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Stormy ‘Space Weather’ for M-dwarf Planets?

Proxima Centauri b, that highly interesting world around the nearest star, is about 0.05 AU out from its primary. The figure leaps out to anyone new to red dwarf stars, because it’s so very close to the star itself, well within the orbit of Mercury in our own system. But these are small, dim stars compared to our Sun, and hugging the star is essential to remain in the habitable zone. That also makes for very short years — Proxima b completes an orbit every 11.2 days.

Guillem Anglada-Escudé and colleagues reminded us in the discovery paper that among the many things we have to ask about this planet is whether or not it has a strong magnetic field. Because Proxima Centauri is known for flare activity, not to mention 400 times the X-ray flux the Earth receives. A magnetic field could help the planet hang on to its atmosphere, but just how strong would it need to be? Like any M-dwarf planet, then, Proxima b seems vulnerable.

This thinking has ramifications much closer to home. We are learning that CMEs have influenced the atmosphere of Mars and may have played a large role in how it evolved. Data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft point in this direction, showing that a CME can compress the Martian magnetosphere, spinning off effects in the ionosphere and below. We are now in the realm of what is being called ‘space weather.’

Think about what happens here on Earth when the Sun throws off one of its enormous storms, a coronal mass ejection, or CME. Outflowing plasma can thoroughly disrupt communications and navigation equipment, with damage to satellites and even power blackouts, all this with our own planet’s magnetic field around us. And although stars like Proxima Centauri are much smaller than ours, it turns out that they are far more active, with CMEs often ten times more powerful.

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Image: On August 31, 2012 a long filament of solar material that had been hovering in the sun’s atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 1450 kilometers per second. The CME did not travel directly toward Earth, but did connect with Earth’s magnetic environment, or magnetosphere, with a glancing blow. causing aurora to appear on the night of Monday, September 3. Credit: NASA GSFC.

Add into the mix the fact that M-dwarfs can maintain high levels of magnetic activity for billions of years. Although Proxima Centauri is thought to be roughly the age of the Sun, about five billion years, it is clearly an active star. All of this is troubling for the prospects for astrobiology.

Now we have new work led by Christina Kay (NASA GSFC and Boston University) that makes an intriguing case. The probability of a habitable zone planet around an M-dwarf being hit by a CME may depend on the plane of the planet’s orbit. The work revolves around modeling of coronal mass ejections from the M-dwarf V374 Peg, allowing Kay and team to assess the effects of CMEs on a planet in the star’s habitable zone. The model is called Forecasting a CME’s Altered Trajectory (ForeCAT), and it predicts how a CME can wind up being deflected.

Now we get into interesting territory, for these models show that CME’s move in particular directions when deflected by the star’s magnetic field. A key factor here is what is known as the Astrospheric Current Sheet, which is the location of the minimum level of the background magnetic field. A strong magnetic field like that of V374 Peg can trap a CME at the Astrospheric Current Sheet, with the deflection depending primarily on the CME mass but also on its speed.

ForeCAT was used to analyze these interactions both in the case of a planet in the habitable zone of V374 Peg as well as a hot Jupiter orbiting a Sun-like star. The paper describes an intriguing result:

For both habitable zone mid-type M dwarf exoplanets and hot Jupiters [orbiting solar-type hosts] the probability of impact decreases if the exoplanet’s orbit is inclined with respect to the Astrospheric Current Sheet. 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.

In other words, planetary orbits that line up with the astrospheric current sheet, which is generally aligned with the star’s equator, have a higher probability of being hit by a CME than planets in higher-inclination orbits. All of this has significant potential for affecting habitability:

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.

At stake here is the minimum planetary magnetic field needed to retain an atmosphere (Kay and team believe a magnetosphere twice the size of the planetary radius is necessary). For mid-type M-dwarfs like V374 Peg, magnetic fields between tens to hundreds of Gauss are required to protect an exoplanet in the habitable zone. This is one to two orders of magnitude more than that of the Earth. The conclusion is stark: “We expect that rocky exoplanets cannot generate sufficient magnetic field to shield their atmosphere from mid-type M dwarf CMEs.”

The good news: The scientists argue that the minimum magnetic field strength will change depending on the M-dwarf’s spectral type, as well as on stellar activity and stellar magnetic field strength changes. Some types of M-dwarf may thus be more likely to retain an atmosphere than dwarfs like V374 Peg. Extending this work in their direction is a compelling next step.

The paper is Kay et al., “Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars,” Astrophysical Journal Vol. 826, No. 2 (abstract / preprint). An AAS Nova essay on this work is also available.

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Comments on this entry are closed.

  • Geoffrey Hillend September 22, 2016, 18:52

    I think Proxima B can still have a thick atmosphere even if it does not have a strong magnetic field. How strong is the solar wind from Proxima A compare to the solar wind of Venus which still has an a thick atmosphere? Venus atmosphere was postulated to be originally be three times thicker billions of years ago. If Proxima B atmosphere has a stronger solar wind maybe it has lost most or all of its atmosphere. Promixa B is 30 percent larger than Earth which means that might have a stronger gravity and higher escape velocity than Earth if it has the same density of Earth. Consquently, it might have a thick atmosphere. It also must have some kind of volcanism which might counter atmospheric loss from the solar wind.
    The solar radiation might not be a problem with a thick atmosphere. A magnetic field will not stop x rays and gamma rays but a thick atmosphere will stop them. If it had a thick greenhouse gas atmosphere early in its history and lost some of its atmosphere, more x rays and ultra violet might reach the surface making it less habitable today? As long as there is liquid water, life could evolve in an ocean and still be protected from ultra-violet radiation. It could also have lost a lot of water due to solar wind and the ultra violet splitting of water into hydrogen and oxygen.
    Finally what it the mass relationship of super Earth to the thickness of its atmosphere in relation to a thick atmosphere? Will a super Earth if it is too massive in the life belt must have a greenhouse effect too high for liquid water to exist due to a much thicker atmosphere than Earth? It might be permanently trapped in a greenhouse effect like Venus.
    If Proxima B is tidally locked how much would that affect it’s ability to capture and retain an atmosphere from the primordial accretion disk due to it s competition with the larger gravity of nearby Proxima A? It will interesting to see what spectroscopy of Proxima b will tell us once it becomes available.

  • Harry R Ray September 23, 2016, 9:41

    It is suprising that no papers(that I KNOW of, correct me if I’m wrong) came out regarding CME direct hits of Mercury and Venus while spacecraft were in orbit around these planets.I assume this is because neither planet has a magnetic field, and the strength of such CME’s can be INFERRED from hypothetically equivalent hitting Earth’s magnetic field. DIRECT data from spacecraft would STILL have been a boon toMODELING what happens when planets like Proxima Centauri b receive a direct hit. The one GOOD thing about these direct hits would be ELIMINATION of thick Hydrogen envelopes around these types of planets when these envelopes were HEATED by MUCH MORE LUMINOUS HOST STARS to the point where the envelopes FILLED the planets’ Hill spheres. CMEs could then PHYSICALLY PUSH such hydrogen OUTSIDE of the HILL SPHERE>

  • Andrew Palfreyman September 23, 2016, 15:51

    There’s a little upside for tidal locking in that the night side has a massive shield against radiation and flares. Now add a well-mixed global atmosphere and – who knows?

  • ljk September 26, 2016, 10:01

    Proxima b is on the short list for a SETI effort by Breakthrough Initiative:

    http://www.dailymail.co.uk/sciencetech/article-3765291/Mark-Zuckerberg-Yuri-Milner-Stephen-Hawking-begin-100-million-search-alien-life-nearby-Earth-like-planet.html

    To quote:

    Studies carried out so far by the project include most of the stars within 16 light years of Earth.

    Breakthrough Listen can collect data over a 10-year period from a network of the world’s most powerful radio and optical telescopes to yield vast, full-sky signal monitoring.

    Search capacity is 50 times more sensitive, cover 10 times more of the sky, 5 times more of the radio spectrum, and at speeds 100 times faster.

    What would Milner do if we did hear signals from an alien civilisation?

    ‘I will take a bottle of champagne out of the fridge and start thinking about the message back,’ he says.

  • xcalibur September 27, 2016, 2:02

    I believe that the presence of a strong magnetosphere is significant for the development of biology. First of all, it provides shelter from radiation, but more importantly it protects the atmosphere and oceans.

    My understanding is that Venus and Mars became inhospitable because their magnetospheres are lacking. On Venus, solar wind evaporated the oceans, broke up water vapor and stripped away hydrogen, leaving behind CO2 which caused a runaway greenhouse effect. On Mars, a similar process stripped away the water and most of the air, leaving behind a cold desert (presumably Mars’ greater distance and lower gravity led to the very different end result).

    These two examples illustrate why a strong magnetosphere (like on Earth) is one of the most important attributes of a habitable planet.

    As I said, it also shelters life from radiation, but this is less of a concern in my mind. First of all, life underground/underwater would be shielded from radiation. Secondly, life shows a remarkable ability to adapt to challenges and stimuli, so I believe that exobiology would adapt to higher levels of radiation (within reason). Therefore, solar flares are not necessarily a barrier to exobiology. Of course, it all depends on the numbers, but if there were an exo-earth with a strong enough magnetosphere to maintain its oceans and atmosphere, and it were subject to periodic flares that irradiated the surface, then I would not rule out a biosphere on that hypothetical planet.

  • Ashley Baldwin October 2, 2016, 7:32

    Although this article is very much dominated by planetary magnetic fields and their potential shielding effect from CMEs ( with limitations ) , a rather more important and somewhat understated but positive corollary is that there is a specific area for CME generation ( as opposed to uniformally about the star ) roughly centred onthe stellar Equatorial area . Approximately at the base of the plane of the ecliptic . The very plane in which planets generally orbit , though not all and not perfectly . This implies that even planets in modestly inclined orbits can avoid a lot of the worst effects of CMEs or indeed miss them all together , certainly experiencing both dimished numbers and effects though , irrespective and probably much greater than any additional protection they might gain from an indigenous magnetic field.

    The other factor that arises from this is that CMEs, of all the various dangerous stellar eminations , appear to be most responsible for planetary atmospheric erosion so anything that mitigates their effect has got to be good in terms of planetary habitability and most of all in M dwarf systems where the “habitable zone” is close to the star and well within the region of synchronous rotation.

    So avoiding CMEs is good and possible too if as a planet your orbit is inclined . Planets with such orbits are also more likely to have greater orbital eccentricity which if not too large can in turn can contribute to non synchronous rotation such as 3:2 or even 2;1 resonances , as seen in Mercury with an orbital eccentricity of 0.2 . This will help produce bigger magnetic fields due to quicker rotation ( down to five days for a hypothetical 2;1 eccentricity in Proxima b) in close in planets which could otherwise expect to be tidally locked.

    It will also increase vital heat transfer about the planet too , helping resist atmospheric collapse likely on the permanently cold night side of any synchronous planet.

    Various simulations have shown that synchronisation can be compatible with planetary habitability , but not many , and it would seem that anything that can prevent it can only be helpful . Especially if it’s linked with other processes that can help habitability such as CME avoidance and increased planetary magnetic field strength.

  • ljk October 11, 2016, 14:39

    Proxima Centauri Might Be More Sunlike Than We Thought

    Release No.: 2016-25

    For Release: Tuesday, October 11, 2016 – 9:00am

    Cambridge, MA –

    In August astronomers announced that the nearby star Proxima Centauri hosts an Earth-sized planet (called Proxima b) in its habitable zone. At first glance, Proxima Centauri seems nothing like our Sun. It’s a small, cool, red dwarf star only one-tenth as massive and one-thousandth as luminous as the Sun. However, new research shows that it is sunlike in one surprising way: it has a regular cycle of starspots.

    Starspots (like sunspots) are dark blotches on a star’s surface where the temperature is a little cooler than the surrounding area. They are driven by magnetic fields. A star is made of ionized gases called plasma. Magnetic fields can restrict the plasma’s flow and create spots. Changes to a star’s magnetic field can affect the number and distribution of starspots.

    Our Sun experiences an 11-year activity cycle. At the solar minimum, the Sun is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun’s surface on average.

    The new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak. However, its cycle is much more dramatic. At least a full one-fifth of the star’s surface is covered in spots at once. Also, some of those spots are much bigger relative to the star’s size than the spots on our Sun.

    “If intelligent aliens were living on Proxima b, they would have a very dramatic view,” says lead author Brad Wargelin of the Harvard-Smithsonian Center for Astrophysics (CfA).

    Full article here:

    https://www.cfa.harvard.edu/news/2016-25

    To quote:

    “The existence of a cycle in Proxima Centauri shows that we don’t understand how stars’ magnetic fields are generated as well as we thought we did,” says Smithsonian co-author Jeremy Drake.

    The study does not address whether Proxima Centauri’s activity cycle would affect the potential habitability of the planet Proxima b. Theory suggests that flares or a stellar wind, both of which are driven by magnetic fields, could scour the planet and strip away any atmosphere. In that case, Proxima b might be like Earth’s Moon – located in the habitable zone, but not at all friendly to life.

    “Direct observations of Proxima b won’t happen for a long time. Until then, our best bet is to study the star and then plug that information into theories about star-planet interactions,” says co-author Steve Saar.

  • ljk October 20, 2016, 9:14

    https://arxiv.org/abs/1610.05765

    The Habitability of Planets Orbiting M-dwarf Stars

    Aomawa L. Shields, Sarah Ballard, John A. Johnson

    (Submitted on 18 Oct 2016)

    The prospects for the habitability of M-dwarf planets have long been debated, due to key differences between the unique stellar and planetary environments around these low-mass stars, as compared to hotter, more luminous Sun-like stars.

    Over the past decade, significant progress has been made by both space- and ground-based observatories to measure the likelihood of small planets to orbit in the habitable zones of M-dwarf stars. We now know that most M dwarfs are hosts to closely-packed planetary systems characterized by a paucity of Jupiter-mass planets and the presence of multiple rocky planets, with roughly a third of these rocky M-dwarf planets orbiting within the habitable zone, where they have the potential to support liquid water on their surfaces. Theoretical studies have also quantified the effect on climate and habitability of the interaction between the spectral energy distribution of M-dwarf stars and the atmospheres and surfaces of their planets.

    These and other recent results fill in knowledge gaps that existed at the time of the previous overview papers published nearly a decade ago by Tarter et al. (2007) and Scalo et al. (2007). In this review we provide a comprehensive picture of the current knowledge of M-dwarf planet occurrence and habitability based on work done in this area over the past decade, and summarize future directions planned in this quickly evolving field.

    Comments: 44 pages, 11 figures, Invited review article accepted for publication in Physics Reports

    Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

    Cite as: arXiv:1610.05765 [astro-ph.EP]

    (or arXiv:1610.05765v1 [astro-ph.EP] for this version)

    Submission history

    From: Aomawa Shields [view email]

    [v1] Tue, 18 Oct 2016 20:00:00 GMT (6558kb,D)

    https://arxiv.org/pdf/1610.05765v1.pdf

  • ljk February 8, 2017, 17:15

    Feb. 8, 2017

    NASA Finds Planets of Red Dwarf Stars May Face Oxygen Loss in Habitable Zones

    The search for life beyond Earth starts in habitable zones, the regions around stars where conditions could potentially allow liquid water – which is essential for life as we know it – to pool on a planet’s surface. New NASA research suggests some of these zones might not actually be able to support life due to frequent stellar eruptions – which spew huge amounts of stellar material and radiation out into space – from young red dwarf stars.

    Now, an interdisciplinary team of NASA scientists wants to expand how habitable zones are defined, taking into account the impact of stellar activity, which can threaten an exoplanet’s atmosphere with oxygen loss. This research was published in The Astrophysical Journal Letters on Feb. 6, 2017.

    Full article here:

    https://www.nasa.gov/feature/goddard/2017/nasa-finds-planets-of-red-dwarf-stars-may-face-oxygen-loss-in-habitable-zones

    To quote:

    The new habitability model has implications for the recently discovered planet orbiting the red dwarf Proxima Centauri, our nearest stellar neighbor. Airapetian and his team applied their model to the roughly Earth-sized planet, dubbed Proxima b, which orbits Proxima Centauri 20 times closer than Earth is to the sun.

    Considering the host star’s age and the planet’s proximity to its host star, the scientists expect that Proxima b is subjected to torrents of X-ray and extreme ultraviolet radiation from superflares occurring roughly every two hours. They estimate oxygen would escape Proxima b’s atmosphere in 10 million years. Additionally, intense magnetic activity and stellar wind – the continuous flow of charged particles from a star – exacerbate already harsh space weather conditions. The scientists concluded that it’s quite unlikely Proxima b is habitable.

    “We have pessimistic results for planets around young red dwarfs in this study, but we also have a better understanding of which stars have good prospects for habitability,” Airapetian said. “As we learn more about what we need from a host star, it seems more and more that our sun is just one of those perfect parent stars, to have supported life on Earth.”