What are the numbers involved in galaxies, solar systems and planets?

I read that there are a hundred billion galaxies in the universe. Firstly, how do scientists know this? 

Secondly, how many solar systems are in each of these galaxies?

Thirdly, how many planets are in each of these solar systems?

7 Answers

  • 1 year ago
    Favourite answer

    The 2016 estimate of the number of galaxies in the observable universe is now 2 trillion.

    Such estimates are made by using a telescope to take a magnified photograph of a very tiny square of sky. Such a photograph is taken over a period of many days so that the light from very distant galaxies will show up in the picture. Then the number of galaxies in that tiny square of sky is counted. Then that count is multiplied by the number of such tiny squares it would take to cover the whole sky. A few samples like this can give a reasonable estimate of the number of galaxies in the whole sky, the observable universe.

    Currently, we cannot even make a good estimate of the number of planetary systems there are, and Certainly not in each of 2 trillion galaxies.

    We have almost no way of estimating how many planets there are. There are a vast number of planets that are not detectable with our current planet-finding methods.

  • 1 year ago

    Trillions, Billions and Millions

  • The quoted number of galaxies is an estimate derived from the known size of the universe, and the observed number of galaxies per volume.

    It is totally unknown how many planetary systems there are. In our own neighborhood of our galaxy a thousand or so planetary systems have been discovered. We don't know if this is typical of other areas of our galaxy, or is typical of other galaxies.

    In any case the vast majority of galaxies are only ever seen as tiny feint blobs of light on long exposure plates, and hence their stars are invisible to us. It is only the few nearest galaxies that can be resolved into stars and hence it is only planetary systems in the nearest galaxies on the side facing us that we need worry about.

    The vast majority of planetary systems we have observed are unstable and therefore we should view planets as temporary residents in most cases, especially smaller planets (those smaller than Jupiter). There appears to be generally a few "SuperJupiters" in the systems we have observed. The number and time of residency of smaller planets is unknown.

    Our own solar system appears to have a unique structure, stable over periods of billions of years, and should not be used as a typical model of planetary systems.

  • 1 year ago

    We DO NOT KNOW. We did not know there really were planets orbiting other stars until 1992. 


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  • 1 year ago

    It's a matter of extrapolation. Surely they did not count every galaxy there is, but the universe looks pretty much the same in every direction. So if you count just a small part and do the math than an average will roll out.

  • Jim
    Lv 7
    1 year ago

    You need to be very careful with the wording, they use "visible universe", but we might only see a minuscule amount of the real universe! Or we might be seeing all of the universe!!

    "All in all, Hubble (telescope) reveals an estimated 100 billion galaxies" but what if there are 100 trillion trillion galaxies in all? We just don't know. Most galaxies seem to form with a black hole in the middle.

    Probably 100 billion solar systems in each, with an average of 5 planets per solar system.

  • 1 year ago

    Keep in mind that the community of planetary scientists did not generally concur in any particular detection of exoplanet(s) until about 25 years ago.  So our knowledge of them remains quite limited.

    A 2012 paper in Nature vol. 481, by A. Cassan et al., speculates that stars with planets are the rule (at least in the Milky Way) rather than the exception.  Of course we cannot possibly see any planet outside the Milky Way, they're just too far away.  You could suppose that galaxies "like" the Milky Way have the same characteristic -- most stars having one or more planets.  At this time, we know of one other star (besides the Sun) that has 8 planets, and several more that have 6 or 7 planets.  Time will certainly increase these numbers.  Although scholarly papers have been appearing, with estimates of the likely number(s) of planets per star, these papers are as speculative now as the "discoveries" of exoplanets were 30 years ago.  We simply don't know the answers.

    How many stars per galaxy?  We don't know that either.  In the Milky Way galaxy, the recent estimate is 200 billion, and some sources cite 100 billion stars "per galaxy" to arrive at a guesstimate of the number of stars in the observable universe.

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