Do all planets rotate around their stars?

Do all planets rotate as they go around their stars? Do they all rotate in the same direction (e.g. clockwise or anticlockwise?) Or does it just depend on what started them rotating in the first place?
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Originally posted @ Medium!

Before we can expand our thinking out to “all planets”, the easiest wayto start looking at planets and how they rotate is to look at our ownsolar system, which we can investigate in far more detail (and far moreeasily) than anywhere else in the Universe. What we see in our own solarsystem is that all of the major planets are rotating around their own internal axis.

We’re wellacquainted with the rotation of the Earth, even if we haven’t thoughtabout it in this way — the Earth’s rotation is what creates our days. Asa result, we all know how long it takes for the Earth to rotateonce — 24 hours. But 24 hours isn’t the rule within our solar system; infact, of all the other planets, only Mars rotates at a similar speed.Mars completes one rotation in 24 hours and 40 minutes; nearly identicalto our home planet.

Venus,our planetary sister gone horribly awry, rotates much slower than theearth — one rotation takes 243 days and 26 minutes; this makes it theslowest rotator in our solar system. Mercury comes in second slowest,and rotates once in 58 days, 15 hours, and 30 minutes. The gas giantsall seem to have similar rotation speeds, all of which are faster thanany of the inner, rocky worlds. Jupiter rotates once every 9 hours and55 minutes. Saturn recently had its rotation speed re-measuredto be 10 hours and 36 minutes. Uranus rotates once every 17 hours and14 minutes, whereas Neptune rotates once every 16 hours and 6 minutes.

Mosteverything in our solar system rotates in the same direction — the samedirection as the Earth. If we had a bird’s eye view of our solarsystem, where we’d flown into space “up” via the North Pole, and lookedback down, most of the planets would be rotating counterclockwise — orfrom the West towards the East.

You can usually remember which way the earth is rotating by thinking about thetime zones; the further east you go, the earlier it is — they’re pushedtowards daylight sooner than the west.

Sowhy does almost everything rotate in the same way? A hint of the answerlies in the Sun. The sun also rotates, and it rotates in the samedirection as the majority of the planets — counterclockwise from ourbird’s eye view. Since the Sun is also following our consistent rotationpattern, we’re going to have to make our way back to the formation ofthe solar system in order to make sense of this.

Our sun formed out of a cloud of gas and dust — enormous molecular clouds arethe only places where, under the persistent pull of gravity over time,material gets dense enough to begin to collapse and form a star (orten). However, during the collapse of the cloud of dust and gas, anyinitial motion in the cloud becomes quite important.

It’svery unusual for objects in the universe to be completely still inrelation to each other (in fact, they would have to be at absolute zeroto prevent any motion), so there’s a little bit of stirring about withinthe cloud that is always present. As the cloud collapses, the averagedirection of motion of the cloud is kept — and as gravity pullseverything closer to the center, the conservation of angular momentumcauses that average motion to speed up. You can do this experiment athome if you have a chair that spins. If you start yourself turning alittle bit with your arms and legs extended, and then pull your legs andarms in quickly, you’ll find yourself spinning much more rapidly thanyou were to start out. Pulling your arms in plays the same role thatgravity is playing in the collapse of the early solar system.

Because therotation speed is so much faster than it was initially, the gas formsinto a flat disk that’s all rotating in the same direction. If you’veever played with a ball of bread dough or silly putty, and spun it onyour finger, you’ll find it flattens out into a thin disk fairlyquickly. The star at the center is therefore being formed in anenvironment where everything is already rotating — as the star collapseseven more, its rotation will increase even more. All the proto-planetsare also forming out of gas which is rotating, so it makes sense thatthey too will absorb the rotation of the disk they formed in. So inprinciple, we expect most planets to form with a rotation that’sconsistent with the star at the center of their solar system.

However,“most everything” isn’t everything — we’ve got two notable exceptionswithin our solar system to the counterclockwise rotation rule: Venus andUranus. Uranus rotates 90 degrees off from everything else. If youconsider the plane of all the planets’ orbits around the sun as a flatsurface, most planets spin as though they were a coin spun on its edge,flicked counterclockwise. Uranus, on the other hand, spins like a beadrolled along the ground, instead of spinning vertically. Venus is evenodder- it spins clockwise. As far as we can tell, this means Venus issomehow upside down.

Whathappened to these planets? The picture I painted above indicates thatthey should have formed with a rotation aligned with every other planetin our solar system. In fact, they probably did form with a rotationthat matched. However, the very early solar system was a violent place,filled with many more proto-planets which were inclined to collide withother objects. Some of these collisions were likely with the earlyversions of the planets which exist today. A particularly bad collisionor series of collisions could cause such an energetic punch to the otherplanet that the planet was effectively tipped over onto its side (inthe case of Uranus) or upside down completely (in the case of Venus.)

Toexpand to the Universe outside of our own solar system; physics is thesame everywhere in the Universe, so solar systems should form in thesame way everywhere. However, we wouldn’t expect every planet to rotatein the same direction as its parent star, since — like our own solarsystem — the planets may have had a very collision-intensive early life.

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What happens when Betelgeuse explodes?

If Betelgeuse explodes right now, could we see it with naked eye? It is over 400 light years away, so you might think that people would see it long after it actually happens?
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Originally posted @ Medium!

Betelgeuse is already one of the brightest stars in the night sky, sitting somewhere around the 8th or 9th brightest star in the night sky. (These lists don’t include the Sun, which is somewhat obviously always the brightest object in the sky.) It sits in the constellation Orion, along with a number of other bright stars, and makes up the left hand shoulder of the warrior. It looks visibly orange in the night sky, and is classified as a red supergiant star, in the later stages of its life. It’s also one of the few stars that’s close enough for us to resolve in more detail than a point source, and the pictures are pretty fun.

If Betelgeuse were to go supernova right now — as in, if you could break physics and travel to the star instantaneously to check on it — you’re absolutely correct to think that it would take us quite a while to notice. Betelgeuse is about 600 light years away from our solar system, so the light traveling from Betelgeuse has about 600 years of travel before it will reach us. If the star had physically exploded in 2015, we wouldn’t spot the light from that explosion until 2615. We’re constantly observing this star (and pretty much everything in the Universe) as it was, a significant period of time ago. This is also why astronomers say that in studying the night sky, we study the past. The more distant the object, the further in the past we observe. 600 light years, in the grand scheme of things, is pretty close; we’re still dealing with our local neighborhood inside our own galaxy.

Supernovae are incredibly bright phenomena. At the brightest point of the explosion, a supernova can outshine the whole galaxy it lives in. A single star has managed to, for a short time, be a brighter source of light than the several billion other stars in its galaxy combined. This is tremendously bright. Supernovae do have a “rising time” of about a week, when the star is increasing in brightness — it stays at its peak brightness for a few days, and then slowly declines into obscurity over a period of a couple of weeks.

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But how bright would Betelgeuse specifically be? We can do some math to work this out, making the assumption that Betelgeuse explodes as a Type II supernova. The exact style of supernova is still up for a bit of debate, depending on the exact rotation speed and mass loss of the star over the next hundred thousand years. Regardless of the exact method of its explosion, all the supernovae options for this star have a peak brightness of approximately the same value, so for a quick calculation that’s good enough to determine what we’d see with the naked eye.

There are two ways of measuring brightness in the astronomy world; the first is absolute magnitude, which is the brightness of the star, as it would be measured from a fixed distance. (It’s arbitrary, but the fixed distance chosen is 10 parsecs, or about 33 light years.) This is trying to get to a measure of intrinsic brightness — as though we could line up everything in the sky at equal distance from us, and compare them to each other that way. We can’t actually measure the brightness of a star this way, but we can apply some corrections based on the distance to the star to get to it. The absolute magnitude of a Type II supernova is around -17. Because astronomers have the worst conventions in the world (for largely “historical reasons”), negative numbers mean brighter objects. The sun has an absolute magnitude of 4.83, which, once we translate out of “magnitudes”, means that the sun is 500 million times fainter than the supernova, when measured at the same distance. This huge difference in relative brightness is why a supernova can outshine an entire galaxy.

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The other method of measuring brightness is a bit more straightforward. It’s the apparent brightness — i.e., how bright does it appear to us as viewed from the Earth. In this frame of reference, more distant objects will always appear fainter, regardless of how intrinsically bright they are. Because Betelgeuse is still fairly distant from us, the apparent brightness would be significantly less than the absolute magnitude. Based on the distance to Betelgeuse, we can work out that the apparent magnitude of the peak of the explosion would be -10. The sun, in apparent magnitude, is the brightest thing in our sky, and is checking in at an apparent magnitude of -26.74. Once again translated out of magnitudes, this means that the Sun as seen from the Earth is a whopping ~5 million times brighter than Betelgeuse’s explosion, so our supernova certainly won’t be anywhere near as bright as our sun in the daytime. That’s not to say you wouldn’t be able to see it — it would definitely be bright enough to see during the daytime, as long as you were looking in the right direction. (After all, you can still see Venus in the daytime, if you know where to look!)

Nighttime will be a different story. The brightness of Betelgeuse’s supernova is about the same as the quarter moon. It would also be about 16 times brighter than the brightest supernova known to have been seen from earth, which occurred in 1006, and was recorded by a number of early civilizations. (An image of what remains of that supernova is shown below.)

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It was said that the supernova in 1006 was bright enough to cast a shadow at night. Betelgeuse, being significantly brighter, would likely also cast shadows — which, if you think about the brightness of a quarter moon, would make sense!

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All that said, Betelgeuse isn’t expected to explode for another 100,000 years or so. We do expect a few supernova in our galaxy every few hundred years, so there are a number of stars that are nearing the ends of their lifetimes within our galaxy. It’s hard to predict exactly when a star will transition from “close to the end of its life” to “exploding in the next week”, so while we expect that none of these will be exploding in the next little while, it’s difficult to predict which one of the stars will be the first to go. In the mean time, we can take wonderful pictures of the more nearby stars, like the one below taken by Hubble, and watch them cast off their outer layers at an incredible rate.

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