Artwork depicting a circumbinary exoplanet orbiting a tightly-spaced pair of stars; their orbits shown as nearly-edge on ellipses in the sky. In this case, the Tattooine-like planet really would very likely orbit in that same plane.
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Artwork depicting a circumbinary exoplanet orbiting a tightly-spaced pair of stars; their orbits shown as nearly-edge on ellipses in the sky. In this case, the Tatooine-like planet really would very likely orbit in that same plane. Credit: NRAO/AUI/NSF, S. Dagnello

Science and Star Wars: Tatooine was a coplanar circumbinary planet

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Mar 23, 2020

In an iconic scene in Star Wars*, young Luke Skywalker ponders his future while gazing across the desert to the setting twin suns of Tatooine.

We now know that it is very likely that his home world orbited in the same plane that the two stars did. Assuming a few things about those stars, that is, and also that the laws of physics in the Milky Way and in that galaxy far, far away are the same. Which they would be.

This is due to new research looking at very young binary stars, ones just a few million years old that are still surrounded by the disk of material from which they formed. In some cases, the plane of the disk is the same as the plane of the orbit of the two stars around each other. But in some it is decidedly not, with the disk tipped at a high angle — usually perpendicular to the plane of the stars!

In the wide binary HD 98800 (left) the stars orbit in and out of the screen (toward and away from you) but the disk is perpendicular to them; in the tighter binary AK Sco (right) the stars and disk orbit in the same plane. Credit: ALMA (ESO/NAOJ/NRAO), I. Czekala and G. Kennedy; NRAO/AUI/NSF, S. Dagnello

In their work, the astronomers used old and new observations of these systems, including with the Atacama Large Millimeter/submillimeter Array (or ALMA), which is very sensitive to the light emitted by these disks, and which also can take extremely high-resolution images of them.

What they found is that the tilts of the disks depended on how close together the two stars orbit each other: If the orbit is tight, and the time it takes the two stars to orbit each other short (30 days or less), the disk tends to be almost exactly in the plane of the stars' orbits. If the stars are farther apart and the orbital period is longer (30 days to a few centuries) the disks are perpendicular to the stars.

Animation of the circumbinary disks around a wide binary (HD 98800, left) and a tight binary (AK Sco, right), showing their tilts.

For example, they used ALMA to observe the three-million-year-old star system UZ Tau which is about 425 light years from Earth. UZ Tau is actually four stars, a pair of binaries orbiting each other! The one to the east is called UZ Tau E (and each star in that binary is called UZ Tau Ea and Eb) and to the west UZ Tau W (itself made up of Wa and Wb).

The stars in the binary system Ea and Eb are both low mass, about 0.93 and 0.28 times the mass of the Sun. They orbit each other in just 19 days on a mildly elliptical orbit, and are surrounded by a lovely disk of gas and dust about 4 billion kilometers in radius (a disk that orbits both stars in a binary like that is called a circumbinary disk).

Careful examination of the data showed that the plane of the stars' orbits and the plane of the disk match to within a few degrees. In fact, looking at other short-period binaries they found the same thing.

But when they looked at binary stars that orbit farther out, the disk is highly tilted. An earlier paper from a different team of astronomers, for example, looked at a different young system, called HD 98800. This too is what's called a hierarchical quadruple system, with two pairs of binary stars orbiting each other. The inner pair (HD 98800 Ba and Bb) has a ring of dust surrounding it, but that ring is perpendicular to the binary orbital plane. The orbital period of that binary is 315 days, much longer than UZ Tau E. Again, the same is true for several other system they examined.

Why does this occur? It's not clear. Computer models looking at how the gas in the disk behaves show that small misalignments in the disk can be amplified by a wider binary, which yanks on it, torqueing it via gravity, and that can cause the gas to flow in a way that eventually makes the entire disk tilt. Why though does that not happen for tighter binaries? That's the question. I'm speculating, but it may have to due with the amount of time a star can tug on a part of the disk that's misaligned; it's longer for stars that are farther apart. But as I said, I'm guessing, and more observations of these kinds of binaries need to be made.

An interesting thing here is that we've seen circumbinary exoplanets; alien worlds that orbit both stars in a binary system. These planets would have formed from the circumbinary disks, so we'd then expect that planets orbiting tight binaries would orbit in that same plane. Well, guess what: the exoplanet hunting satellite Kepler found just that! Unfortunately, Kepler isn't sensitive to planets that orbit at a high angle, so we don't know about planets orbiting more widely space stars. Hopefully future missions can fill in that gap.

A binary sunset on Tatooine. There’s a lot of science hiding in this shot. Credit: Disney/Lucasfilm

… which brings me back to Tatooine. It's canon that the two stars are more or less like the Sun. That one iconic shot gives us clues about their separation: They look to be about two star-diameters apart edge-to-edge, so three center-to-center. The Sun is 1.4 million kilometers wide, so the centers of the stars (which is how we measure distances for orbits) are at least four million km apart.

Now, we can't say if that's how far apart they really are because we don't know where Tatooine was in its orbit at the time. Perspective can make them look close together when they may be widely spaced if we see them down the long axis of an elliptical orbit. But, if that was their widest separation then they're just a few million kilometers apart, and their orbit would only take a day or two to complete.

Even if we assume the stars really are farther apart, they can't be too far apart or else Tatooine's orbit wouldn't be stable; each star would yank on it every close pass, destabilizing the orbit. So I think it's safe to assume the two stars are pretty tight, making Tatooine a coplanar circumbinary planet.

Kinda rolls of the tongue, doesn't it?

And if that's the case, that means they're close enough together that Tatooine would orbit very nearly on the same plane! If it did orbit exactly on the same plane, then twice a year it would see the stars eclipse each other, which would be incredibly cool. Literally, too, since one star's heat would be blocked by the other, and the planet's temperature would dip considerably.

[Correction (23 March, 2020 at 15:15 UTC): OOPS! What I said above is incorrect! As Bad Reader Gerrit van Langeveld pointed out to me, the stars orbit each other every couple of days, so Tatooiners would see an eclipse roughly every day! That's pretty dang amazing, and would have profound effects on the planet. However, if the planet's orbit were tilted by just a few degrees (which the science says is still possible) then the two stars would usually "miss" each other and eclipses would be more rare, like how solar eclipses are rare here on Earth. My apologies for any confusion!]

How amazing would that be to see and experience?

And it makes one thing Luke said very wrong. Tatooine may not be the shining center of the Universe, but it would still be a really incredible place to visit.

But given what we do see of it, yeah, I think I may actually agree with him. I wouldn't want to live there. I'm not really fond of womp rat anyway.


*Sure, some of you may argue that it's actually A New Hope, but I saw that movie in the theaters when it came out, several times, and it was just called Star Wars, so get off my lawn.

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