update our thinking a bit and consider the possibility of visitors to such star systems.
If an ETI is advanced enough for interstellar travel, exploration, and settlement, they might prefer red dwarf systems for the following reasons:
1. A lack of native inhabitants who could interfere with their goals one way or another. Perhaps even down to the cellular level.
2. There are a LOT of red dwarf stars to choose from.
3. Going on the theory that aliens who do want to perform METI but would rather not give away their home system(s) to any potentially unfriendly recipients, locating their transmitter beacons in an out-of-the-way and essentially desolate star system would presumably reduce such threats.
Just food for thought.
If the aliens’ home star was brighter, they would have to let go of bioforming any planet to recreate a local version of their homeworld. However, I see no reason why such an advanced civilization would not live in habitats that matched their homeworld environments. They would spread out around the M_dwarf and know that the potential for maintaining a very lengthy civilization (or sequence of civilizations) was very long indeed.
While we may talk about seeding sterile worlds with terrestrial life, how much more interesting to build gigantic habitats with existing ecologies and allowing the life forms to evolve over millions of years.
And to return to a well worn theme, why assume that ETI is biological at all? I still like Clarke’s (not original) expressed view that mind is the most precious thing in the universe, and that it doesn’t matter what its substrate is. Whether uploaded wetware minds or AGI, I suspect this is the means to allow interstellar migration. Embodied minds and servant machines could build whatever they wanted as long as there is a suitable energy source. M_dwarfs are attractive in that regard. However, if they have also mastered fusion then they may dismantle stars or gas giant planets to power their civilizations for even longer periods of time.
Unless there is a great filter just in front of us, our technology might indeed seem like magic in a millennium or two, and who knows what our capabilities will be and what we as a species [plural by then?] will be like (physically and mentally)?
If they are uploaded and live in Dyson swarms, they might prefer OB-stars instead. Maximized energy output, and still very good lifespan even by human technological timescale. If they prefere to live fast, several MYrs is enough to grow, mature and scatter seeds to other young cluster on the supernova wind. By some metrics, 1 MYr at O-star is 100000 times better than 10 Gyrs around M-dwarf with the billion times less bolometric luminosity!
I often wonder, is there a way to check if that dust around Eta Carinae is sentient…
They make surprisingly good gravilens with their focal lines quite close and well a very nice nuclear reactor if somewhat cantankerous.
I am wondering is tidal stress from the central sun and from the other planets would widen the habitable zone.
Peter Watts has the right general idea. If light speed cages are limiting (this may be even more the case for nonbiological life with shorter doubling times), and even if they are not, the interstellar species with the greatest range will be those that travel fast, far, and long before quickly blooming and scattering seeds far and wide.
That’s what we’re likely to run into in our local neighborhood. That’s the kind of visitor that makes the most sense.
Anyone have the details on the Pandora exoplanet mission? NASA gives a brief but no details on M dwarf atmospheres and star relationship but nothing on size of the objective, date or the 20 nearby red Dwarfs selected. Can not be too big for 20 million.
Elisa Quintana , Pandora’s effusive and amiable PI will be launching a mission website hosted by GSFC within a few weeks . Plenty to read about.
Watch out for that site – and others too, closer to home ..,,.
I think double planetary systems might be most habitable.
Sol sort of has two: Earth-Moon and Pluto-Charon.
Moon is 1/80th mass of Earth, some could say it doesn’t count as double planetary system.
And Pluto-Charon is too far from the Sun. Say it was at Mars distant.
And replaced our Moon with Mars size planet?
Earth-Mars could each be habitable at Mars distance. And both could
be habitable at Earth distance.
And possible each could habitable at Venus or Jupiter distance.
It seems major factors in Earth having life is plate tectonic activity and having the Moon.
It’s possible Mars has life underground or as likely, as dead as tomb.
One could say the Sol system has vast amount water it, but seems other systems could far more water in them. So rather having enough water as one could say about Sol. Maybe having a lot more water, is generally related to being a star system with more habitable planets.
Lots going on ! For a $20 million cubesat I’m guessing at an 8inch aperture max – but still more than enough for larger planets close in to nearby , bright stars.
The Pandora mission Principal Investigator , Elisa Quintana , of the NASA Goddard Spaceflight Centre has assured me that Pandora will have its own website up and running imminently . Watch that space .
In terms of M dwarf exoplanets and local ‘ space weather’s effect on atmospheres ’ – particularly via EUV and FUV , this is the subject matter of the recently shortlisted finalist foe latest Nasa Small Explorer concept round ($120-145 plus launch and operations ) ‘ESCAPE’ 0.5m UV telescope.
Winner decided next autumn for a 2025 launch and two year baseline mission.
If you Google ‘NASA Exopag ‘ and go into its meetings’ archive for last June, there is a comprehensive presentation by its PI , Professor Kevin France of the University of Boulder.
Here are a few related links
Characterizing Atmospheres of Transiting Earth-like Exoplanets Orbiting M Dwarfs with James Webb Space Telescope
https://arxiv.org/abs/2101.04139
The Copernican Principle Rules Out BLC1 as a Technological Radio Signal from the Alpha Centauri System
https://arxiv.org/abs/2101.04118
Persistence of Flare-Driven Atmospheric Chemistry on Rocky Habitable Zone Worlds
https://arxiv.org/abs/2101.04507
Super-Earths, M Dwarfs, and Photosynthetic Organisms
https://arxiv.org/abs/2101.04448
Here is an exoplanet in a three-star system…
https://www.space.com/kepler-space-telescope-2nd-exoplanet-candidate-confirmed
The Copernican Principle Rules Out BLC1 as a Technological Radio Signal from the Alpha Centauri System.
Typical earth centered universe BS, the opposite opinion is that intelligent life is common and the BLC1 signal is the galactic communities introduction to a virgin civilization. Why else would the signal be received at midnight on the only night the earth is closets to Proxima Centauri. They know we are here and they have for a long time. Think about it, this happens only once a year every year on April 29 for Proxima Centauri!
Why do you think Astrophysicist Sofia Sheikh from Penn State University is going to be looking for the same signal on April 29, 2021?
Was this mystery radio signal really from Proxima Centauri?
Posted by Paul Scott Anderson in SPACE | December 23, 2020
“Astronomers with Breakthrough Listen first detected the signal on April 29, 2019, using the Parkes radio telescope at Parkes Observatory in Australia.”
https://earthsky.org/space/wow-signal-2020-blc1-proxima-centauri
The work by Amir Siraj and Abraham Loeb; “The Copernican Principle Rules Out BLC1 as a Technological Radio Signal from the Alpha Centauri System” has a rather large flaw in it. The stars included in the article have an effective temperatures between 4800 K and 6300 K. This only covers the Main Sequence dwarf stars from F8V through G to K3V and leaves out all stars from K4V to M9V. The K4V to M9V includes close to 90% of all of the oldest stars in our galaxy. This make the study biased toward favoring solar G type stars and is not following Copernican Principle Rules.
In other words Proxima Centauri a M5.5V star, cannot have a beacon on it’s planets because it is not included in the stars in this study.
The use of Sun-like stars implies that K and G type stars are only systems capable of having radio-transmitting civilization, but M dwarfs may be just as capable. First the assumption that M dwarfs burn planets near them to a crisp is based on the sun centered idea that solar flares and CMEs are coming from near the equator of the star. The late M dwarfs are fully convective and may have the stronger magnetic activity near their poles as in Jupiter. The large volcanic planets near M dwarfs may also act as the Io plasma torus at Jupiter and create trillions of kilowatts of power that cause flaring and CME near the M dwarfs poles. This would keep the hard radiation away from the planet’s orbital plane and may also cause plasma and charge particles to be contained in the flux tubes formed between the volcanic planets and the M dwarf. Large scale magnetic reconnection may take place when two or more volcanically active planets flux tubes pass each other.
If this is the situation then that would give six or more times the number of planets that would be capable of evolving or available for colonizing by intelligent life.
This is why I favor the possibility of a galactic community that may have signal beacons near our solar system in our galaxy. These transmitters would send out the signal at specific times that coincided with near passage of the earth each year to the nearby star systems. The shock effect on civilizations would be greatly reduced if time and location precise signals with only limited, dull, low data information was sent…
Since red dwarfs conserve heat and light thru convection, Henry is right to downgrade their HZ’s as I do. If I hold my hand over a stove, the heat I feel is not directly from the stove. That heat is air between my hand and the stove. Unlike a sun, a red dwarf does not radiate energy. Instead, it remixes itself and does not “share” much heat and light. A sun exhausts itself by pouring out photons, which heat its surroundings. Radiant heat heats the Earth; not the surface heat of the sun. Red dwarf surface heat is irrelevant here. Planets close enough to receive any measurable heat from red dwarfs will be overwhelmed by tidal effects. Consider recent measurements of Gleise 1132 b. An original atmosphere was burned away by x-rays from a red dwarf. Volcanism on this “super-venus” ejects water, only to see that water broken down by x-rays. Henry is right; no HZ.