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Thread: How many habitable exoplanets in the galaxy?

  1. #1
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    How many habitable exoplanets in the galaxy?

    https://www.sciencedirect.com/scienc...79610718300798

    This paper makes this claim:

    "Perhaps the most important astronomical data relevant to the
    theory of cosmic life to emerge in the past decade are the detections
    of habitable exoplanets planets outside of our solar system. The
    total estimated tally of such Earth-like planets in our Milky Way
    galaxy alone now stands at 100 billion, and with 100 billion or so
    galaxies in the observable universe the grand total stands at 10^22
    ."

    Does this sound right? Firstly, have habitable exoplanets even been detected? Secondly what is the estimated number of such planets:

    Given that 100 billion is about the median value for the number stars estimated in the galaxy, and then recognising that many of these stars are going to be high mass short lived stars that are not suitable for life, or even planetary formation, also many are red dwarves and smaller that may have very narrow photic zones, or be subject to flares or other instabilities, I wonder if that estimate is not out by at least a couple of orders of magnitude, unfortunately in the text of the paper there are only a couple of old references to what is no doubt a vibrant and active field of research.
    Last edited by Swift; 2018-Apr-23 at 12:23 PM.

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    To be pedantic, some planets have been detected at a distance from a star where liquid water could occur at about 1 bar pressure, but insofar as I am aware, there has been no detection of an atmosphere on these planets. may be high by several orders of magnitude.
    Last edited by swampyankee; 2018-Apr-24 at 12:12 AM.

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    How many explanets have been detected at all? I. e. objects that were formerly planets but have evolved to some other type of objects?
    What types of objects can planets evolve to? Stars/brown dwarfs? Satellites? Dwarf planets?

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    yes i saw that..but i don't think i can edit the title

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    Quote Originally Posted by transreality View Post
    yes i saw that..but i don't think i can edit the title
    But I can
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    One thing to watch out for is the definition of "habitable".

    In this context it means liquid water could exist on the planet surface, given atmospheric pressure in the correct range.

    it does not mean that you could go and live there necessarily.

    Having got that out the way, 100 billion seems quite reasonable. There is a paper out there which estimated the average sun-like star has more than one habitable planet.

    This is not unreasonable when you think about it. Venus could've been just a bit further out, or Mars could've been a big bigger, and we could have three habitable planets right here in the solar system.

    Then there is the fact that most stars are red dwarfs, and at the present they are assuming there is nothing stopping red dwarf planets being habitable, by the above definition.

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    Quote Originally Posted by kzb View Post
    One thing to watch out for is the definition of "habitable".

    In this context it means liquid water could exist on the planet surface, given atmospheric pressure in the correct range.

    it does not mean that you could go and live there necessarily.

    Having got that out the way, 100 billion seems quite reasonable. There is a paper out there which estimated the average sun-like star has more than one habitable planet.

    This is not unreasonable when you think about it. Venus could've been just a bit further out, or Mars could've been a big bigger, and we could have three habitable planets right here in the solar system.

    Then there is the fact that most stars are red dwarfs, and at the present they are assuming there is nothing stopping red dwarf planets being habitable, by the above definition.
    I'd also add, at least to some extent, that it doesn't matter. If the 100 billion estimate is high by several orders of magnitude, we are still talking about maybe 10 to 100 million planets in our galaxy. That is up from the total of one habitable planet and eight total planets that we knew existed for most of my lifetime.
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    Quote Originally Posted by Swift View Post
    I'd also add, at least to some extent, that it doesn't matter. If the 100 billion estimate is high by several orders of magnitude, we are still talking about maybe 10 to 100 million planets in our galaxy. That is up from the total of one habitable planet and eight total planets that we knew existed for most of my lifetime.
    There is a difference though, and that is we know for a fact Earth is inhabitable.

    We don't know for a fact that any of these exoplanets within their star's habitable zone are actually habitable.

    The estimates of numbers of earth-like planets depend on extrapolations from the known exoplanet population.

    The experiment to detect true Earth analogues is yet to be executed successfully. It's not clear to me that even TESS will detect an Earth around a sun-like star?

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    Quote Originally Posted by kzb View Post
    Venus could've been just a bit further out, or Mars could've been a big bigger, and we could have three habitable planets right here in the solar system.
    if the solar system had been subject to search from a nearby dynamically favourable location, under the current criteria, would we have three 'inhabitable planets' detected?

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    Quote Originally Posted by transreality View Post
    if the solar system had been subject to search from a nearby dynamically favourable location, under the current criteria, would we have three 'inhabitable planets' detected?
    Currently? Probably two; if I recall correctly, the inner edge of the "habitable zone" is about 0.95 AU.

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    Quote Originally Posted by transreality View Post
    if the solar system had been subject to search from a nearby dynamically favourable location, under the current criteria, would we have three 'inhabitable planets' detected?
    I don't think any planet in the solar system would have been counted by Kepler (maybe Mercury?)

    Kepler required three transits, which in the case of Earth is of course three to four years' observation. It broke before it could do this. (I have a recollection there is a later paper which relaxed the criteria somewhat, but I am still not sure that Venus Earth or Mars would've been detected).

    Current detection methods (except microlensing) are heavily biased towards short-period planets.

    TESS has funding to operate for two years, so I can only think they must have relaxed the three-transit rule if they want to detect Earth-like planets around sun-like stars.

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    Quote Originally Posted by swampyankee View Post
    Currently? Probably two; if I recall correctly, the inner edge of the "habitable zone" is about 0.95 AU.
    But do we know what criteria are used for exoplanets?

    The 0.95AU figure comes from modelling what would happen to Earth, if it's orbit were moved. So it is specific to planets of similar size and composition to Earth.

    Different planets may do well closer in. Planets much lower in water, so the greenhouse effect cannot runaway. Maybe if Venus and Mars orbits were swapped we'd have three habitable planets.

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