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Featured B A terrestrial, temperate planet around Proxima Centauri

  1. Aug 29, 2017 #41
    It would be extraordinary good luck if the nearest stellar system to ours harbored a planet compatible with life that originated on Earth.
    There is not even anything in our solar system that is like Earth, though Mars and Venus could have developed that way, but they didn't.
  2. Aug 29, 2017 #42
    In the specific case of Proxima Centauri, the Very Large Telescope Interferometer (VLTI) was used to confirm the interferometric measurements of Proxima Centauri obtained with the two 8.2 meter VLT Antu and Melipal telescopes. These interferometers can produce higher resolution astronomical images than any other type of telescope. Once the interferometric observations have been made it is combined with other measurements, such as the star's mass, radius, effective surface temperature, inclination, and luminosity then comparisons are made with stellar evolutionary models, such as MESA. It is the most accurate means we have today for determining the age of main sequence stars.

    New evolutionary models for pre-main sequence and main sequence low-mass stars down to the hydrogen-burning limit - Astronomy and Astrophysics, Volume 577, May 2015
    Modules for Experiments in Stellar Astrophysics (MESA) - The Astrophysical Journal Supplement Series, Volume 192, Number 1, December 2010
  3. Aug 29, 2017 #43
    I agree. The only advantage that Proxima Centauri b has is its mass. A more massive exoplanet would also imply a denser atmosphere, at least initially. Using our own solar system as a model, it is a fairly safe inference to assert that at least at one time in its history Proxima Centauri b should have had an atmosphere. Atmospheres on planets and moons seem to be fairly common once you reach a certain mass. Whether or not it still has its atmosphere seems highly unlikely after 4.85 billion years and its proximity to its star, but I would not rule it out entirely.
  4. Aug 29, 2017 #44
    A Day at the Beach on Proxima Centauri b
    Intended to be taken with your tongue firmly planted in your cheek. :wink:
  5. Sep 19, 2017 #45
    Those Z numbers, are they ratios of metalicity compared to our sun?
  6. Sep 19, 2017 #46
    Have all three stars been analyzed enough to say for sure whether there are planets around A and such?
  7. Sep 19, 2017 #47
    I am not so sure about this.
    Look at this this way: if you shrink Earth to a 1 meter ball, oceans on it would be barely ~1mm deep in the deepest points! Our "vast" oceans are insignificant when you look at the whole planet. IOW: Earth actually has very little volatiles as a fraction of overall mass.

    It's hard to imagine that having this little volatiles is some sort of typical thing. I expect that for planets around Earth mass, some will be much drier than Earth, and (importantly for this case) some will have much *more* water and other volatiles than Earth. With oceans, say, 4x deep as Earth, a planet can survive many billions of years of atmosphere loss and still not dry out.

    We don't know where Proxima b is on the "water" scale. It may well be an "ocean world". We need more data. Spectroscopy would be great.
  8. Sep 19, 2017 #48
    To demonstrate the point:
  9. Sep 19, 2017 #49
    Hi nik:

    I am not sure I understand the point you are demonstrating. Is it that there is expected to be a wide variability with respect to H2O/mass ratio for a planet or moon?

    I would expect there to be an issue about how this variability depends on average body surface temperature. Europa is much colder than Earth, and it would therefore be expected that a greater fraction of a body's original H20 content would survive for billions of years on a colder body, even if it has less mass.

    Perhaps some appropriately educated person might be able to create a plausible model showing how much H20 there was on Earth, Mars, and Europa say 4,500,000,000 years ago and how it came to be what is there today, based on mass, and temperature.

  10. Sep 20, 2017 #50
  11. Sep 20, 2017 #51
    Yes, Z = 10dex
  12. Sep 20, 2017 #52
    Each of the three stars have been analyzed, but not the solar systems of these stars. There is a suspected planet orbiting Alpha Centauri B, but it has not yet been confirmed. One of the problems is that if there are planets orbiting any of these three stars they are not: A) Transiting their parent star; and/or B) Massive enough to create a noticeable wobble in their parent star. That certainly does not rule out the possibility of planets orbiting these stars, it just makes detecting them more difficult.

    It should also be noted that Alpha Centauri A and B maintain an average distance of ~11 AU from each other, which puts a limit on the size of the solar system each star can have. The distance between Alpha Centauri AB and Proxima Centauri is 0.21 light years (13,280 AU) which gives Proxima Centauri much larger solar system possibilities.

    An Earth-mass planet orbiting α Centauri B - Nature, Issue 491, November 8, 2012 (free preprint PDF)
    Last edited: Sep 20, 2017
  13. Sep 20, 2017 #53
    I was considering Proxima Centauri b's proximity (making it tidally locked) and the fact that its parent's star has the equivalent x-ray emissions as Sol, despite being 588 times less luminous. As of right now we just do not have enough information to make a determination either way. We can only infer possibilities based upon the information we have and that information is clearly incomplete. Hence, I would not rule anything out yet.

    Stringent X-Ray Constraints on Mass Loss from Proxima Centauri - The Astrophysical Journal, Volume 578, Number 1, October 10, 2002. (free)
    NASA Finds Planets of Red Dwarf Stars May Face Oxygen Loss in Habitable Zones - NASA Article, February 8, 2017
  14. Sep 20, 2017 #54
    In that article I would read it as fractional mass of metals. The mass ratio of non metals to hydrogen in the Alpha Centauri at the time they formed is written (Z/X)i. The ratio of metals to to hydrogen on the surface is written (Z/X)s

    As apposed to [Fe/H] which would be the logarithm of the ratio of the ratio of iron to hydrogen in alpha centauri to iron to hydrogen in the sun. ?:)
    [Fe/H] = log10((Z/X)/(Zsun/Xsun))
    [Fe/H] = log10(Z/X) - log10(Zsun/Xsun)
  15. Sep 20, 2017 #55
    Hi nik:

    I confess I have not been paying attention to exoplanet spectroscopy, so I did not know that the technology has advanced for this to begin to be an active aspect of current astronomy. Your remark prompted me to search for this topic and I found several interesting sources including:
    I am wondering is anyone knows about any efforts to use this technology for Proxima b.

  16. Sep 20, 2017 #56
    I agree. Metalicity is not the same as the iron to hydrogen ratio. Metalicity encompasses everything that is not hydrogen or helium. That includes iron, but also oxygen, carbon, silicon and everything else on the periodic table. Nevertheless, a star with a 0.21 dex is going to have 1.621 Z⊙. Which means that the star has a 62.1% higher ratio of non-hydrogen and non-helium elements than our sun. However, it should not be taken to mean that Proxima Centauri has a 62.1% higher Fe/H ratio.
  17. Sep 20, 2017 #57
    Like this? linked to this paper.

    I suspect people are already spending a lot of energy fighting about where to aim JWST. Choose between 1) beautiful awe inspiring pictures of nebula and other fireworks, 2) images of distant galaxies and details of the creation of the universe, 3) some debatable evidence that a rock which we thought was dead could really be dead.
  18. Sep 20, 2017 #58
    Check this paper. Are they saying Alpha Centuari A currently has surface composition 71.5% Hydrogen, 25.8% Helium and 2.7%"metals"? They list (Z/X)i as 0.0384 which can work if you adjust the rounding.
  19. Sep 20, 2017 #59
    The paper gives Alpha Centauri AB the initial helium mass fraction (Y) of 30% ± 0.8% and an (Z/X)i of 0.0459 ± 0.0019. Given that X + Y + Z = 1, that would make Alpha Centauri A's surface composition 66.93% hydrogen, 30% helium, and 3.07% other metals. Using the Skylab data from 1979, the sun is composed of 73.46% hydrogen, 24.85% helium, and 1.69% other metals. Which should have given Alpha Centauri A a Z⊙ = 1.817 or 0.259 dex instead of the 0.20 dex I posted above. Or they were using a different composition for our sun than the one I listed.

    The Sun's Vital Statistics - Stanford Solar Center
  20. Sep 20, 2017 #60
    The "i" is for initial. To me that would mean the gas cloud that formed both stars. Nothing we could see now.

    On table 1 they list [Fe/H] as 0.20 under Alpha Centuari A.
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