Far out, man

Egg in the UniverseThe thing about Al’s coffee shop is that there’s generally a good discussion going on, usually about current doings in physics or astronomy.  This time it’s in the physicist’s corner but they’re not writing equations on the whiteboard Al put up over there to save on paper napkins.  I step over there and grab an empty chair.

“Hi folks, what’s the fuss about?”

“Hi, Mr Moire, we’re arguing about where the outer edge of the Solar System is.  I said it’s Pluto’s orbit, like we heard in high school — 325 lightminutes from the Sun.”

The looker beside him pipes up.  “Jeremy, that’s just so bogus.”  Kid keeps scoring above his level, don’t know how he does it.  “Pluto doesn’t do a circular orbit, it’s a narrow ellipse so average distance doesn’t count.  Ten percent of the time Pluto’s actually closer to the Sun than Neptune is, and that’s only 250 lightminutes out.”

Then the looker on his other side chimes in.  Doing good, kid.  “How about the Kuiper Belt?  A hundred thousand objects orbiting the Sun out to maybe twice Neptune’s distance, so it’s 500 lightminutes.”

Third looker, across the table.  You rock, Jeremy.  “Hey, don’t forget the Scattered Disk, where the short-period comets drop in from.  That goes out to 100 astronomical units, which’d be … 830 lightminutes.”

One of Cathleen’s Astronomy grad students can’t help diving in despite he’s only standing nearby, not at the table.  “Nah, the edge is at the heliopause.”

<several voices> “The what?”

“You know about the solar wind, right, all the neutral and charged particles that get blown out of the Sun?  Mass-density-wise it’s a near-vacuum, but it’s not nothing.  Neither is the interstellar medium, maybe a few dozen hydrogen and helium atoms per cubic meter but that adds up and they’re not drifting on the same vector the Sun’s using.  The heliopause is the boundary where the two flows collide.  Particles in the solar wind are hot, relatively speaking, compared to the interstellar medium.  Back in 2012, our outbound spacecraft Voyager 1 detected a sharp drop in temperature at 121 astronomical units.  You guys are talking lightminutes so that’d be <thumb-pokes his smartphone> how about that? almost exactly 1000 lightminutes out.  So there’s your edge.”

Now Al’s into it.  “Hold on, how about the Oort Cloud?”

“Mmm, good point.  Like this girl said <she bristles at being called ‘girl’>, the short-period comets are pretty much in the ecliptic plane and probably come in from the Scattered Disk.  But the long-period comets seem to come in from every direction.  That’s why we think the Cloud’s a spherical shell.  Furthermore, the far points of their orbits generally lie in the range between 20,000 and 50,000 au’s, though that outer number’s pretty iffy.  Call the edge at 40,000 au’s <more thumb-poking> that’d be 332,000 lightminutes, or 3.8 lightdays.”

“Nice job, Jim.”  Cathleen speaks up from behind him.  “But let’s think a minute about why that top number’s iffy.”

“Umm, because it’s dark out there and we’ve yet to actually see any of those objects?”

“True.  At 40,000 au’s the light level is 1/40,000² or 1/1,600,000,000 the sunlight intensity we get on Earth.  But there’s another reason.  Maybe that ‘spherical shell’ isn’t really a sphere.”

I have to ask.  “How could it not be?  The Sun’s gravitational field is spherical.”

“Right, but at these distances the Sun’s field is extremely weak.  The inverse-square law works for gravity the same way it does for light, so the strength of the Sun’s gravitational field out there is also 1/1,600,000,000 of what keeps the Earth on its orbit.  External forces can compete with that.”

“Yeah, I get that, Cathleen, but 3.8 lightdays is … over 400 times closer than the 4½ lightyear distance to the nearest star.  The Sun’s field at the Cloud is stronger than Alpha Centauri’s by at least a factor of 400 squared.”

“Think bigger, Sy.  The galactic core is 26,000 lightyears away, but it’s the center of 700 billion solar masses.  I’ve run the numbers.  At Jim’s Oort-Cloud ‘edge’ the Galaxy’s field is 11% as strong as the Sun’s.  Tidal forces will pull the outer portion of the Cloud into an egg shape pointed to the center of the Milky Way.”

Jeremy’s agog.  “So the edge of the Solar System is 1,000 times further than Pluto?  Wow!”

“About.”

“Maybe.”

~~ Rich Olcott

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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