Dear Urban Astronomer

Do all planets spin the same way, or are there some planets where the sun rises in their equivalent of West? Does it depend on the particular solar system? What about the stars in a galaxy — do they all spin in the same direction? Do all galaxies spin the same way, or do they even spin at all?

Regards

Amanda Dominy (extracted from Facebook conversation)

Dear Amanda

First, the short answer: All rotations and revolutions in a planetary system are in the same direction, except for when they aren’t.  These motions are not necessarily in the same direction between neighbouring stars, even stars born from the same gas cloud.  The stars themselves do move through a galaxy in the same direction in a roughly circular orbit, except for when they don’t.

So that’s all cleared up then.  But just in case you’re still in the dark, here’s a more detailed answer:

How planets form

Current models of planetary formation haven’t changed much since astronomers first turned their minds to the subject.  You start with a vast cloud of dust and gas, several light years across.  It is made mostly of Hydrogen, but contains some Helium and a smattering of heavier elements ejected from the atmospheres of dying stars.  The individual molecules of this cloud are in constant motion, bouncing and colliding off each other, which causes the gas to have an internal pressure.  However, the gas is so spread out and diffuse that it might as well be a vacuum (In fact, the very best vacuums we’re able to pump out in laboratories on Earth contain more material per unit volume than these clouds).  This means that the gas’s pressure is so low that it cannot beat its own gravity, and the cloud does not dissipate completely.  Instead, it floats around space, stretching and distorting from passing gravity fields and its own internal currents, like a child’s soap-bubble.  Eventually, something will happen to increase the pressure on the cloud; perhaps a collision with another cloud, or the shockwave of a nearby supernova.  Astrophysicists are still untangling the exact processes here, but you end up with a region of gas that is now dense enough to have an appreciable gravity field.  More gas starts to fall in, further increasing the mass and density.  Star formation has begun!

In practice, though, thanks to the swirling turbulence of these clouds, you find many hundreds of these dense knots forming, but we’ll just focus on one of these protostars for now.  Because the gas is moving and swirling, it doesn’t just fall in a straight line to the centre of mass, as you might expect.  Instead, thanks to the  principle of Conservation of Momentum, the individual molecules will tend to miss the centre and swing around, in an orbit much like a comet’s.  However since there are so many molecules, and the density is increasing, most of them will not have a clear path.  They will collide with each other, creating new swirls and increasing the turbulence, until eventually the entire mass begins to swirl in one direction, averaged out from the sums of all the individual motions.  Further in-falling gas gets swept up into the vortex, and the end result is that all the matter becomes concentrated in a vast flat disk, centred on a core of material that will eventually become a star.

Now similar processes start happening within this disk.  Specks of dust within the gas begin clumping together, and breaking apart as they collide.  Some clumps manage to stick together through a mechanism that’s not fully understood (get used to that phrase… Astronomy is going through a golden age of rapid discovery, leading to a great many questions that still need answering) but that probably includes electrostatic attraction or weak chemical bonds, until they too are able to begin gravitationally attracting material inwards.  These are the seeds of planets, and some will grow to enormous sizes while others never get larger than a pebble.  But the important thing to remember is that, just like the original gas cloud, the infalling debris already has momentum which must be preserved.  Since it’s already concentrated into a single plane, that motion is all in the same direction, but it’s not all moving at the same speed.  Kepler’s laws tell us that the closer a body is to the object it orbits, the faster it has to move to stay in orbit.  This means that the material on the star-ward side of the protoplanet is moving faster than the protoplanet itself.  As it falls towards the centre of the protoplanet, it pulls further from the star, causing its speed to slow down.  Material starting on the other side of the protoplanet has the opposite experience:  It starts slow and speeds up.  The result, which goes counter to common sense, is that the protoplanet begins rotating in the same direction as its orbit.  Therefore, all planets spin in the same direction as their orbits, and all the orbits are in the same direction.  In our Solar System, this direction is anti-clockwise when viewed from the side that can see Earth’s north pole.

The Nice model

So far, so good.  But if we observe our own solar system, we find that things aren’t quite so clear cut.  Comets fly in at all angles and directions, many of the larger planet’s moon’s orbits revolve the wrong way, Venus rotates in the wrong direction, and Uranus is actually flipped over onto its side so that it rolls along rather than rotating neatly like all the other planets.  The theory explained above doesn’t allow for any of this, so what’s going on?  The answer that most astronomers have accepted was proposed as recently as the year 2005.  A group of scientists in the French town of Nice published a series of papers in Nature (here, here and here) which suggested that the early Solar System, for all its neat regularity, was quite unstable.  It involves planets and asteroids hurling each other around space, resulting first in spectacularly violent chaos, but eventually leaving us with the scarred yet stable solar system that we live in today.  This scenario has become known as the Nice model (and personally, I think it’s one of the coolest scientific theories ever!)

If we imagine our own Solar System many billions of years ago, when it was so new that it was still crowded with gas and dust that hadn’t yet been used to make the Sun or any of the planets, we find that it looked very different to today.  The most obvious difference would be that all the planets were in different places than where we find them today.  But something happened to move them around, and that something is gravity.  Each planet exerts a tiny tug on all the other planets which, as Newton explained more than three centuries ago, is stronger for large planets but weaker if those planets are further away.  If the orbits of planets that aren’t too far from each other have any kind of simple mathematical relationship (for example, if one planet takes twice as long to orbit the Sun as another), then they will keep arriving at the same spot relative to each other at regular intervals.  This means that the tiny, weak attraction they have for each other can start to add up.  Over a few tens of millions of orbits, that little tug adds up to quite a powerful nudge, and the planets move.  In this way, the gas giants (Jupiter, Saturn, Uranus and Neptune) all shuffled around, moving close and further from the Sun until they eventually settled into stable orbits.  As they did so, however, they played havoc with the smaller objects in the Solar System.

Small rocky planets like Earth got thrown all over the place, and we know that there was at least one collision when a planet the size of Mars smashed into Earth.  Earth itself survived, but huge amounts of rock, instantly melted into lava by the force of the impact, were thrown out into space.  Much of this material would have either fallen back to the surface or been lost into space, but some of it settled into a ring around the Earth which then (by a process that we’ve now explained twice in this article!) accreted into a single body: the Moon.  This explains why Earth and the Moon are made of such similar materials yet have such different structures, and why the Moon’s orbit is at such a crazy angle – it’s more than 20° off-kilter from the rest of the Solar system.

Venus and Uranus may also have suffered similar collisions, leaving them with their weird rotations, but they could also simply have been torqued into new rotations:  a closely passing planet could have applied a very strong gravitational pull for a short period, at an angle way off their axis of rotation.  If you’ve ever removed a bicycle’s wheel and spun it while holding the axle in your hands, you’ve experience the strange and powerful gyroscopic effects that happen if you try to move the axis, and those same forces could have moved Uranus’ axis of rotation by around 90° and Venus’ by a full 180°.  Unfortunately, we don’t have a lot of evidence to show that it was one or the other, so the collision vs torquing debate rages on.

Rotating stars and galaxies

So that’s what happens in a star system.  But the orientation of planets within one star system does not have any connection with the orientation of planets within a neighbouring system, even if they’re related.  Remember that the progenitor gas cloud was turbulent, with currents swirling in all directions.  So, while each of the hundreds of stars formed from that cloud has their planets revolving in a direction caused by the currents in their neighbourhood, no two stars were subject to the same currents and so the different planetary systems all rotate in different directions and at different angles to each other.  That said, astronomers studying planetary nebulae ejected from dying stars near the centre of our galaxy recently found that they tend to line up rather neatly with each other, and with the galaxy itself.  They’re still puzzling out why this could be.

But when we step back even further, we find that galaxies as a whole do spin.  Or more specifically, the hundreds of billions of stars that compose them are all moving in a roughly circular orbit around the centres of those galaxies.  They have to, otherwise they would all just fall inwards, same as the planets in our solar system have to orbit the Sun.  Thanks to the generally chaotic motions of clusters of freshly born stars, and random gravitational interactions between stars as they age, any individual star could be moving in any direction, at almost any conceivable speed.  But the overall trend is towards a consistent roughly circular orbit around the core, or at least it is in a spiral galaxy.

Galaxies have one final wrinkle though, which complicates this picture:  They move through space and interact with each other.  But they are not solid objects, and the cumulative effect of a collection of hundreds of billions of independent stars gravitationally attracting a different collection of hundreds of billions of stars creates some frankly bizarre shapes, and can leave a galaxy with entire regions moving consistently in otherwise unpredictable directions.  And all of that brings us to my original short answer: “Yes, they move in the same way, except when they don’t”.