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I Astronomical improvements needed to discover a ex-solar moon

  1. Jun 17, 2016 #1
    I am hopeful that some PF participants can suggest specific plausible future improvements to astronomical instruments sufficient to detect a moon similar in size to Earth's moon revolving around a Earth-size planet revolving around a distant sun.
  2. jcsd
  3. Jun 18, 2016 #2
    Perhaps we should wait till some big projects get online...probably something that would make direct imaging of extrasolar moons possible. At least, Kepler satellite is not quite enough.
    Besides, though terrestrial moons should have larger albedo than gas giants have, gas giants are normally much bigger, so transit methods might not be available.
    It's just my guess, I'm not professional on this...
  4. Jun 18, 2016 #3
    Hi GW:

    Thanks for your post. I am also not a professional, but I do have some thoughts, although I have no way of judging if they are plausible.

    Kepler finds planets by the very slight dimming of the brightness of a star during the transit of a planet. The area of a view of our moon's disk is about 7.4% of the area of the earth's disk. If the distant planet and the planet's moon were aligned during a transit, then the reduction in the star's brightness would be about 7% less than when there was no alignment. What improvement in a Kepler-like instrument would be able to detect this? I understand that such an alignment would occur only about 0.6% of the transits. It there were one transit a year, it would take about 150 years of observation to detect one alignment.

    If it were possible to actually photograph an image of the distant star during a transit with sufficient detail to make out a moon's darkness together with the planet's, fewer transit observations might be sufficient to confirm a moon was present. Perhaps a PF participant might suggest plausible improvements in telescopes that would be sufficient to do this?

    Last edited: Jun 18, 2016
  5. Jun 18, 2016 #4


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    The relative sizes of the Earth and Moon are unusual, so it really can't be used as a baseline for detecting extra solar moons. The Earth only has 3.67 times the radius of the Moon while the next closest is between Neptune and its largest moon, and that is a ratio of 18.3 to 1.
  6. Jun 18, 2016 #5
    Hi @Janus:

    Thank you for your post. I apologize for not being clearer in my original question.
    I am aware the our moon is quite unusual in the solar system. My question is motivated by an interest in the search for extra-terrestrial life on extra-solar planets. Some (not a large fraction of) researchers about the origin of life have discussed the plausibility that our unusual moon may have been a necessary contribution to life evolving here, for a variety of different reasons. As astronomical explorations look for Earth-like extra-solar planets, it might be useful to know how frequently such planets have an over-large moon like ours.

  7. Jun 18, 2016 #6
    Only because Pluto has been demoted.
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