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Homework Help: Planetary system being born

  1. Nov 7, 2012 #1
    Hey guys so I'm kind of stuck on the second part of this question, here it is.

    It’s possible to make a rough estimate of how often planetary systems form by making some basic assumptions. For example, if you assume that the stars we see have been born at random times over the last 10 billion years, then the rate of star formation is simply the number of stars we see divided by 10 billion years. The fraction of stars with detected extrasolar planets is at least 5%, so this factor can be multiplied in to find the approximate rate of formation of planetary systems.

    1. Using these assumptions, how often does a planetary system form in our galaxy? (Our galaxy contains at least 100 billion stars.)

    answer was 2 year/planetary systems

    2. How often does a planetary system form somewhere in the observable universe, which contains at least 100 billion galaxies?

    (In seconds between planetary systems being born)

    Need help with this second part, thanks!
    Last edited: Nov 7, 2012
  2. jcsd
  3. Nov 7, 2012 #2


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    Well, remember that plenty of stars die off in just a few million years, and that small dim stars, which are by far the most common stars, are very hard to detect. Also, current exoplanet research points to well over 5% and can be expected to increase as our detection methods improve.
  4. Nov 9, 2012 #3
    Correct on the 2 year/ planetary system(per 1 galaxy)--otherwise known as .5 planetary systems/year or 1 ps every 2 years.

    That is for our galaxy.

    Now, under the assumption that our galaxy is like the other 100 billion in the observable universe, then in each galaxy we expect to see .5 ps's per year(per galaxy). So how many per year do we expect to see for all galaxies?
  5. Nov 9, 2012 #4
    We must remember what we see is millions if not billions of years old and since then star formation may have slowed along with the formation of planetary systems. This paper http://www.naoj.org/Pressrelease/2012/11/05/index.html proposes that the universe has already made most of the stars it will and thus planetary formation could be slowing, just a thought.
  6. Nov 10, 2012 #5
    I agree, but I believe the question is just of a general type.
    I went off the assumption that the OP is in a class that is assuming a constant rate at this point. Otherwise, we would need a proposed "deceleration rate".
  7. Nov 11, 2012 #6


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    I'm never quite sure what to think in the light of that fact. Which is more relevant to us - what we see at present or what is actually 'going on' at the present? That's a nebulous thing too (jeez this topic is full of unintended puns). It stresses the brain to consider that what is happening at our limit of visibility at this time (i.e. what an observer, half way between us will observe at a time in our future that corresponds to half the time for light to get to us from that limiting distance) will only reveal itself to observers here (should there be any) in 46 billion years or so.

    Humans are obsessed with the notion of intelligence 'out there' and that is one (possibly the main?) reason for interest in other planetary systems. But our innate grasp of the time scales involved is very limited (based only on experience of communications and exploration on Earth). Just how relevant would the detection of an intelligence be if it turned out to exist at a distance, say, of 1000LY - or 1,000,000LY? Unless there is someone out there within a few 'light decades' it would be very hard to build up any kind of relationship.

    Otoh, I suppose it would be interesting to find evidence which would confirm that we are not unique. It would certainly spoil the day for a lot of religious organisations.
  8. Nov 16, 2012 #7
    The answer for the second part was 6.0*10^-4 seconds between planetary systems being born. Sorry for the late reply.
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