We’ve looked at the factors that are problematic for life around red dwarf stars for some time now, focusing on tidal lock (in which one side of the planet always faces the star) and stellar flare activity, which could dramatically affect life on the surface. A new paper from Vladimir Airapetian (NASA GSFC) and colleagues homes in on the latter problem, offering the idea that we should re-shape our notion of the habitable zone to include space weather.
A planet in the habitable zone of any kind of star, according to the definition used most commonly today, is one on which liquid water could exist on the surface. But is this painting the habitable zone with too broad a brush? Because if we allow X-ray and extreme ultraviolet emissions into the picture — these are common on red dwarf stars, and especially on younger ones — then even clement temperatures at a planetary surface may not be enough.
The problem: Stellar eruptions like flares and, in their most extreme form, coronal mass ejections, may degrade the atmosphere of nearby planets. This is bad news for planets around M-dwarfs, because to be in the habitable zone in terms of surface temperatures around these small stars, they must orbit in relatively close proximity. The Airapetian model takes note of the effect, as the lead author of the paper on this work notes:
“When we look at young red dwarfs in our galaxy, we see they’re much less luminous than our sun today,” Airapetian said. “By the classical definition, the habitable zone around red dwarfs must be 10 to 20 times closer-in than Earth is to the sun. Now we know these red dwarf stars generate a lot of X-ray and extreme ultraviolet emissions at the habitable zones of exoplanets through frequent flares and stellar storms.”
None of this is news to those who have been tracking habitability issues around red dwarfs, but this new habitability model offers a close look at what can go wrong and suggests that the flare and emission problem may be more dangerous than we have previously estimated. High flare activity can cause atmospheric molecules to fragment into atoms, while continuing bombardment can ionize atmospheric gases, separating electrons that can be lost to space.
Ion escape is the process of positively charged ions being drawn out of the atmosphere. The Airapetian model looks at oxygen ion escape, which can be tracked even on Earth, although on a much smaller scale. Superflares of the kind found on young red dwarfs, the model finds, can cause loss of atmospheric oxygen, hydrogen and nitrogen. And losing hydrogen and oxygen from the atmosphere could eliminate a young planet’s water supply.
Image: In this artist’s concept, X-ray and extreme ultraviolet light from a young red dwarf star cause ions to escape from an exoplanet’s atmosphere. Scientists have developed a model that estimates the oxygen ion escape rate on planets around red dwarfs, which plays an important role in determining an exoplanet’s habitability. Credit: NASA Goddard/Conceptual Image Lab, Michael Lentz, animator/Genna Duberstein, producer.
The paper proposes, then, that we extend our idea of a habitable zone in the case of stars that show X-ray and extreme ultraviolet radiation levels 7 to 10 times the average emissions from our Sun, at which point the effects of space weather come into play. A planet like Proxima b, orbiting 20 times closer to its star than the Earth is to the Sun, clearly falls within the space weather range, and the authors estimate oxygen loss would occur in its atmosphere within 10 million years.
Add to this the effects of the star’s stellar wind and the conclusion is stark: Proxima b is most likely not a habitable world. “We have pessimistic results for planets around young red dwarfs in this study,” says Airapetian, “but we also have a better understanding of which stars have good prospects for habitability.” Could life emerge at a later date in red dwarfs whose stellar activity has slowed over time, and if so, what mechanisms could replenish a planetary atmosphere? There is much to ponder here as we assess red dwarf habitability.
The paper is Airapetian et al., “How Hospitable Are Space Weather Affected Habitable Zones? The Role of Ion Escape,” Astrophysical Journal Letters Vol. 836, No. 1 (abstract). This NASA news release is also useful.
I regard this as inconclusive for Proxima Centauri. Proxima is not a young star – it has a firm age estimate of 6.5 ± 0.2 Gyr (i.e., 2 billion years older than our Solar System). Red dwarfs are thought to calm considerably as they age and their rotation slows, and Proxima fits that, with a very slow 82 day rotation period. This paper deals with events early on, not so much about events today.
There is speculation that Proxima may have originally been closer to Alpha Centauri AB, and that Proxima b may have been further away from Proxima, both star and planet migrating in their early history. If that is the case, the atmosphere may not have been blasted away early on. In any case, the _Earth’s_ atmosphere appears to have been blasted away by early impacts, and was regenerated from volcanic outgassing. If the Earth can do it, so (potentially) can Proxima b.
Proxima Centauri isn’t particularly calm though: it’s an active flare star despite its age and the long rotation period.
There seems to be a substantial difference in the activity behaviour between the early-type red dwarfs and fully-convective stars: early-type red dwarfs become inactive at rotation periods longer than about 26 days, while red dwarfs later than M5 become inactive at rotation periods longer than about 86 days and also have slower spindown rates which further prolongs the active lifetime. As a result, I don’t rate fully-convective stars as being good prospects for finding habitable planets. Proxima is a fully-convective M6 star and its 82-day rotation period is below the threshold, so the flare activity is not unexpected.
Look at Figure 3 here. Proxima is a pretty active radio flare star, but its X ray luminosity is a good 2 orders of magnitude below the active young red dwarfs.
https://arxiv.org/abs/0901.1860
I regard the question of planetary habitability as something that will have to be solved by direct observation, which with Proxima b we have a reasonable chance of doing.
https://arxiv.org/abs/1608.07345
Touché. Few if any are even old enough to have reached the slow rotation rates that are postulated to lead to relative quiescence . Even then the full convection driven unpredictable activity contribution of their distributed magnetic fields is poorly understood.
This is another reason in favor of the rarity of life in the universe. Maybe GK stars are able to support lifebearing planets and rarely an early M dwarf can do the same. What size star makes for the best chances of hosting a living planet— G or K, as F and M seem suboptimal for habitability?
If it took 4 billion years of stable stellar lifetime for large , surface dwelling multi cellular organisms to appear on Earth and if the avoidance of habitable zone tidal locking is important to this process too then the spectral class stars K4 ( just ) – F8 meet both these criteria. F9 stars can stay on the main sequence for about the Earths current 4.5 billion years so could theoretically have hosted human life too. Although radiating more ultra violet than the sun , interaction with O2 in an Earth like atmosphere should actually produce a more substantial ozone layer ( Kasting et al ) and ironically lead to less UV reaching the surface of any hab zone planet than Earth- although the resultant lower mutation rate wouldnt necessarily be a good thing for evolution driven biodiversity )
Based on what is actually known, G stars are still the best bet. But K stars could be good too since there are more of them and they remain on the main sequence longer than Gs.
Would a magnetic field around the planet mitigate some of the risk from space weather, like earth’s magnet field?
Yes, but then there’s the problem of the slow rotation rate with tidally locked planets. In general, slow rotation would produce weak field strength.
Zuluaga et al have shown that substantial magnetic fields can still be produced by slow rotating tidally locked planets . Assuming they have an iron core and Earth like silicate mantle . Key features that allow this are convection in an outer liquid iron/nickel core and high viscosity in the lower mantle to facilitate heat transfer outward to drive this convection . This would obviously drive plate tectonics and help with secondary atmosphere production too. The bigger the planetary Core / mass fraction, CMF, the bigger the field too . ( Earth= 0.32, Mars =0.23)
Bigger terrestrial planets produce bigger magnetic fields in general but the key to optimal protection is a dipole field like Earth with the surface magnetic field strength less important than the stand off distance of the magnetosphere from the planet , expressed in terms of planetary radii ( protecting the atmosphere from erosion ) . The bigger the better with up to 4 Re about the best achievable ( but much less than a rotational Dynamo field ) . The plasma pressure of the atmosphere beneath helps to resist the stellar wind too.
Bigger terrestrial planets (2-7Me) with large , hot ( entirely liquid cores) are prone to creating strong but multipolar magnetic fields which although strong have gaps that undermine the protection they offer the planetary atmosphere and even the best dipole fields created in thus fashion leave considerably more of the planetary poles exposed than via a rotational Dynamo. .
There does seem to be an optimal planetary mass for core convection dipole field creation which lies around 2Me. Proxima b lies within this mass range but best possible field or not the withering stellar flux of nearby Proxima b would overwhelm even such an optimal field .
Planetary rotation rates of up to one day provide the maximal additional Dynamo effect for an even greater dipole field magnetosphere though even this would be pushed back to the surface in the face of Proxima’s regular and current extreme CMEs, XUV and X Rays outbursts .
If the planet is a moon around a hot Jupiter, the primary’s high magnetic field might help mitigate the loss of gases.
If (a big if) life can emerge in the subsurface oceans of icy moons, then this might be the preferred target for the search for life as these environments are going to be more robust against these solar processes.
A magnetic field would help reduce the space weather or solar wind which consists of protons and electrons which become trapped in Earth’s magnetic field called the Van Allen belts. However, we have yet to find an Earth like planet with a strong magnetic field that does not have a Moon like our Earth which is postulated to be the result of a Mars sized object which collided with our Earth and is responsible for Earths large Iron core and the rotation or angular momentum. Some of the iron came out of the Mars sized object and went into Earths core. Venus does not have a rotation fast enough to make a magnetic field and it has no Moon.
Also only a large atmosphere can shield x-rays and ultra violet rays which need ozone made from oxygen to shield them. The solar wind might have stripped much of the atmosphere but maybe not all of it due to volcanism. It still could have thick atmosphere though but most likely a sterile one due to lack of oxygen, ozone.
It will be interesting to see if any water vapor is present in the atmosphere which might indicate if any surface water or oceans exits. I think that spectroscopy of Proxima B will be able to tell us much about it’s atmosphere once it becomes available.
The ebb and flow of M dwarf habitability !
There is no two ways about it. On the face of it fully convective little Proxima Centauri is a yappy, snappy, Jack Russell of a star. Not exactly ideal for organic molecule based “life as we know it “( which is where we have to start looking given we wouldn’t likely recognise life as we don’t know it !) .
I would agree that now it seems to have been confirmed that the star is indeed gravitationally bound to Alpha Centauri in a very long period high eccentricity orbit that Proxima b could indeed have been caught from the Primary Stars’ at perihelion , late on in its history ,thus reducing its exposure time to space weather and especially through the active pre and early main sequence epoch. If it is indeed terrestrial ( as seems likely ) with a mass around twice Earth , even if tidally locked it should retain both a liquid nickel/iron outer core and a plastic mantle. Convection in the former can still generate a generous magnetic field even in the absence of fast rotation ( though the magnetosphere would not extend near so far out as an Earth like field thus more easily pushed back by stellar activity and also exposing more of the planetary poles ) and convection in the latter would help drive secondary atmosphere producing plate tectonics over an extended period ( aided by a higher heat of formation than smaller Earth further supplemented by a larger radioisotope load ) .
Provided the planet had a decent starting water content . It’s looking as if most of Earth’s early water didn’t in fact come from comet or asteroid bombardment ( it would take millions of hits to provide what we see today ) but was always there internally , conveniently hidden away from any dessicating / photodissociating stellar activity until needed to replenish any loss.
A similar situation on significantly larger Proxima b or any similar M dwarf planet could lubricate the plate tectonics described above for the billions of years till stellar “senescence quiescence” .( Kite et al) Would this be enough to cope with ongoing loss due to space weather meantime ? An inclined orbit might also help remove the planet from the equatorial plane of the star which seems to send out the most CMEs . Although according to msini this would increase the planet’s mass significantly . Although it would likely still remain terrestrial up to 4-5 Me the higher density might squash all important both core and mantle convection . Alternatively the extra mass could help in retention of a primordial hydrogen atmosphere whose eventual erosion might create a ” habitable evaporated core ” )
There have been countless simulations both for and against the potential habitability of M dwarf planets . No doubt there will be more with repudiation assuredly following on from this well crafted work. All the simulations are as good as the available data but still only simulations . As was found at the outset with ” hot Jupiters”, exoplanetary science has a propensity to confound expectation so I’m sure once suitable planets have been characterised in detail by planetary night side transit spectroscopy ( M dwarf hab zone planets are most likely to be too close for direct imaging “day side” spectroscopy except perhaps nearby Proxima b) we may yet be surprised -Maybe via JWST from a TESS discovery . Refraction limits how far this technique can reach into planetary atmospheres , most especially for Sun like stars , but for M dwarfs and especially from M2 and later should allow spectroscopy extending down into the all important troposphere .
I agree, we have too few examples at this time of planets larger then earth but if we look at the solar system we have many active worlds. Starting with Venus, a tidally locked planet that has 1600 major volcanoes or volcanic features many of which are still active. Every planet going up in size and mass from Uranus to Jupiter are very active and dynamic and we have another example in Io, like Venus that is tidally locked but very active volcanically. We have no examples between the earths mass and Uranus 14.5 earth mass and Proxima b mass could be anywhere between 1.25 and 3 earth mass. When we find the out what its orbital inclination, diameter and mass is, we should be able to model its dynamics. As for most of these studies, they are based on a earth centered universe!
http://volcano.oregonstate.edu/oldroot/volcanoes/planet_volcano/venus/intro.html
http://www.esa.int/Our_Activities/Space_Science/Venus_Express/Hot_lava_flows_discovered_on_Venus
Venus had plate tectonics and regular volcanoes till about 3 billion years ago . A combination of progressive tidal locking shut down its rotational magnetic Dynamo and loss of plate tectonics as it lost its lubricting water load led to shut down of its core convection ” thermal “magnetic field too . With no tectonics This created an alternate ” stagnant lid” system by which its internal heat is shut in over millions of years building up to periodic cataclysmic melting of its entire surface to release the pressure . ( Kite et al 2008, Zuluaga et al 2014)
The big difference between Earth and Venus is the large moon, could Venus have had plate tectonics if it had a large moon like earth? The early earth’s moon was close to and kept the earth rotating at a faster rate then today. http://curious.astro.cornell.edu/about-us/37-our-solar-system/the-moon/the-moon-and-the-earth/31-how-close-was-the-moon-to-the-earth-when-it-formed-intermediate. I would be interested to know if there has been any studies of exomoons and how close to a red dwarf a superearth could keep it bound. Could a large moon keep the superearth from becoming tidally locked
One of the unusual aspects of plate tectonics is that most of the large islands and island chains are to the east of the main continental land- masses. If you look at a topographic map showing the continental shelves the large islands and island chains to the east of the continents are separated from them. Could the earth’s moon have over time exerted enough influence to have caused this? The moon’s easterly movement in it’s orbit and it ability to raise tides in both the earth’s oceans and crust would seem to me to indicate so. This would be one of the things that we will find out as a better understanding and observation of exoplanets becomes available in the future.
I fail to understand what you are getting at here. Island chains are usually caused by movements of an ocean plate over a mantle hotspot. The direction of the chain is dependent on the direction of movement by the plate. The plates are in turn driven by spreading and subduction edges.
What mechanism are you thinking of that links the Moon to this mechanism?
Large islands- island chains: Madagascar, SE Asia, Philippines, Japan, New Zealand, Cuba, Haiti. Look at a good map without the oceans. I am not talking about hotspots but the separation of major parts of the continental crust to the east of that each continent. I have a clear understanding of plate tectonics, but you need to look and open your mind a little, it is very obvious!
https://theearthexpanded.files.wordpress.com/2014/11/ocean-floor-map-300.jpg
Zuluaga et al in 2014 have attempted this already for planets with masses ranging from 0.5 Me – 7 Me , 1-4 Re using Earth and a variety of established exoplanets as models . They assume an Earth like make up of an iron/nickel solid/ liquid core and silicate mantle .
If most M stars flare and leach HZ Planets of their Atmospheres and water Then:
The type of animal life that can arise and survive in M type stars seems very
limited. Any primitive life arising with the early atmosphere would subsequently have to adapt to Low Atmospheric pressure and No Water, a transition many times tougher than Reduction – Oxydation transition on the Earth.
Any surviving life will probably exist the within a few miles underground.
Feeding on chemotrophic single cell colonial “mats”. I doubt they would last
long, as the inherent H20 of the crust will be eventually be used up/ejected
as part of vulcanism.
This is a very interesting topic. The paper being discussed here provides a sober dose of realism in the face of what seems like overly enthusiastic wishful thinking, with broadly defined HZs around stars of all classes. SciFi has conditioned us to hope to find life all over the place. But space in general (and not just the moon) is a harsh mistress.
Otoh, still hoping to pull Proxima b and other M orbiters out of the fires of total inhospitality, here I go, grasping at another straw: Consider something that could give at least a few M star HZ planets the magnetic fields they would need; a glancing giant impact that spins them up.
Our system tells us that giant impacts due occur, leading to all kinds of outcomes. They have been suggested as causes for the Earth-Moon system, the retrograde rotation of Venus, and Mercury having an overly large core. If three out of four of our system’s rocky planets have taken big hits then many exoplanets will have as well.
So I ask, if a planet in an M’s HZ is spun up by the right type of collision, could it retain a protective magnetic field for long enough to hold onto its water and air?
A better bet would be capture of Proxima b ( from one of Alpha Centauri A or B at perihelion of Proxima’s long and eccentric orbit ) long after Proxima Centauri entered the main sequence so as to minimise its long term exposure to the hostile stellar wind, especially during the star’s uber active adolescence .
This would also allow the ideal sub one day rotation rates required to produce the biggest possible Dynamo induced dipole magnetic field ( Lopez-Moralez et al ) and allow a large standoff magnetosphere to last as long as possible before inevitable tidal locking at 0.05 AU.
An optimal 2Me Proxima b would also provide additional core thermal magnetic field protection and a larger tectonically driven atmosphere also held tighter by its greater gravity.
Still a push though as this paper shows just how harsh the space weather of close in “habitable zone ” planets is around late M dwarf stars .
The author has an extensive background exploring this area around a variety of different spectral classes from red dwarf to red giant stars .
Interesting, but capture of Proxima’s b planet away from one of the system’s much more massive A and B stars right into the sweet spot of Proxima’s HZ at 0.05 AU seems unlikely. Perhaps more possible if b had once orbited outside of both A&B, but even then it seems like a stretch. An extremely difficult problem in orbital mechanics, I would think.
Also, at least according to Wikipedia’s Proxima article, there is some as of yet unconfirmed evidence of other Proxima planets further out. If true that complicates the capture picture even more. In general, except for rogue planets, I think it will be very much more likely that planets will have formed around whichever star they currently orbit.
Don’t forget the collision thought to be responsible for Mar’s northern lowlands.
Excellent point Roy. So in our system it may be likely that ALL the rocky worlds have suffered very massive collisions. Since planets form via accretion it then can be taken as a given that any rocky world will have had a complex history of collisions, including ones big enough to alter rotation rates.
Should this(and future)studies put the final nails in the coffin to the existence of life as we know it on planets orbiting ACTIVE M dwarf stars, a NEW question arises: Would we even be ABLE to TERRIFORM them? I have gone on record hoping that there is NO DNA based life on Proxima b so that we can make it Earthlike ENOUGH to sustain a human colony without any shred of guilt. We need to find out a LOT MORE about Proxima b to determine whether terraformation is feasible.
Quote by Ashley Baldwin: ” If it is indeed terrestrial ( as seems likely ) with a mass around twice Earth , even if tidally locked it should retain both a liquid nickel/iron outer core and a plastic mantle. Convection in the former can still generate a generous magnetic field even in the absence of fast rotation ( though the magnetosphere would not extend near so far out as an Earth like field thus more easily pushed back by stellar activity and also exposing more of the planetary poles ) and convection in the latter would help drive secondary atmosphere producing plate tectonics over an extended period ( aided by a higher heat of formation than smaller Earth further supplemented by a larger radioisotope load ) . ”
A magnetic field needs more than just convention currents in the core. It needs the twin rotating currents caused by the coriolis deflection of a fast rotation like in our Earth which has two rotating cylindrical currents pointing North and South in the liquid core. Venus does not have this so it has a bow shock interaction with the solar wind which is deflected by it. Planetary geologists have hypothesized that Venus has no plate tectonics because it has no oceans oceans to cool of the mantle and make a mobile crust possible. The convention currents in the Mantle get to hot and melt the entire surface after 100 million years. https://www.psi.edu/epo/faq/venus.html
This of course happens with a planet with a big greenhouse effect like Venus which has an atmosphere mostly of CO2 so the temperature never lets an ocean form. If proxima B has a mass twice that of Earth it might have a thick atmosphere even though much of it has been lost by being stripped away by the solar wind so the atmosphere might not be the only problem. It probably does not have the same greenhouse effect and is not nearly as hot as Venus The crust can’t also be too thick or plate tectonics won’t work. The question is does Proxima B have an ocean and liquid water on it’s surface. We could assume that it might have also plate tectonics if it did. It certainly should have some volcanism due to it’s size especially being tidally locked since tides also might effect volcanism to a small degree. I agree it must have a large iron core. A lot of factors are involved here and some are still only theoretical since we don’t have any other planets other than our solar system as a model such as super Earths, but we can get a more better estimate of Proxima B’s with spectroscopy and an accurate measurement of its mass.
Zuluaga et al ( 2014) have developed models that show magnetic fields can be produced thermally by convection in the liquid iron/nickel cores of terrestrial planets provided there is mantle convection. These fields can be both dipole ( as with Earth) and with a stand off magnetosphere of several planetary radii ( and thus able to protect a putative atmosphere from stellar flux erosion) . Not so potent as a conventional rotational based field but still significant and especially so in slow rotating tidally locked planets .( Morales -Lopez work has shown that the biggest fields are produced by rotation rates of up to 1 day with rapid tail off thereafter ).
All liquid , hotter cores can produce large multi pole fields which offer much less protection and as with the dipole fields leave a large exposed polar area too. Their model suggests a 2Me terrestrial planet has the potential to produce the biggest field ( lower mantle viscosity at the core mantle boundary determines convection away from the core and consequent field duration ) though not nearly enough to withstand an active late M dwarfs flux.
The solar wind has a bow shock interaction with Venus atmosphere because the magnetic field is not strong enough to deflect so it’s atmosphere deflects the solar wind.
Question: Are higher stellar metallicities a possible route that could lead to stronger magnetic field strength for at least some planets? For example, Proxima’s metallicity is 0.21 dex, which means I think (and please correct me if this is wrong) that Proxima’s spectrum shows that it has 10^0.21 times, or about 62% more iron than our Sun does. Couldn’t this lead to bigger and hotter cores in some planets?
I disagree with the idea that a planet without a fast rotation can have a magnetic field only from convection. Venus has convection by why does it not have a magnetic field which is very weak and does not block the solar wind. Charged particles like electrons produce a magnetic field when they move in circles like the helical motion in Earth’s liquid core and like a nail with a wire wrapped around it with an electric current moving through the wire. Electrons do produce some electromagnetic radiation if they move straight.
It might help if the planet has an extra large core like Mercury but it could be a super Earth. I still think the kinetic energy from the angular momentum of rotation is needed to have a strong magnetic field. For example: Jupiter has a strong magnetic field from rotation which caused the coriolis deflection of the metalic liquid hydrogen inside it. There are electric arcs as a result of the rotation and coriolis deflection which make radio waves in the decametric frequency and a strong magnetic field. It is called liquid metallic hydrogen because it is so dense it behaves like a metal and conducts electricity due to the ionization pressure of the atoms.
Excuse me Venus has only a very weak magnetic field
Venus has no measurable magnetic field
The question unaddressed here is – How long in a red dwarf’s history do the violent flares continue? If we come upon a planet whose star is say a billion years old, does the star still have the dangerous flares? I understand that only youthful M stars do this. What then is “youthful?” In this position, I agree with Harry Ray, that finding abiotic but terraformable planets my be much more desirable than finding live planets as far as human use is concerned. Of course, if the flares continue, the planet would be of no use. If not, adding a denser atmosphere, or finding a planet further away from the center of the habitable zone and adding greenhouse gases to extend the zone would make the planet much easier to deal with. This assumes that red dwarf systems have lots of frozen ice and nitrogen in their outer reaches.
RDs like Proxima stay on the MS for around a TRILLION years! It is now thought to be 6.5 Billion, so I’d say it hasn’t moved out of the terrible two’s yet!