Is there a lurker in the Solar System?

The Solar System is much bigger than we learned in school, with a more complicated history.

pillars-with-vortex
NASA’s 2014 edition of “The Pillars of Creation,”
plus a speculative addition

This famous photograph shows a portion of the Eagle Nebula, about 7,000 lightyears away from us but still within the Milky Way Galaxy.  The nebula is a diffuse mass of dust, gas  (mostly hydrogen atoms, of course) and hundreds of stars aborning.  Spectroscopic red- and blue-shift data could prove me wrong, but to my eye those “pillars” are exactly what you’d expect to see if each had formed around a vortex such as I described in my previous post.  Those two bright rings look very much like solar nebulae, don’t they?

If that’s what the rings are, then the region between them should be even emptier than normal interstellar space (estimated at one hydrogen atom per cm³ or about 30 atoms per fluid ounce if you swing that way).  If you’re floating in vacuum and a whole solar system’s gravity is pulling you towards it, then that’s where you’ll go.  Interstellar space will be emptier without you.

By the way, space between galaxies is a million times emptier than space between stars.

The solar nebula hypothesis does a decent job of explaining the familiar structure — an inward succession of gas giants, then an asteroid belt, then rocky planets, all orbiting within a degree or so of their common Plane of The Ecliptic.  When the Sun lit up 4.6 billion years ago, its fierce light stripped hydrogen and other light elements from the region where the inner planets were coalescing.  Those atoms fled outward to the asteroids, the gas giants and beyond.  An eon later, the rocky planets collected water and other volatiles from impacting comets and such.

But some incoming objects, especially the long-period comets, seem to have no respect for the Ecliptic.  They come at us from all angles, an oddity that led Ernst Öpik and Jan Oort to suggest that the familiar planar Solar System is in fact enclosed by a spherical shell of loosely-held objects, ready to pelt us at any time from wherever they happened to be.

No-one’s yet seen that shell, but statistical models suggest it’s huge.  Earth is one Astronomical Unit (AU) from the Sun.  Neptune, our farthest-out known planet, orbits at about 30 AU.  Researchers think the Oort Cloud starts somewhere near 2,000 AU and runs out to 20,000 or more.  Some suggest it may contain material from other solar systems.

Astronomers also think the Cloud contains something like a trillion objects, pebble-size up to planetoids or bigger.  In a volume that large, the average distance between objects is about 30 AU.  When NASA’s New Horizon spacecraft finally flies through the Oort Cloud 900 years from now, accidentally colliding with something shouldn’t be a problem.

outer-orbits
I drew the Oort Cloud much too small
compared to the purple Kuiper Belt Object orbits
(adapted from the Batygin-Brown paper)

In between the familiar Solar System and the Oort Cloud there’s a whole zoo of objects we’ve only started to glimpse in the past 25 years.  The Kuiper Belt is a doughnut of about 100,000 bodies that stay close to the Ecliptic Plane but orbit from just beyond Neptune’s orbit out to about twice as far.  (By my calculation the average distance between the rocks is, you guessed it, about 30 AU.)  These guys are heavily influenced by Neptune’s gravity and thought to be leftovers from our solar nebula.  Most short-period comets seem to come from the Kuiper Belt.

Recently, CalTech astrophysicists Konstantin Batygin and Michael Brown, drew attention to a half-dozen objects with orbits that were strangely similar.  Unlike the other thousand-or-so Kuiper Belt Objects characterized so far, these all

  • go further than 250 AU from the Sun despite getting as close as 50 AU
  • have a perihelion (the point of closest approach to the Sun) at about the same equatorial latitude (see the diagram)
  • (the kicker) the perihelion drops below the ecliptic by about the same amount.

The authors account for these observations, and more, by hypothesizing a Planet 9 that roams out beyond the Kuiper Belt.  They call it “a mildly inclined, highly eccentric distant perturber.”  I know what you’re thinking, but in the paper those are technical terms.

~~ Rich Olcott

Gettin’ kinky in space

Things were simpler in the pre-Enlightenment days when we only five planets to keep track of.  But Haley realized that comets could have orbits, Herschel discovered Uranus, and Galle (with Le Verrier’s guidance) found Neptune.  Then a host of other astronomers detected Ceres and a host of other asteroids, and Tombaugh observed Pluto in 1930.whirlpool-44x100-reversed

Astronomers relished the proliferation — every new-found object up there was a new test case for challenging one or another competing theory.

Here’s the currently accepted narrative…  Long ago but quite close-by, there was a cloud of dust in the Milky Way galaxy.  Random motion within it produced a swirl that grew into a vortex dozens of lightyears long.

Consider one dust particle (we’ll call it Isaac) afloat in a slice perpendicular to the vortex.  Assume for the moment that the vortex is perfectly straight, the dust is evenly spread across it, and all particles have the same mass.  Isaac is subject to two influences — gravitational and rotational.

making-a-solar-nebula
A kinked galactic cloud vortex,
out of balance and giving rise
to a solar system.

Gravity pulls Isaac towards towards every other particle in the slice.  Except for very near the slice’s center there are generally more particles (and thus more mass) toward and beyond the center than back toward the edge behind him.  Furthermore, there will generally be as many particles to Isaac’s left as to his right.  Gravity’s net effect is to pull Isaac toward the vortex center.

But the vortex spins.  Isaac and his cohorts have angular momentum, which is like straight-line momentum except you’re rotating about a center.  Both of them are conserved quantities — you can only get rid of either kind of momentum by passing it along to something else.  Angular momentum keeps Isaac rotating within the plane of his slice.

An object’s angular momentum is its linear momentum multiplied by its distance from the center.  If Isaac drifts towards the slice’s center (radial distance decreases), either he speeds up to compensate or he transfers angular momentum to other particles by colliding with them.

But vortices are rarely perfectly straight.  Moreover, the galactic-cloud kind are generally lumpy and composed of different-sized particles.  Suppose our vortex gets kinked by passing a star or a magnetic field or even another vortex.  Between-slice gravity near the kink shifts mass kinkward and unbalances the slices to form a lump (see the diagram).  The lump’s concentrated mass in turn attracts particles from adjacent slices in a viscous cycle (pun intended).

After a while the lumpward drift depletes the whole neighborhood near the kink.  The vortex becomes host to a solar nebula, a concentrated disk of dust whirling about its center because even when you come in from a different slice, you’ve still got your angular momentum.  When gravity smacks together Isaac and a few billion other particles, the whole ball of whacks inherits the angular momentum that each of its stuck-together components had.  Any particle or planetoid that tries to make a break for it up- or down-vortex gets pulled back into the disk by gravity.

That theory does a pretty good job on the conventional Solar System — four rocky Inner Planets, four gas giant Outer Planets, plus that host of asteroids and such, all tightly held in the Plane of The Ecliptic.

How then to explain out-of-plane objects like Pluto and Eris, not to mention long-period comets with orbits at all angles?outer-orbits-1

We now know that the Solar System holds more than we used to believe.  Who’s in is still “objects whose motion is dominated by the Sun’s gravitational field,” but the Sun’s net spreads far further than we’d thought.  Astronomers now hypothesize that after its creation in the vortex, the Sun accumulated an Oort cloud — a 100-billion-mile spherical shell containing a trillion objects, pebbles to planet-sized.

At the shell’s average distance from the Sun (see how tiny Neptune’s path is in the diagram) Solar gravity is a millionth of its strength at Earth’s orbit.  The gravity of a passing star or even a conjunction of our own gas giants is enough to start an Oort-cloud object on an inward journey.

These trans-Neptunian objects are small and hard to see, but they’re revolutionizing planetary astronomy.

~~ Rich Olcott

Smoke and a mirror

Etna jellyfish pairGrammie always grimaced when Grampie lit up one of his cigars inside the house.  We kids grinned though because he’d soon be blowing smoke rings for us.  Great fun to try poking a finger into the center, but we quickly learned that the ring itself vanished if we touched it.

My grandfather can’t take credit for the smoke ring on the left — it was “blown” by Mt Etna.  Looks very like the jellyfish on the right, doesn’t it?  When I see two such similar structures, I always wonder if the resemblance comes from the same physics phenomenon.

This one does — the physics area is Fluid Dynamics, and the phenomenon is a vortex ring. We need to get a little technical and abstract here: to a physicist a fluid is anything that’s composed of particles that don’t have a fixed spatial relationship to each other. Liquid water is a fluid, of course (its molecules can slide past each other) and so is air.  The sun’s ionized protons and electrons comprise a fluid, and so can a mob of people and so can vehicle traffic (if it’s moving at all).  You can use Fluid Dynamics to analyze motion when the individual particles are numerous and small relative to the volume in question.

Ring x-section
Adapted from a NOAA page

You get a vortex whenever you have two distinct fluids in contact but moving at sufficiently different velocities.  (Remember that “velocity” includes both speed and direction.)  When Grampie let out that little puff of air (with some smoke in it), his fast-moving breath collided with the still air around him.  When the still air didn’t get out of the way, his breath curled back toward him.  The smoke collected in the dark gray areas in this diagram.

That curl is the essence of vorticity and turbulence.  The general underlying rule is “faster curls toward slower,” just like that skater video in my previous post.  Suppose fluid is flowing through a pipe.  Layers next to the outside surface move slowly whereas the bulk material near the center moves quickly.  If the bulk is going fast enough, the speed difference will generate many little whorls against the circumference, converting pump energy to turbulence and heat.  The plant operator might complain about “back pressure” because the fluid isn’t flowing as rapidly as expected from the applied pressure.

But Grampie didn’t puff into a pipe (he’s a cigar man, right?), he puffed into the open air.  Those curls weren’t just at the top and bottom of his breath, they formed a complete circle all around his mouth.  If his puff didn’t come out perfectly straight, the smoke had a twist to it and circulated along that circle, the way Etna’s ring seems to be doing (note the words In and Out buried in the diagram’s gray blobs).  When a vortex closes its loop like that, you’ve got a vortex ring.

A vortex ring is a peculiar beast because it seems to have a life of its own, independent of the surrounding medium.  Grampie’s little puff of vortical air usually retained its integrity and carried its smoke particles for several feet before energy loss or little fingers broke up the circulation.

To show just how special vortex rings are, consider the jellyfish.  Until I ran across this article, I’d thought that jellyfish used jet propulsion like octopuses and squids do — squirt water out one way to move the other way.  Not the case.  Jellyfish do something much more sophisticated, something that makes them possibly “the most energy-efficient animals in the world.”

jellyfish vortices

Thanks to a very nice piece of biophysics detective work (read the paper, it’s cool, no equations), we now know that a jellyfish doesn’t just squirt.  Rather, it relaxes its single ring of muscle tissue to open wide.  That motion pulls in a pre-existing vortex ring that pushes against the bell.  On the power stroke, the jellyfish contracts its bell to push water out (OK, that’s a squirt) and create another vortex ring rolling in the opposite direction.  In effect, the jellyfish continually builds and climbs a ladder of vortex rings.

Vortex rings are encapsulated angular momentum, potentially in play at any size in any medium.

~~ Rich Olcott