The prospects for life around M-dwarf stars, always waxing and waning depending on current research, have dimmed again with the release of new work from Christina Kay (NASA GSFC) and colleagues. As presented at the National Astronomy Meeting at the University of Hull (UK), the study takes on the question of space weather and its effect on habitability.
We know that strong solar flares can disrupt satellites and ground equipment right here on Earth. But habitable planets around M-dwarfs — with liquid water on the surface — must orbit far closer to their star than we do. Proxima Centauri b, for example, is roughly 0.05 AU from its small red host (7,500,000 km), while all seven of the TRAPPIST-1 planets orbit much closer than Mercury orbits the Sun. What, then, could significant flare activity do to such vulnerable worlds?
Image: Artist’s impression of HD 189733b, showing the planet’s atmosphere being stripped by the radiation from its parent star. Credit: Ron Miller.
Working with Merav Opher and Marc Kornbleuth (both at Boston University), Kay has homed in on coronal mass ejections (CMEs), the vast upheavals that throw stellar plasma into nearby space. While red dwarf stars are significantly cooler than our G-class Sun, their CMEs are thought to be far stronger because of their enhanced magnetic fields. From the paper:
Stellar activity tends to increase with the size of the stellar convection envelope (West et al. 2004) and stellar rotation rates (Mohanty et al. 2002; West et al. 2015), although the activity saturates for sufficiently high rotational velocity (Delfosse et al. 1998). For mid- to late-type M dwarfs (M4 to M8.5) the activity saturates at higher rotational velocities than for early-type M dwarfs, and above M9 the activity levels decrease significantly (Mohanty et al. 2002). Accordingly, most M dwarf stars will have significantly enhanced stellar activity as compared to the Sun.
A strong planetary magnetic field could counteract at least some of the resulting flare activity, but even there a possibility of strong erosion of the atmosphere remains. And if the planet is tidally locked, some recent work suggests little to no magnetic field can be expected.
The paper outlines the process when a CME hits a nearby planet. One problem is extreme ultraviolet and X-ray flux (XUV) which can heat the upper atmosphere and perhaps ionize it. If such radiation gets through to the surface, it can damage any potential life-forms there. While it turns out that an M-dwarf habitable zone planet receives an order of magnitude less XUV flux than Earth when the star is quiet, the flux jumps as high as 100 times Earth’s during the star’s frequent flare activity.
Significant CME activity also makes the planet much less likely to retain its atmosphere as a shield for life on the surface. A CME compresses whatever magnetosphere the planet has, and in extreme cases, say the authors, can exert enough pressure to shrink the magnetosphere to the point where the atmosphere can be seriously eroded.
Modeling an Astrospheric Current Sheet
Kay’s team modeled the effects of theoretical CMEs on the red dwarf V374 Pegasi, using a tool Kay developed for CME modeling called ForeCAT. They found that the strong magnetic fields of the star produce CMEs that can reach the so-called Astrospheric Current Sheet, where the background magnetic field is at its minimum. The same effect occurs with our Sun, when solar CMEs are deflected by magnetic forces toward the minimum magnetic energy.
At the Sun, the Heliospheric Current Sheet — the local analog to a different star’s Astrospheric Current Sheet — is a field that extends along the Sun’s equatorial plane in the heliosphere and is shaped by the effect of the Sun’s rotating magnetic field on the plasma in the solar wind. The HCS separates regions of the solar wind where the magnetic field points toward or away from the Sun.
Let’s dwell on that for a moment. Here’s what a NASA fact sheet has to say about the Heliospheric Current Sheet:
The sun’s magnetic field permeates the entire solar system called the heliosphere. All nine planets orbit inside it. But the biggest thing in the heliosphere is not a planet, or even the sun. It’s the current sheet — a sprawling surface where the polarity of the sun’s magnetic field changes from plus (north) to minus (south). A small electrical current flows within the sheet, about 10−10 A/m². The thickness of the current sheet is about 10,000 km near the orbit of the Earth. Due to the tilt of the magnetic axis in relation to the axis of rotation of the sun, the heliospheric current sheet flaps like a flag in the wind. The flapping current sheet separates regions of oppositely pointing magnetic field, called sectors.
Image: The Heliospheric Current Sheet results from the influence of the Sun’s rotating magnetic field on the plasma in the interplanetary medium (solar wind). The wavy spiral shape has been likened to a ballerina’s skirt. The new work uses a software modeling package called ForeCAT to study interactions between CMEs and the Astrospheric Current Sheet around the red dwarf V374 Pegasi. Credit: NASA GSFC.
Kay and team have modeled the Astrospheric Current Sheet expected to be found around M-dwarfs like V374 Pegasi. The authors find that upon reaching the ACS, CMEs become ‘trapped’ along it. Planets can dip into and out of the ACS as they orbit. A CME moving out into the Astrospheric Current Sheet around an M-dwarf can cancel out a habitable zone planet’s local magnetic field, opening the world to devastating flare effects. The upshot:
We expect that rocky exoplanets cannot generate sufficient magnetic field to shield their atmosphere from mid-type M dwarf CMEs… We expect that the minimum magnetic field strength will change with M dwarf spectral type as the amount of stellar activity and stellar magnetic field strength change, and that early-type M dwarfs would be more likely to retain an atmosphere than mid or late-type M dwarfs.
The authors calculate that a mid-type M-dwarf planet would need a minimum planetary magnetic field between tens to hundreds of Gauss to retain an atmosphere, values that are far higher than Earth’s (0.25 to 0.65 gauss). CME impacts as numerous as five per day could occur for planets near the star’s Astrospheric Current Sheet. The only mitigating factor is that the rate decreases for planets in inclined orbits. The paper notes:
The sensitivity to the inclination is much greater for the mid-type M dwarf exoplanets due to the extreme deflections to the Astrospheric Current Sheet. For low inclinations we find a probability of 10% whereas the probability decreases to 1% for high inclinations. From our estimation of 50 CMEs per day, we expect habitable mid-type M dwarf exoplanets to be impacted 0.5 to 5 times per day, 2 to 20 times the average at Earth during solar maximum. The frequency of CME impacts may have significant implications for exoplanet habitability if the impacts compress the planetary magnetosphere leading to atmospheric erosion.
So we have much to learn about M-dwarfs. In particular, how accurate is the ForeCAT model in developing the CME scenario around such stars? As we examine such modeling, we have to keep in mind that magnetic field strength will change with the type of M-dwarf we are dealing with. Based on this research, only early M-dwarfs are likely to maintain an atmosphere.
The paper is Kay, Opher and Kornbleuth, “Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars,” accepted at the Astrophysical Journal (preprint).
Comments on this entry are closed.
The ebb and flow of M dwarf star suitability for habitability. First one way then the other. This paper even acknowledges that for the same mid M dwarf star should a “hab zone” planet have even a small inclination to the equatorial plane of the star ( where the CME concentration is worst ) then the CME rate could drop from five plus per day to less than one every other day. Regardless of magnetic field interaction. With M dwarfs age for a start is critical too in terms of CME activity dropping off , particularly for earlier M dwarfs at around 1 billion years . CME activity peaks for M6 dwarfs , but even stars of this class like Barnard’s star display markedly reduced activity at around 5 billion years . With expected main sequence lifetimes of a trillions of years or so . Even the biggest most quiescent M dwarfs ( which are not fully convective and thus lose their potent and chaotic magnetic fields much sooner than the star cited here, and with greater luminosity can push that hab zone out as far as 0.4 AU ) lasting ten times as long as the Sun.
Although as we all know, M dwarfs have an extended active pre main sequence period too, from work done on the TRAPPIST-1 planets it also looks as if these are formed beyond the “ice line ” before migrating in from distance much later , thus avoiding the worst of the stellar activity prior to entering the proximal “habitable zone” . Given Kepler has shown that M dwarfs have a preference for forming Earth sized terrestrial planets almost independent of stellar “metallicity ” ( and thus age ) , and that these stars represent 70 % of all stars , I think it likely that at least some will harbour life of some sort. It may be easy to put forwards obstacles to this , but it is equally easy to envisage plenty of circumstances in which these could be bypassed .
Role on TESS, PLATO,JWST and the ELTs!
excellent study, since it confirms my own thoughts about little red suns with a few scorched rocks whizzing around them in 9-day “years”. If we want to find interesting life forms, we’ll find them on habitable zone planets moving around yellow suns in a dignified manner, say, 250-500 day years. Sara Seager is wasting her life. So sad.
‘Sara Seager is wasting her life. So sad.’
The seeking of knowledge is not wasted time, what is sad is seeking time to waste.
There is at least one planet (f) around TRAPPIST-1 (a M-dwarf) that has all the characteristic of an ocean planet with a rocky core and a dense atmosphere. This counter-example contradicts all the theories – including the one presented in this article – saying that the frequent flares from M stars should completely erode the atmosphere of any tidally-locked planet to the point where no life can ever exist on its surface.
Therefore it’s too early to rule out completely all M-dwarves in our search for extraterrestrial life.
Scientists used to think life could not exist at the bottom of Earth’s oceans or in boiling hot acidic springs, but guess what? And certain microbes were found swimming in the reactor pools of nuclear power plants that were able to handle radiation levels that were over three thousand times beyond what a human could tolerate before being fried into bacon.
So maybe life cannot exist on worlds circling red dwarf suns, but as I noted above we have been surprised before. And oh yeah, it was quite recent that astronomers realized exoplanets can have stable orbits around binary stars – just another example.
The accretion theory of planetary production itself has only gained precedence in the last thirty years or so. Before that many plausibly believed that the solar system had been created by a “passing star” ( something that still has traction in numerous other theories ) dragging out mass from the Sun that then condensed into planets . Hot Jupiters more recently defied all planetary formation simulations as have Super Earths and mini Neltunes more recently still . I think it’s fair to say that once there is detailed spectroscopic characterisation of temperate terrestrial planets , even around M dwarfs , then we are in for yet more surprises .
Omniscient are you? How lucky for you.
Certainly not, but hopefully lucky enough to see some interesting exoplanetary atmospheric spectroscopy within the next decade .
On the one hand we have M-dwarf star’s CMEs potentially eroding a planets atmosphere.
On the other hand we have evidence for heaps of mini Neptune / large super Earths whose atmospheres are thought to be too thick to be habitable.
I’m beginning to think that the most likely “habitable” planet around M-dwarfs is going to be a large super Earth whose previously thick atmosphere has been stripped down to something more life friendly.
My worry with this scenario would be that the heat of the ‘evaporation’ process would create conditions that would destroy organic molecules before they could form complex life.
On the plus side there will be plenty of room for intelligent beings to occupy.
The heat of the evaporation process just shifts the habitable zone out a little.
Nice to know that, while the necessary field to protect against the CME’s is higher than plausible for a natural field, it’s quite feasible to supply technologically. So if we did find a suitable planet, we could keep it that way.
If say we had an Earth mass and solar irradiance equivalent at the start of the contraction phase by the end of it a billion years later the planet would be in the deep freezer. It will most likely keep a magnetic field due to lower chance of tidal locking and could have life under the ice, this life could emerge much, much later as the star goes through its main sequence and gets hotter.
Now if say we had a large planet that ‘evaporates’ to an Earth mass and solar irradiance one it would most likely have no magnetic field due to tidal locking and no organics required for life due to thermal/chemical destruction.
Red Dwarfs are looking more and more like life producing stellar dead zones at least for complex life.
Put a substantial terrestrial class moon in orbit around a Neptune of Mini Neptune planet ( both of which have shown to be relatively frequent around M dwarfs ) and it tidally locks to the planet not the star . It may thus rotate quickly enough to stir up an atleast partially protective magnetic field from its outer convective core ( which could create a reasonable field in its own right if the mantle is suitably convective too) .
Given the high absorption of M dwarf near infrared radiation by even 0.5 bar CO2 atmospheres , there is also potential to reduce the required instellation below even 25 % Earth thus pushing the habitable zone further out perhaps even beyond 0.4 AU for an M0 star . ( further still for a hydrogen rich “habitable evaporated core ” planet ) Thus bringing in the inverse square law to offer additional mitigation of CMEs etc. Stretch into late K class and it could even go beyond 0.5 AU and out if tidal locking territory , or not for billions of years anyway . ( Leconte et al (2015 ) have also shown that substantial CO2 atmospheres can resist orbital synchronisation in their own right also )
Put all these together and we have a good deal of protection against anything nasty coming from the host star without even considering any mitigation provided by the nature of said star itself. ( larger mass, greater age / lower rotation and consequent magnetic field etc )
‘Put a substantial terrestrial class moon in orbit around a Neptune of Mini Neptune planet…’
Unfortunately a moon around these close in planets would be unstable as the star facing LaGrange point is quite close to the planet.
Not necessarily , especially not for any hab zone planet/moon sitting in the outer hab zone of larger M dwarfs ( and smaller K dwarfs thinking of the wider definition of ” red dwarf “) . Where the gravitational zone of influence of any planet orbiting a larger star , the Hill radius , r , = a x cube root (m/3M ) with a= the planetary semi major axis , m= planetary mass and M = stellar mass . Even the hot Super Earth CoRoT-7b orbiting at just 0.017 AU from a 0.91 Msun star has a Hill radius of 61000 Kms , six times the planetary radius . Gravitational tidal heating provided from a larger planet might even help extend the hab zone for an erstwhile moon even further .
Here is a neat quick hill radius calculator, from it we can see there is not a lot of room between the Hill and the Roche limit for some stable moon systems.
Nice find. Thanks . Ever the optimist I still think there is latitude for stable terrestrial moon around the Mini-“maxi “Neptunes in the outer hab zones of early M and late K “red dwarfs ” at the least .
Ironically when exomoons are finally discovered it’s likely to be around red dwarfs ,via transit photometry as its sensitivity increases most likely up to and during an extended PLATO mission. TESS hab zone planets will likely be too close in to hold onto a moon as per Hill/Roche as you point out , though if they’re there it should be easier to tease their signatures out of the deeper transits associated with larger TESS planets .
We’re currently at that twixt and between stage were theory and simulation is running ahead of practical observation sensitivity .
So why do the Trappist1 planets in the habitable zone have low densities?
They formed from volatile rich accretion disk outside of the “ice line” before migrating inwards at a much later time . Thus avoiding a lot of the worst pre and post main sequence stellar activity of what is likely to have been lively star even by M dwarf standards.
It must be frustrating to discover that the exoplanets that are the easiest for us to find, are the ones that are the least likely to harbor life. I wouldn’t advise giving up, however.
There’s a hell of a lot of them though , with many not nearly as active as the example star cited here either through age or mass or both. With a propensity for forming Earth sized terrestrial planets too. If you count “late” K class stars as “red dwarfs” as well ( as is often done ) then things improve further still, with potentially up to 80% of all stars falling in this class.
And we still keep assuming that the types of ETI we have the best chance of detecting are still living on dirty old planets.
Just like the rich humans on this planet, the really advanced aliens are off somewhere in the really nice sections of the galaxy, probably living in structures of their own making and not very big on advertising their existence. Or perhaps they ARE the structures they built.
We need to think outside the box even if it leads us to more dead ends. SETI still keeps focusing on ideas going back to Project Ozma from 1960, namely that aliens not too different from us are sitting on Earthlike planets circling yellow dwarf suns broadcasting away because they want to make contact because that is what an altruistic scientist would do.
Well if such beings do exist and they are conducting such METI projects, they certainly are not being very obvious to us. Then again look at the history of human SETI and you will see that it probably would take someone doing some massive, constant transmitting right at our Sol system to get our attention.
No, we keep assuming that we will find ETI everywhere that we look.
Ashley Baldwin I wouldn’t assume the theory that the TRAPPIST-1 planets had to form outside the snowline and migrate inward is a general principle of the smaller red dwarf stars. It is highly speculative since it says that the planets had to form sequentially, but not all at the same time like planetary accretion theory.
I like the idea of the migrating millimeter and centimeter sized particles, but since the force of gravity is stronger than thermal emission, these sized particles would not have to migrate to the ice line to come together to form a planetesimal. It sounds like the theory says that these sized particles have to be cleared out by thermal emission from the life belt and closer to the star than the life belt, the water vapor line where the temperature is high enough for water to only be in a vapor. It’s not a bad idea if that is what is meant.
With planetary accretion theory, the planetesimals and the star form at the same time so by the time the star is born, there already are planetesimals the size of our Moon. I am not an expert on planetary accretion theory, but I don’t think that thermal emission plays a significant role in the formation of planetesimals before the star is born or even after. I could be wrong of course but there still is the problem of ultra violet light It sounds like the TRAPPIST-1 migration theory says that the millimeter and centimeter particles could only accumulate inside the snow line where water is frozen. They wouldn’t need thermal emission to move the particles out of the life belt, the liquid water line, or water vapor line where it is too hot and close to the star for water to exist as a liquid.
Accretion theory suggests that planets could just as well form very close to a red dwarf. I could be wrong, and maybe things are different with smaller stars. It seems logical with a smaller star and less gravity, planets would form nearer to it. There still is the problem of the space weather though, the x-rays, ultra violet, and CME’s, and cosmic rays.
Technological limitations of both transit photometry and Doppler spectroscopy have so far favoured discovery of close in planets around smaller M dwarfs with small related “hab zones ” like Proxima b and the TRAPPIST planets . It will interesting when , as seems likely , terrestrial planets are discovered in the hab zones of larger M dwarfs extending out towards 0.5 AU . Debra Fischer’s 100 Earths project is due to start later this year when the EXPRES high res spectroscope becomes operational on the Discovery telescope . Given its chromospheric activity reducing software it is optimised for discovering just such planets . TESS should push the hab zone discovery field out a bit and PLATO will take it to its maximum.
What would be the optimal design for a large space station that could comfortably play host to terrestrial life but be in orbit around a burping red dwarf star? It seems like these things are orbited by lots of good construction material. It might not generate its own lifeforms, but I bet we could make a cozy home there.
I assume that silicon solar cells would degrade pretty fast, but the station would need some robust and hopefully efficient method of generating power. Could coronal mass ejections themselves be harvested by giant antennas and capacitors?
This is different:
Stellar Chemical Clues As To The Rarity of Exoplanetary Tectonics
Press Release – Source: astro-ph.EP
Posted July 5, 2017 1:58 PM
Earth’s tectonic processes regulate the formation of continental crust, control its unique deep water and carbon cycles, and are vital to its surface habitability.
A major driver of steady-state plate tectonics on Earth is the sinking of the cold subducting plate into the underlying mantle. This sinking is the result of the combined effects of the thermal contraction of the lithosphere and of metamorphic transitions within the basaltic oceanic crust and lithospheric mantle. The latter of these effects is dependent on the bulk composition of the planet, e.g., the major, terrestrial planet-building elements Mg, Si, Fe, Ca, Al, and Na, which vary in abundance across the Galaxy.
We present thermodynamic phase-equilibria calculations of planetary differentiation to calculate both melt composition and mantle mineralogy, and show that a planet’s refractory and moderately-volatile elemental abundances control a terrestrial planet’s likelihood to produce mantle-derived, melt-extracted crusts that sink. Those planets forming with a higher concentration of Si and Na abundances are less likely to undergo sustained tectonics compared to the Earth.
We find only 1/3 of the range of stellar compositions observed in the Galaxy is likely to host planets able to sustain density-driven tectonics compared to the Sun/Earth. Systems outside of this compositional range are less likely to produce planets able to tectonically regulate their climate and may be inhabitable to life as we know it.
Cayman T. Unterborn, Scott D. Hull, Lars P. Stixrude, Johanna K. Teske, Jennifer A. Johnson, Wendy R. Panero
(Submitted on 30 Jun 2017)
Comments: Submitted. 18 pages, 7 figures, 1 Table
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1706.10282 [astro-ph.EP] (or arXiv:1706.10282v1 [astro-ph.EP] for this version)
From: Cayman Unterborn
[v1] Fri, 30 Jun 2017 17:31:51 GMT (1111kb,D)
Exoplanets and SETI
Press Release – Source: astro-ph.EP
Posted July 10, 2017 8:19 PM
The discovery of exoplanets has both focused and expanded the search for extraterrestrial intelligence.
The consideration of Earth as an exoplanet, the knowledge of the orbital parameters of individual exoplanets, and our new understanding of the prevalence of exoplanets throughout the galaxy have all altered the search strategies of communication SETI efforts, by inspiring new “Schelling points” (i.e. optimal search strategies for beacons).
Future efforts to characterize individual planets photometrically and spectroscopically, with imaging and via transit, will also allow for searches for a variety of technosignatures on their surfaces, in their atmospheres, and in orbit around them. Even in the near-term, searches for new planetary systems might even turn up free-floating megastructures.
Jason T. Wright
(Submitted on 7 Jul 2017)
Comments: 9 page invited review
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1707.02175 [astro-ph.EP] (or arXiv:1707.02175v1 [astro-ph.EP] for this version)
From: Jason Wright
[v1] Fri, 7 Jul 2017 13:55:03 GMT (13kb,D)
Radio Exploration of Planetary Habitability: Conference Summary
Press Release – Source: astro-ph.EP
Posted July 10, 2017 8:18 PM
Radio Exploration of Planetary Habitability was the fifth in the series of American Astronomical Society’s Topical Conference Series.
Notable aspects of the conference included the interdisciplinary nature of both the topics and the intellectual breadth of the participants, the diversity of approaches to studying this topic presented by recent discoveries and of the participants themselves, the expanding meaning of the topic of “star-planet interactions,” and the expectation of an increasingly statistical approach to the topic.
Potential areas of future research include the actual extent to which planetary magnetic fields shield planetary atmospheres; the planetary dynamo process itself, particularly once multiple extrasolar planetary magnetic fields are confirmed; and “planet-star interactions.”
A major major topic of the conference concerned observational opportunities, highlighted by a number of new or upcoming, specialized observatories to observe exoplanets especially at radio wavelengths. This article summarizes these main points of the conference and expands briefly upon these potential avenues for future investigation. A future meeting on this topic, given the variety of data sets being generated over the next few years, is warranted.
T. Joseph W. Lazio (JPL, CIT), A. Wolszczan (Penn. State Univ.), M. Güdel (Univ. Vienna), Rachel A. Osten (STScI), Jan Forbrich (Univ. Vienna), M. M. Jardine (Univ. St. Andrews), P. K. G. Williams (CfA)
(Submitted on 7 Jul 2017)
Comments: Five pages; conference Web site: this http URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:1707.02107 [astro-ph.EP] (or arXiv:1707.02107v1 [astro-ph.EP] for this version)
From: Joseph Lazio
[v1] Fri, 7 Jul 2017 10:17:07 GMT (134kb)
Seemingly strange radio signals from a red dwarf star spark interest at Arecibo
by Alan Boyle on July 13, 2017 at 8:14 pm