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Figuring out which stars an exoplanet's poles point to

  1. Jun 6, 2017 #1
    I know Earth's north pole points to Polaris and during the 26,000-year precession cycle, it will also point to Demeb, Vega, and Alpha Draconis. But...

    Is there a reference that'll help me figure out what Proxima Centauri b's most likely pole star candidates would be? I know, so many variables, including planetary axis tilt, etc. I just want something like a 3-d star map that might help, or a genius astronomer who might want to point me in the right direction (dad pun intended).

    Part of my back story involves an artefact, an obelisk, discovered at one of Proxima b's poles that takes on religious significance for a lot of people who see it as a pointer. This way to first contact. (I know it makes no sense that an alien society could even find a planet's pole that pointed other explorers in the right direction, but we're talking about a central religious tenet here.)

    So, yeah, any help appreciated. Thanks.
  2. jcsd
  3. Jun 6, 2017 #2


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    Of which none are known. This makes it impossible to determine where the planet's poles are pointing to.
    The planet was found using stellar radial velocity measurements. This is sufficient to confirm a planet, and determine its orbital period, semi-major axis, and eccentricity, but not what is the inclination of its orbit, let alone of its axis.
  4. Jun 6, 2017 #3
    I knew there'd be no way to see axis tilt, didn't know we wouldn't know orbit inclination.

    Would there be a way to fudge the unknowns and determine north/south stars from there? Or, am I better off copping out and just making up names?

    I'd prefer if I could name relatively plausible host stars for the habitable precession planets.
  5. Jun 6, 2017 #4


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    They really could be almost anything. Just pick any pair of stars opposite one another in the sky. The good part of not having the inclination data is that nobody can get picky and tell you that what you've chosen is not possible.

    Not to derail the thread, but the what now?
  6. Jun 6, 2017 #5
    Sorry. That's shorthand for the hypothetical planets that would orbit the stars along the precessional axis/what the poles point to in space. I'm self-taught, trying to figure this stuff out as I go, so my terminology will not be kosher sometimes. My apologies.

    I'll see if I can figure something out in Celestia as per your recommendation, Bandersnatch. Does anyone have any other recommendations for seeing the galaxy in 3D?
  7. Jun 6, 2017 #6


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    NASA's Eyes on Exoplanets is nice for easily finding currently-known planets. Just don't take the visualisations provided as more than artist's impressions (including inclinations of orbits of planetary systems).
  8. Jun 7, 2017 #7
    Thanks for the help, Bandersnatch. It took me a while, but I think I'm on the right track now. I had a bit of a breakthrough when I stopped long enough to check just how much space was between Polaris and Vega. It's a considerable amount of wiggle room. I thought I had to show a real narrow line that could only take in 3 or 4 possible stars.

    So I used Celestia to fly out to Proxima and pointed my field of view toward Orion, and I figure pretty much anything I see onscreen is fair game. Tons of opportunity.

    Also, the Eyes program kicks ass. Glad to have it now.
  9. Jun 15, 2017 #8

    stefan r

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    Proxima b is likely to be a tidal locked planet. If so then the pole stars will be perpendicular to the orbit. Proxima b was discovered by observing variations in proxima's velocity toward and away from earth. So it should be not too far from perpendicular. The orbit cannot be exactly lined up with earth or we would see transits so you have a lot of wiggle room. Proxima's coordinates are 14:30 and -63 degrees from earth. Look for anything 90degrees off of that. Maybe 70 to 85 degree angles.

    One possibility around 14:30 RA and 27 north and the other pole at 2:30 RA and 27 south. Arcturus is at 14:15 and 19 degrees on earth. That will shift a few degrees because Proxima sees Arcturus at a different angle. So if Proxima b orbits on a plain tilted 10 to 15 degrees away from earths view then Arcturus will be dead nuts at the pole.

    You can rotate the whole system if you do not like Arcturus. So for example you can use aquila and hydra 0 north and 8:30 and 20:30, Altair is bright and is at 19:50 and 8 north. So only a few degrees off.

    If the inhabitants see in visible light then alpha centuri will be visible in a the sky with proxima. Most of astronomy would be done by something like polar explorers (although on proxima they are dark side explorers). The sun would be in Cassiopeia and would rise and set over 11.2 "days".

    Many of the constellation would look similar to earth. Orion would still be Orion.
  10. Jun 16, 2017 #9
    With a tide-locked planet, is there any wiggle room for where the pole would point over the course of a several-thousand year (as it does for Earth)? If so, I'm liking Arcturus as a supposed destination, what with some of the nearby visible objects being globular clusters many Kpc away. That could lead to some intense debate.
  11. Jun 16, 2017 #10


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    With a tidally-locked planet you're pretty much stuck with the poles pointing to just one spot (i.e. no precession).
  12. Jun 16, 2017 #11
    Oh well. Then again, who's to say Proxima B's the only rock in that system? It's just the only one we've seen so far. What are the chances there could be a C or D planet that we're as-yet unaware of? Especially if they're on the smallish side?
  13. Jun 16, 2017 #12
    Since Proxima B was discovered due to it's gravitational effect on the host star, that means it would be the largest planet in the system.
    However there could very well be smaller planets which cannot be detected by that method.
    If alien astronomers were investigating the solar system using this method they would probably conclude that it has one very large planet and possibly two or three others around half that size.
  14. Jun 16, 2017 #13
    As has been covered, there is no way of knowing right now, so you're going to have to make something up. If you need help, check out SpaceEngine. It's got a mix of real (cataloged) and procedurally generated (like Minecraft) objects. Load it up, fly to Proxima b, pick a direction, and name a star. Looks like you can even disable the procedurally generated ones, if you've got to have that extra realism.
  15. Jun 16, 2017 #14
    Wow, the trailer for that makes Celestia look like a big ol' pile o' puke. Downloading now. Even if I don't see this story idea through to fruition, holy crap this is the kind of software that can really inspire a sci-fi writer. Thanks for the recommendation.
  16. Jun 16, 2017 #15

    stefan r

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    I downloaded earlier this week and it does not work on my computer. :(

    There is a lot of room for wiggle. Libration has been suggested as part of a mechanism for life forming. If the planet is librating a lot then you get the "sun" rising and setting over a wide area. That does not change the pole at all.

    Ice grows thicker on land than over water. Compare current Antarctica and north pole. Also compare North and South during recent ice ages. When there is an ocean below the ice, added pressure displaces water somewhere else. Since the dark side gets no direct light from Proxima the ice sheet can be many miles thick. So, for example using current earth continents, if the dark spot started in Columbia/Brazil the glacier would build up in the amazon basin. The weight eventually shifts South America toward the south pole and Antarctica into the light. An alternate example would be starting the dark spot off the coast of Africa. The African glacier would keep growing and tilt the planet north until Africa crosses the pole into the sunny side and starts melting. The thickness of ice needed to start a shift depends on the internal geology of the planet. "Ice" is also a general term. No reason to assume only water.

    The dense core has to move inside of the planet in order for the poles to move around. I do not know geology well for earth. Exoplanet geology is not very well understood. A lot of geo activity will generate heat which will melt the underside of a glacier.

    The continents on earth move around relative to each other. Not on a "thousands of years" time scale but they do move. Buried obelisks could survive as long as dino teeth.
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