Friday, June 2, 2017

Donut Shaped Planets


It's so very tempting to give a post like this the subheading along the lines of "Homer's Haven".

There's been some chatter in the science field lately about synestias - short term stages in the development of an earth-like planet where the dust and rock of accretion takes on a donut shape.

Note that all three objects have the same density.
You couldn't walk on the surface of the synestia shown here.
By itself, this isn't particularly groundbreaking, but it does shed some light on the possibility of donut shaped planets.  We science fiction writers are always on the lookout for fun new planets on which to place our space princesses, four armed green aliens, and fighting men of earth, and a toroid shaped planet seems tailor made for adventure.  By its very nature, it is an alien setting.  Gravity varies over its surface.  The day-night cycle varies depending on where you stand.  The seasons are affected by how great it's tilt is off of the solar ecliptic.  The paths a moon (or moons!) might take get way-out wild.

The donut moon is worth a paragraph.  While the moon could dive through the donut hole in huge figure-eights, or even just bob up and down inside of it, both orbits are very unstable.  Far more likely is a moon rotating around the rim, parallel to the tilt of the planet.  On the other hand, these orbits are stable enough to allow for artificial satellites for spying, communication, and GPS systems.  They'd need more nudging than their Terran counterparts.

So why haven't we seen more of these?

Here's a video with a few explanations:

If you can't watch the video or just can't wait, here's a few interesting bullet points to consider:
  • In order to maintain its stability as a hoop, a planet with earth approximate gravity would have to rotate once every few hours.  A 24 hour day might have four to six sunrises and sunsets.  That sort of bright-dark cycle would play havoc with normal life.
  • Gravity would vary by 200% as you travel from the equator (the line of smallest diameter through the hole or largest diameter around the outside of the hoop).
  • To drop this planet in the habitable zone of the star, you need a big, fat blue star.
  • The seasons on a planet with an axial tile similar to earth would run very hot and very cold.
  • Continental drift would 'smoosh' the tectonic plates up as they approached the inner ring, and stretch them out as they drifted back around to the outside of the ring.  That would result in massive mountain ranges inside the donut hole.
All in all, not a very friendly place, but a decent writer should be able to work with it.  Continental drift is not a requirement for a colonizable planet, nor are moons.  They real problem as I see it is the lack of a liquid-solid core.  Without that, you've got no magnetic field, and no protection from solar radiation.  Which means life on a donut planet just isn't feasible.

And I guess that's why we haven't seen many stories set on planets like this.  The kind of authors prone to think about the geometry and gravity and tides on a place like this are hard enough science guys to throw up their hands and walk away.  To drop a planet like this in your story, you'd have to be one of those guys capable of juggling just enough math and science in your head to make it plausible while being loose enough with science to ignore how all of the data screams, "This is not SCIENCE!"  You'd have to be crazy to use one of these in a story.

Which reminds me, it's a planet called Antioch, and it's one of the last lines of defense for those tasked with protecting the Sacred Planet against the ravages of the Demented.


  1. Hal Clement's "Mission of Gravity" comes to mind with your posting.

  2. That's pretty slick. Non-spheroid worlds seems to me like an under-utilized idea. Aside from Ringworld there isn't a whole lot out there.