Out of line

Astrophysicists have a fairly clear idea about how planetary systems form.

The whole thing starts with a nebula -- a cloud of interstellar gas and dust,  mostly made of hydrogen and helium -- that begins to contract under the influence of gravity.  Assuming it's large enough, that compaction raises its temperature; and because almost always, the cloud as a whole had some angular momentum to start with (i.e. it had a net spin around the nebula's center of mass, even if a small one) its rotational rate increases as the collapse proceeds.  That increase in spin rate flattens the cloud out -- think of a whirling blob of pizza dough in the hands of someone who knows how to make the perfect pizza crust -- resulting in a concentrated mass in the center (the future star) and a "protoplanetary disk."

The disk is never perfectly uniform, so the higher gravitational pull of the denser parts draws in more material, making them denser still -- a classic example of positive feedback.  Those lumpy bits form the planets, ultimately gaining sufficient mass to gravitationally clear the regions around their orbits.  When the star becomes dense and hot enough to initiate fusion, the light and heat blow away lighter elements (hydrogen and helium), leaving the inner regions enriched in heavier elements like carbon, silicon, magnesium, nickel, aluminum, and iron.

This model explains two things; why the planets in the Solar System all have relatively circular orbits that are aligned with each other and with the spin plane of the Sun, and why the inner planets (Mercury, Venus, Earth, and Mars) are dense and rocky, while the outer ones (Jupiter, Saturn, Uranus, and Neptune) are gas giants.

But.

When we get too confident, nature has this awkward way of saying, "You think you understand everything?  Ha.  A lot you know."  Back in the 1990s people looking for exoplanets started finding what are now nicknamed "hot Jupiters," which are gas giants locked in a tight orbit around their host stars.  Hot Jupiters seem to be pretty common; on the other hand, it may just be that they're simple to spot.  Given their size and mass, they are going to be easier to pick up both by the transit method (the dip in a star's brightness as its planet crosses in front of it) and the wobble method (stars having a slight back-and-forth "wobble" as the star and its planet orbit their common center of gravity; this effect is more pronounced for larger exoplanets and ones with closer orbits).  

So how does a gas giant form, and remain stable, so near to its host star?  Wouldn't the light and heat of the star blow away the lightweight gases, as they seem to have done in our own Solar System?

The answer is "we're not sure."

Another spanner in the works comes from planets that are misaligned -- that have rotational axes or orbital planes skewed with respect to the rotational plane of the star.  There are two examples in our own Solar System; Venus (which actually rotates backwards as compared to the other planets; its day is longer than its year) and Uranus (which lies on its side -- its rotational axis is tilted 82 degrees with respect to its orbital plane).

Neither of these has been explained, either.

But weirdest of all is when a planet's orbital plane is out of alignment with both the star's rotation and the orbits of other planets in the system.  This, in fact, is why the topic comes up; a paper this week in the journal Astrophysics presents some strange new data on the system AU Microscopii, suggesting that the planet AU Microscopii c has its orbital plane tilted by 67 degrees with respect to everything else in the system.  So as the other two planets, and the star itself, are all moving in a nicely aligned fashion, AU Microscopii c is describing these wild loops above and below the system's orbital plane.

You might be wondering how they figured out the orientation of the rotational axis of the star, since most stars look like points of light even in large telescopes.  And this part is really cool.  It's called the Rossiter-McLaughlin effect.  As a star rotates, from our perspective half of the star's disk is heading toward us while the other half is heading away.  So the light from the part that's coming toward us gets slightly blue-shifted, and the light from the other half is simultaneously red-shifted.  Now, imagine a large planet crossing in front of the star, orbiting in the same direction as the star is rotating.  First the blue-shifted part of the light will be partially blocked, then the red-shifted part, resulting in a spectrum alteration that will look like this:

[Image licensed under the Creative Commons Amitchell125, Animation of the Rossiter-Mclaughlin (RM) effect, CC BY-SA 4.0]

So we know the rotational plane of the star from the Rossiter-McLaughlin effect, and the orbital planes of the planets from the direction of the star's wobble.

And they don't line up.  At all.

This completely confounds our models of how planetary systems form.  Did a close pass by another heavy object yank one of the planets out of alignment?  Or an actual collision with something?  (That's one guess about why Uranus's axis is tilted.)  The answer is still "we don't know."  What seems certain is that the configuration is gravitationally unstable.  AU Microscopii is thought to be a young star, on the order of 24 million years old; the Solar System is over five hundred times older than that.  As I described in a post a couple of years ago, long-term stability usually requires some kind of orbital resonance, where the gravitational pull of planets acts to reinforce their trajectories, keeping them all locked in a tight celestial dance.  So it seems like the weird loop-the-loop described by AU Microscopii c is unlikely to last long.

But it's also an orbit that, based on what we know, shouldn't have happened in the first place.  So maybe it's not a good idea to place bets on what it's going to do in the future.

In any case, it's yet another example of how far we have to go in our understanding of the universe we live in.  That's okay, of course; it'd be boring if we had it all figured out.  Science is like some benevolent version of the Hydra from Greek mythology; for every one question we answer, we create nine more.

I think the scientists are going to be busy for a very long time to come.

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