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Habitable Binary Worlds, and their characteristics

  1. Feb 26, 2014 #1
    I'm working on a SF story that takes place on Binary Planets orbiting a single star. I've worked out their mean distance from each other (702,904.853 km) from their star (167,323,260 km or 1.1 AU) and their orbits around the common barycenter(48 days), and the parent star(432 days). For simplicity's sake they are identical to Earth in terms of mass, and size, but the arrangement still leaves a lot of questions.

    1.A full "moon" on each world would be significantly brighter (8Xs brighter) but how long would it last? 5 days? 6? For reference would it be like dusk or dawn or almost daylight?

    2. Tidal actions. Would the oceans and other large bodies of water have dramatic tides and waves or would the presence of a body of equal mass cause almost no tidal action?

    3.Satellites. I'm not sure the system could support a natural satellite, but artificial satellites and space stations would they have their orbits affected by the presence of such a large object so close?

    4.Eclipses. I'm sure they would happen with some regularity, like once a month, but would they be more extreme in terms of duration, or amount of light blocked?

    5.View. At twice the distance, but more than double the size of the Moon, we could see our twin, and it's continents, weather patterns and major features, but how soon could we detect intelligent life? Would we be able to look up and see the lights of large cities when the twin was in eclipse? or would it take picking up a radio signal to confirm life?

    I'm sure there's more, but that's all I got for now.(and I'm going for Plausible so far end of the curve works for me)
     
  2. jcsd
  3. Mar 1, 2014 #2

    Bandersnatch

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    Hi, Hainted.

    As far as the duration goes, just multiply our Moon's behaviour by 48/28ths, or about 1.5 times for simplicity.

    The planets would not be 8 times brighter. You need to consider the distance(twice the Moon's orbital radius corresponds to 1/4th resulting brightness), the size(about four times the radius means 16 times brighter) and the albedo(Earth's is about three times that of the Moon). 1/4*16*3=12 times brighter than the Moon.

    Still, that is nowhere near the Sun, which is some 400 000 times brighter than the full Moon.

    I'm sure you could read a book by the full 'moon' in your scenario, but the sky would most likely look black all the same. There'd be a bigger halo near the satellite, obscuring more stars than the Moon does.

    Remember, that to visualise the size of the planets you can always find some pictures of Earth made from the Moon, and imagine it being half as big radius-wise(so, four times smaller by area).
    Of course, keep in mind, there are no atmospheric effects on the Moon.

    As long as there is no full tidal locking, tidal effects will manifest.
    At twice the distance and about 80 times the mass of the Moon, the tidal acceleration would be about 10 times higher than we get.

    Natural satellites are out of the question. They would destabilise in their orbits rather quickly in the geological time scale.
    Artificial satellites are feasible. Some corrections to keep them in their designated orbits would be necessary every now and then, just as it is in the case of our satellites.

    They would only happen once a month if the plane of the orbit of the two planets lied in the same plane as their mutual orbit around the star.
    This is rather unlikely, so the eclipses would be best thought of as similar in frequency to those we have.
    The planets having twice the angular radius of the Moon as seen on the sky would not be that much of a difference. Think twice the eclipse's duration. Maybe somewhat higher frequency as well.
    Still, just as with our eclipses, these would only be visible in a narrow band of land whenever they happen. It would never be a world-wide event.

    I don't know about #5.

    You could also try a forum search. I remember these kinds of binary planet setups, with similar questions, popping up every now and then.
     
  4. Mar 1, 2014 #3
    "As long as there is no full tidal locking, tidal effects will manifest.
    At twice the distance and about 80 times the mass of the Moon, the tidal acceleration would be about 10 times higher than we get."

    Trying to visualize. Are you saying the waves would be massive, and fast, or that high tide/Low tide would be very extreme?

    "They would only happen once a month if the plane of the orbit of the two planets lied in the same plane as their mutual orbit around the star.
    This is rather unlikely, so the eclipses would be best thought of as similar in frequency to those we have.
    The planets having twice the angular radius of the Moon as seen on the sky would not be that much of a difference. Think twice the eclipse's duration. Maybe somewhat higher frequency as well.
    Still, just as with our eclipses, these would only be visible in a narrow band of land whenever they happen. It would never be a world-wide event."

    I know there's no up or down in space, but you're saying if they orbit the star on a horizontal ellipse(like a orrery model) that my two worlds orbit intersects that plane at an angle?
     
  5. Mar 1, 2014 #4

    Bandersnatch

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    The tides would have higher amplitude. I don't know how much higher you should expect, since the actual difference between low and high tides is determined mostly by the local landmass shape - i.e., whether the water gets siphoned into narrow bottlenecks or not. That's why here on Earth you can get as high as ten metre tides, whereas in the open ocean the water level is raised merely by half a metre(iirc).

    All the bodies in the solar system orbit roughly in the same plane. But there are differences of a few degrees. All the orbits are inclined a bit(as measured from the plane of Earth's orbit), each at a different angle. Same with satellites. Our Moon's orbit, for example, does not lie in the plane of the ecliptic(Earth's orbit), but is inclined about 5°. That's why we don't get solar eclipses every lunar month.
    It is statistically improbable for your two planets NOT to be inclined to the orbital plane as well. But you could ask your readers for a bit of a suspension of disbelief here, if you really want frequent eclipses.

    Download Celestia(http://en.wikipedia.org/wiki/Celestia), or other free planetarium software to see how the orbits look in practice.
     
  6. Mar 1, 2014 #5
    Would it be impractical/impossible for one to be inclined above the plane, and one below? Just trying to get a visual in my head.
     
  7. Mar 1, 2014 #6

    Bandersnatch

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    The two planets share the orbit around their common barycentre. Their orbits have to be in the same plane.
    This plane, being inclined to the plane of their mutual orbit around the star, will see each of the two planets spend half of the 'month' above the ecliptic, half below.
     
  8. Mar 1, 2014 #7
    Brainstorming:
    Early telescopes. As evidence: high scale of deforestation :D
     
  9. Mar 1, 2014 #8
    Telescopes,Hadn't thought of that! or the deforestation. I had been thinking more on how societies would have changed. "Where do the Gods live? There!"
     
  10. Mar 1, 2014 #9
    Of course there might be controversies which huge changes in landscape are just a natural process.

    By occasion: if you want some satellites flying around, why did not you choose just two big moons out of many orbiting around a gas giant? (I have a mixed feelings about stability of such binary configuration)
     
  11. Mar 7, 2014 #10
    It was correctly computed that full planet would be 12 times brighter than full Moon, because 4 times bigger in area and 3 times lighter albedo.
    Would it be advisable to read in that light?
    Full Moon is 0,27 lx.
    So we are looking at slightly over 3 lx.
    What is the context?
    "Dusk" or "dawn" means very little. During dusk and dawn the illumination changes from 400 lx or so to night.
    Look at targets for street lights, like these British ones:
    http://discovery.ucl.ac.uk/1430/1/Walk21Fujiyama.pdf
    Basically, that 3 lx is usual nighttime light level on minor streets. Main streets are usually brighter.
    If you built a town on Moon (full Earth is 43 times brighter than full Moon, and no clouds on your glass roof/window) you might decide to omit streetlights... but you still would want lamps in your living rooms and on working tables.

    Duration? That´s another matter.
    Not only is full Moon dark compared to Earth - 3 times darker - but Moon dims hugely when Moon is not full. See
    http://articles.adsabs.harvard.edu//full/1916ApJ....43..103R/0000114.000.html
    Half Moon covers half the area of full Moon - but gives only 1/9 the light, so is 4,5 times darker again than full Moon, and 14 times darker than full Earth.
    Moonlight is halved when Moon is 30 degrees from full, and still covers 93 % of full Moon area. Moon at 30 degrees from new - further from twilight than Mercury ever gets - has 1/15 the area of full Moon, but 1/250 the light.

    Now, unlike Moon (and Mercury), Venus remains dazzlingly bright as a narrow crescent.

    How bright is crescent or half or gibbous Earth?
    Pluto and Charon manage to not perturb Nix and Hydra. So, inner satellites might be possible.
    One obvious thing: since the planets are equal in size, either of them is bigger than the other´s umbra, yet smaller than penumbra.

    On the eclipsing planet, the other planet can never be totally eclipsed, unlike Moon. In the middle of central eclipse, the penumbrally eclipsed circle would stay visible.
    The umbra on the eclipsed planet would be big - unlike the umbra of Moon on Earth. Therefore umbral eclipses would be common and long in duration (whereas antumbral/circular eclipses would be impossible). Also since the non-eclipsed atmosphere is far away and under horizon, an observer would not see light scattered by local atmosphere. Yet since the Sun is not far under the horizon of the other planet, the observer would see lots of light refracted by the atmosphere of the oter planet.
     
  12. Mar 11, 2014 #11
    So the planet doing the eclipse gets to see a shadow fall across, but not cover the planet completely, while the eclipsed planet sees a dark disk surrounded by light?(Early morning and overtime so brain's not fully engaged.)
     
  13. Mar 11, 2014 #12
    Yes to both.
    The penumbra width on Earth-Moon distance is about 3500 km. So Earth, diametre 12 700 km, casts an umbra about 9200 km wide. When Moon is partially eclipsed, being about 3500 km across, the edge of Earth´s umbra is curved, but bigger than Moon, so that Moon can enter into umbra and be completely eclipsed.
    Moon casts umbra on Earth that is at most 300 km across, and most often does not reach Earth. During a total eclipse, the totally eclipsed spot on Earth would be tiny and hard to see by naked eye, but there would be a bigger area in penumbra, that would be detectable by naked eye.

    So, in case of your Earth-sized planet nearly twice the distance to Moon, during eclipse the planet should be about twice the size of Moon as always, with Moon sized umbra on it, surrounded by penumbra also as wide as Moon.

    Earth atmosphere refracts a lot of light. If you look at dawn or dusk from ground, Sun would show appreciably flattened disc on horizon. Well, geometrically it is under horizon! The refraction on horizon is 35 minutes. When a viewer on ground sees lower edge of Sun touch horizon, it actually is 35 minutes below horizon, while the top of the Sun is actually 5 minutes below horizon, is refracted by 30 minutes and seen 25 minutes above horizon.

    If you looked at a sunset from space outside atmosphere, then the light ray that grazes the horizon was refracted by 35 minutes on its way down to ground. It therefore has to be refracted by another 35 minutes on its way out of atmosphere again. That means 70 minutes total.

    If Moon passes the centre of Earth´s umbra during a total lunar eclipse then Earth is just 60 minutes radius. This means that the edge of Sun´s disc is just 45 minutes under Earth edge. So thanks to refraction, an observer on Moon would see all sunrises and sunsets around Earth - the discs of setting and rising Suns refracted into a ring just a few km wide. It would be a bright ring, too narrow to resolve by naked eye.

    Totally eclipsed Moon is usually easily visible in night sky - visible because brightly illuminated. The illumination of totally eclipsed Moon by refracted sunlight is estimated as slightly brighter than full moonlight, in the centre of umbra... and getting much brighter than that near the edge of umbra.

    So yes. A total solar eclipse by a small body having a thick atmosphere would feature a bright ring around a dark disc.
     
  14. Mar 12, 2014 #13
    But would it affect the new moon? In other words would it truly go dark or would a new "moon" just be dark in the middle with a ring of light on the outside where the planet's shadow doesn't reach?
     
  15. Mar 12, 2014 #14
    Are you talking about:
    new moon?
    solar eclipse (that requires new moon)?
    full moon?
    lunar eclipse (that requires full moon)?
     
  16. Mar 12, 2014 #15
    No just a regular cycle of the moon from full to dark to full again. Would it go completely dark?
     
  17. Mar 13, 2014 #16
    We don´t see the darkness of new Moon anyway - and no, new Moon is actually NOT completely dark!

    A nearly new Moon, whether waxing or waning, is a narrow crescent. But it also is near Sun. Which means that it is hard to see because
    1. Sun is nearby and dazzles
    2. even if Sun is somehow obscured, blue sky still is brighter than crescent Moon
    3. even if Sun is below horizon, the dawn or dusk sky background outshines the narrow crescent
    Worse, as mentioned, Moon is black to begin with, and dramatically fades further away from full.
    Yet when narrow crescent is seen in dark skies, it also features relatively noticeable ashen light. Because a new Moon is illuminated by full Earth.

    An Earthlike Moon would not, or would only slightly, fade as a narrow crescent. It would grow less bright as it shrinks in area, but remain bright in surface brightness. So it would shine through day or twilight sky where Moon does not. In addition, as it approaches the Sun, there would be twilight visible... first at the tips of the horns, but as the disc further approaches the Sun and the crescent narrows, brightening and growing from the horns all around the disc.
     
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