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I Small stars host water worlds

  1. Nov 5, 2016 #1
    It has always been my impression than spectral type M stars were notorious for being flare stars. As a result of their small radius and relatively low effective surface temperature, the Habitable Zone has to be relatively close to the surface of the star and small in size. As a result, it would seem to preclude exoplanets with a viable atmosphere in the Habitable Zone of such stars. However, I find that my initial impression may not have been correct.

    A sample of 23,253 stars in Walkowicz et al. (2011), with effective temperature Teff less than 5,150°K (which includes spectral type K and M main sequence stars) and a surface gravity log g > 4.2 was examined for flares over a time period of 33.5 days. Of those 23,253 stars, only 373 stars were identified as having obvious flares. Some stars had only one flare, while others showed as many as fifteen. The strongest events increased the brightness of the star by 7% to 8%. That is a population of only 1.6%. Granted, 33.5 days is not a very long period, but that is still not the percentage of flare stars I originally assumed would be associated with spectral type M stars.

    Althougth, the Walkowicz et al. (2011) study does add a caveat that younger stars tend to be more active than older stars. What that means in the case of spectral type M main sequence stars, which could potentially live for trillions of years, I am not exactly sure. In a universe that is only ~13.78 billion years old, is there such a thing as an "old" spectral type M main sequence star?

    Furthermore, Walkowicz et al. (2011) explains that observations made of solar flares from spectral type M main sequence stars tend to be 10 to 1,000 times as energetic as solar flares. For the 1.6% of flare stars this pertains to, that would seem to rule out any possibility of an exoplanet within the Habitable Zone from having an atmosphere, much less liquid water on its surface. However, that still leaves 98.4% of spectral type M main sequence stars that are not flare stars.

    Since the above study, Alibert et al. (2016), used 0.1 M, 1.004 R, and 2,935°K Teff for its initial conditions, I was able to estimate the "conservative" Habitable Zone, using Kopparapu et al.(2014), as being between 0.14 AU and 0.38 AU, with the frost-line/snow-line (160°K) at 0.79 AU. Interestingly, they found through their simulation that planets with low mass (specifically < 0.4 M) tend to be totally devoid of water.

    Alibert et al. (2016) also assumes that the mean surface temperature of the simulated exoplanet is no colder than 273.16°K (0.01°C; 32.02°F) with a minimum atmospheric pressure of 611.657 pascals (6.11657 mbar; 0.00603659 atm; 0.088713417 psi) at "sea level."

    Formation and composition of planets around very low mass stars - arXiv 1610.03460
    A terrestrial planet candidate in a temperate orbit around Proxima Centauri - Nature 536, 437-440 (August 25, 2016), doi:10.1038/nature19106 (arXiv free preprint)
    White-light flares on cool stars in the Kepler Quarter 1 Data - The Astronomical Journal, Volume 141, Number 2, January 13, 2011 (free article)
    Evolution of protoplanetary disks: constraints from DM Tauri and GM Aurigae - Astronomy & Astrophysics, Volume 442, Number 2 (November 1, 2005) (arXiv free preprint)
    Flares on active M-type stars observed with XMM-Newton and Chandra - Thesis by Urmila Mitra Kraev, University College London [PDF]
    Habitable Zones Around Main-Sequence Stars: Dependence on Planetary Mass - The Astrophysical Journal Letters, Volume 787, Number 2 , May 15, 2014 (free article)
    Last edited: Nov 5, 2016
  2. jcsd
  3. Nov 10, 2016 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
  4. Nov 11, 2016 #3
    I would just like to thank you. I love reading about habitat zones, especially the ones in our relative proximity.
  5. Nov 11, 2016 #4
    Because I had my initial perception of red dwarf stars and the viability of their habitable zones altered by the study I posted above, I did some further research into the subject matter in order to get better insight.

    The “conservative” liquid water Habitable Zone (LW-HZ) around red dwarf main sequence stars is extremely close (< 0.5 AU) due to the very low effective surface temperature of the star. As a result, any exoplanet within 0.5 M to 4.162 M will most likely be tidally locked. Due to this tidal locking, the magnetosphere of the exoplanet will be diminished, allowing Coronal Mass Ejections (CMEs) and the strong solar winds to slowly erode the atmosphere of the exoplanet. An exoplanet without an atmosphere cannot support liquid water on its surface.

    Larger exoplanets (> 4.162 M) would have a stronger magnetosphere, even though they may also be tidally locked, and therefore may be able to retain some of its atmosphere. However, exoplanets greater than 1.599 R may not be terrestrial/rocky.

    While the Alibert et al. (2016) paper above does take into account the irradiation of the star in some of their simulations, they do not take into consideration tidal locking and the diminished effect it has on the magnetosphere of Earth-like exoplanets within the LW-HZ, or the erosive effect the CMEs and strong solar winds have on the exoplanet's atmosphere.
    Interestingly, I came across a couple of articles recently in Astrobiology that suggests that CMEs may be an important factor for life on terrestrial/rocky exoplanets within the LW-HZ. They do take into consideration tidal locking and the weakening effect this has on the exoplanet's magnetosphere.
    However, the study goes on to conclude that if there is a sufficiently strong enough magnetosphere to prevent complete atmospheric erosion, and high concentrations of CO2 in the atmosphere, an Earth-like (0.5 M to 4.162 M) exoplanet in the LW-HZ of a red dwarf main sequence star may be able to support life.

    One other consideration needs to be factored into the climate of Earth-like exoplanets within the LW-HZ of red dwarf main sequence stars: Assuming the exoplanet is able to retain at least 6.11657 mbar of atmospheric pressure and a temperature of 273.16°K at the surface of the exoplanet, a tidally-locked exoplanet may find the vast majority of its liquid water trapped in ice-sheets on the dark side of the exoplanet, while the daylight side is a vast desert. The "Water-Trapped Worlds" article listed below provides some interesting possibilities to consider.


    UV habitable zones around M stars - Icarus, Volume 192, Issue 2, December 15, 2007, Pages 582–587 (arXiv free preprint)
    Water-Trapped Worlds - The Astrophysical Journal, Volume 774, Number 1, August 16, 2013 (free article)
    Magnetospheric Structure and Atmospheric Joule Heating of Habitable Planets Orbiting M-dwarf Stars - The Astrophysical Journal, Volume 790, Number 1, July 3, 2014 (free article)
    Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for the Habitability of Terrestrial Exoplanets. I. CME Impact on Expected Magnetospheres of Earth-like Exoplanets in Close-In Habitable Zones - Astrobiology, Volume 16, Number 10, October 2016, Online ISSN: 1557-8070 (free article)
    Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for the Habitability of Terrestrial Exoplanets. II. CME-Induced Ion Pick Up of Earth-like Exoplanets in Close-In Habitable Zones - Astrobiology, Volume 16, Number 10, October 2016, Online ISSN: 1557-8070 (free article)
    Most 1.6 Earth-Radius Planets are not Rocky - The Astrophysical Journal, Volume 801, Number 1, March 2, 2015 (free article)
    Red Dwarf Planets Face Hostile Space Weather Within Habitable Zone - Astrobiology Magazine, June 11, 2014
    Last edited: Nov 11, 2016
  6. Nov 11, 2016 #5


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    Nice summary, Glitch.

    I'm pretty sure this is incorrect, unless you meant radius rather than mass - but even then I'd like to see where it's coming from.
  7. Nov 11, 2016 #6
    Yes, I meant radius, not mass. My mistake. I have corrected it. Thanks for pointing it out.

    Source: Most 1.6 Earth-Radius Planets are not Rocky - The Astrophysical Journal, Volume 801, Number 1, March 2, 2015 (free article)
  8. Nov 11, 2016 #7


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    You should perhaps reconsider other instances you've used mass there too - the range 0.5-1.5 M⊕ for a likely water-hosting exoplanet does not follow now, since you can easily get over 5 earth masses on the far end of the rocky planet range.
  9. Nov 11, 2016 #8
    I used the 0.5 because Alibert et al. (2016) concluded that low-mass exoplanets (< 0.4 M) could not support liquid water. As to the high end, according to Weiss et al. (2014) (see equation #3), you can only get a mass of 4.162 M if you limit the radius to 1.599 R.
  10. Nov 11, 2016 #9


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    That's their statistical fit to data - it has a large scatter (see the rms error). Just look at the table of observables they used.
  11. Nov 11, 2016 #10
    Granted, they use a very limited set of observable planets to formulate their equation, but is there anything better? At least it uses real-world examples as opposed to pure speculation.
  12. Nov 13, 2016 #11


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    The point is, it's a statistical analysis rather than a physical model. It's erroneous to use it to make categorical statements about how much mass a planet 'can' have per given radius. All you can say, is that in a large sample the trend is to fit that model.
    E.g., if you'll find in the sample two planets with the same radius, but with widely differing masses (due to density differences), the equation would be (roughly speaking) fitted to draw a line going through the average of those values. It would be silly to then claim that a planet of that radius can only have that average value as its mass.
  13. Nov 15, 2016 #12
    This is a very interesting topic, and Glitch has done a great job summarising some of the main question. I work on these topics in my own research so I am always interested in getting into any discussion on it. I have some comments about what has been written so far.

    Another factor is the smaller radii of low mass stars, which means that they have smaller surface areas. Both that and the lower effective surface temperatures means that the stars are less luminous and therefore a planet must be closer in to get the same flux of light from the star. What is interesting though, is that the lower effective temperature actually means that in order for the planet to have an appropriate surface temperature for water, it needs less light than it would if it orbited a G star like the Sun. The difference is in the spectrum and the fact that photons with different energies heat the surface a different amount. The details, I do not know, but this is a result of the climate modelling done in studies like Kopparapu et al.(2014) cited above. Concisely put, a planet orbiting a cooler star gets more surface heating per unit input energy than it would orbiting a hotter star.

    It is very interesting, but it is all still very speculative. There is an idea floating around right now that very active (magnetically) stars produce winds that are dominated by coronal mass ejections. This is based primarily on observations of a correlation between flares and CMEs on the Sun, and observations of high flare rates, and very strong flares, on very active stars. No CMEs have ever actually been observed coming from any of these stars. In the habitable zones of M stars, we would expect these CMEs to be very dense and therefore influence the atmospheres of planets. However, how they will influence the atmospheres is still very difficult to say. I think we need to develop a better theoretical understanding of atmospheric loss mechanisms.

    Anyway, I want to write some more on this, but I have got to go back to work. Maybe later.
  14. Nov 18, 2016 #13
    I tried to keep speculation to a minimum. The second astrobiological study, Lammer et al. (2016), is where I derived the conclusion you cited:
    As I mentioned at the very beginning of the thread, this is not the conclusion I would have arrived at initially. However, having read the sources I posted above, I have come to a completely different understanding. An exoplanet within the LW-HZ of a red dwarf main sequence star that is able to retain its atmosphere has a much greater chance of supporting life, as opposed to an exoplanet that has its atmosphere eroded away.
  15. Nov 18, 2016 #14
    Yes, you of course came to the correct conclusions based on the paper. What I meant is the arguments in the paper that CMEs can cause a significant amount of atmospheric mass loss are quite speculative. The reason is that it is unclear that these strong continuous streams of CMEs even exist since they have never been observed coming from any star except the Sun and have not been predicted from physical models. The idea comes simply from analogies with the solar flare-CME correlation.
  16. Nov 18, 2016 #15
    Actually, CMEs/solar flares have been observed on Proxima Centauri. We have known Proxima Centauri was a flare star since 1951. They have also observed that Proxima Centauri has a solar cycle every 7 years, where sun spot activity covers as much as 20% of the star's surface. A paper was released on this subject just last week.

    Solar-Like M-Class X-ray Flares on Proxima Centauri Observed by the ASCA Satellite - American Association for the Advancement of Science, Volume 268, Issue 5215, pp. 1327-1329, June 2, 1995
    Optical, UV, and X-Ray Evidence for a 7-Year Stellar Cycle in Proxima Centauri - Monthly Notices of the Royal Astronomical Society, November, 11, 2016 (arXiv free preprint)
  17. Nov 18, 2016 #16
    Sure, many flares have been observed on many stars, but CMEs (i.e. the actual ejected material) have not been detected. There have been some attempts, and some hints for CMEs, but nothing concrete.
  18. Nov 18, 2016 #17
    Isn't the mechanism that creates solar fares the same mechanism that produces CMEs - a strong magnetic field? Therefore, if a star produces strong solar flares, is it not almost certain that the same star will also produce CMEs? I understand that there has been no direct observations of CMEs on other stars, but the inference that they exist is extremely strong on stars that produce strong solar flares. To dismiss them as pure speculation merely because they have not been directly observed would defy reason.
  19. Nov 19, 2016 #18
    The mechanisms are quite poorly understood and the link between flares are CMEs are also quite poorly understood. Both are a result of the magnetic field, and more specifically, both are a likely to be a result of energy release from magnetic reconnection. However, a flare on the Sun is not necessarily followed by the release of a CME, and a CME is not necessarily preceded by a flare. For a good study on this, check out Aarnio et al. (2011). They analyse 6700 CMEs and 12,000 flares on the Sun and find only about 800 that are correlated. This is partly due to difficulties in correlating flares and CMEs; as far as I understand, the observatories that detect CMEs have to block out not just the Sun, but also a large region around the Sun, so they only see CMEs after they have already traveled quite far and it is quite difficult to trace them back to their sources. Interestingly, I heard (but don't have a reference at the moment) that the largest flares on the Sun are always followed by a CME.

    Based on some of the correlations that can be seen in Aarnio et al. (2011), e.g. Fig 15, it is natural to assume that there are more CMEs and larger CMEs coming from active stars. I doubt many people would doubt this, but the magnitude of this increase is very unclear. One theoretical reason to think that there might not be is that active stars have very strong magnetic fields. Specifically, they have magnetic fields that have strong dipole components, meaning that the CMEs, once released from the surface, might have not be able to escape the corona through the strong magnetic field. I know some people are working on this now, but it is unclear. There have also been attempts to observe CMEs that have been unsuccessful (e.g. Leitzinger et al. 2014). Another difficulty is just how much of the energy produced in the star can be used for CMEs. Drake et al. (2013) found that when they took simple solar scaling laws, like those in Aarnio et al. (2011), and applied them to active stars, they got CMEs that were removing something like 10% of the available energy from the star. The energy was going into lifting the material away from the star's gravity and the kinetic energy that the CMEs had after reaching escape velocity. This huge number is quite unrealistic.

    The point is, we don't know that CMEs are in fact a danger for planetary habitability for M stars because for all we know, the planets don't get hit by CMEs very often. This can be true, even if they are getting hit more often than the Earth is.

    Aarnio et al. (2011) "Solar Flares and Coronal Mass Ejections: A Statistically Determined Flare Flux - CME Mass Correlation" (http://adsabs.harvard.edu/abs/2011SoPh..268..195A)

    Drake et a. (2013) "Implications of Mass and Energy Loss due to Coronal Mass Ejections on Magnetically Active Stars" (http://adsabs.harvard.edu/abs/2013ApJ...764..170D)

    Leitzinger et al. (2014) "A search for flares and mass ejections on young late-type stars in the open cluster Blanco-1" (http://adsabs.harvard.edu/abs/2014MNRAS.443..898L)
  20. Nov 19, 2016 #19
    We haven't seen evidence of CMEs at stars other than the Sun, and probably Proxima.
    However there is no reason to assume the Sun (and Proxima) are very unusual, so it's likely that CMEs are a common feature of stars.
    Not having observed them at other stars is more likely a limitation of our presently available instruments.
  21. Nov 19, 2016 #20
    Sure, but what I wrote is not that the existence of CMEs from other stars is speculative. The point is that the existence of strong CME dominated winds from other stars is speculative. Our solar wind is very far from being dominated by CMEs but there is speculation that more active stars shoot off so many CMEs that the wind is basically just one continuous stream of them.
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