## rotation of planets

why should the planets rotate?
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 Mentor Because they were rotating when they formed. Edit By rotate do you mean orbit (the Earth orbits the Sun in a year) or revolve (the Earth revolves about its axis in a day)?
 Because they have angular momentum.

## rotation of planets

 Quote by D H Because they were rotating when they formed. Edit By rotate do you mean orbit (the Earth orbits the Sun in a year) or revolve (the Earth revolves about its axis in a day)?
I mean rotating about their own axis.
 As NeoDevin said: 'because they have angular momentum' The planets formed out of a roughly continuous disk of gas around the sun; as the planets form (complex process overall), the probability of having near-zero angular momentum is extremely small. This is amplified by the many large-scale collisions that happen in the later formation-history of planets.
 If you were to calculate on a dusty gascloud collapsing, you would decipher it into "multi-particles motion" eg. 1000 elements heading for the same gravity-center. 1. As they close in on the center, density rises and chances of collisions increase. 2. The gascloud also orbits something else: planetary formations usually orbits a protostar. This means that the parts of the collapsing dustcloud that are closest to the protostar have a higher orbital velocity than the farthest (This is actually the "Coreolis force"). 3. So when they collide, the dustgrains in the outer cloud are gaining speed, as they are drawn inwords, while the inner part of the cloud are drawn outwords, loosing velocity 4. This creates the rotation when they accidentially collides. So the rotation is not only a question of momentum-probability due to random collisions, but is actually an inevidable consequence of the diffenerences in orbital momentums in the inner and outer part of the collapsing cloud.
 Btw: This is not only the case for planetary formation, but for everything that orbits something. This is also why stars rotate, and why they rotate in the same orientation of the galactic plane. Ever wondered why we can see so many exoplanetary transits, when the chance of seeing an eclipse should be small? It's because they all rotate in roughly the same orientation, for the same reasons as my above answer I hope i helped you, rather than confused you :O) Henrik
 Blog Entries: 2 Recognitions: Gold Member There is a hypothesis currently that assumes that our solar system is the remnant of a super nova. Not sure if that agrees with a dusty gas cloud collapsing.
 Hi Andre. Well actually it is a well established theory, that there was a type II supernova prior to the creation of the Sun. This is quite normal for starcreations; When heavy stars that usually only lives <1mio years are born, they explode and tricker a secondary wave of starformations in the surrounding interstellar clouds. The new hypothesis is that there also was a type Ia supernova somewhere around the creation. This theory is based on findings of some characteristic isotopes in meteorites. Se http://news.uchicago.edu/news.php?asset_id=2087 This will not effect smaller scale collapses, such as individual star- and planetary creations. The interesting thing about this theory is in my opinion, that it makes Earth special, and could thus give us something to look for, when we are trying to find extra-terrestial life. /Henrik

Mentor
 Quote by Kometkaj This is also why stars rotate, and why they rotate in the same orientation of the galactic plane.
Not true. The plane of the ecliptic is inclined by about 63 degrees with respect to the galactic plane. Other star systems, who knows? It's pretty much random.

 Ever wondered why we can see so many exoplanetary transits, when the chance of seeing an eclipse should be small? It's because they all rotate in roughly the same orientation, for the same reasons as my above answer
As I mentioned above, the orientation of a star system is pretty much random. Suppose some other Sun-like star truly has an Earth-like planet in an Earth-like orbit. Since the orientation of one star system with respect to another is pretty much random, the chance of seeing that planet transit its star is only about 1/200. This low probability is the driving reason why the Kepler mission is looking at more than 100,000 stars. They want to ensure that a null result (failing to find any Earth-like planets in Earth-like orbits about Sun-like stars) is truly significant.

 Quote by D H .. the orientation of one star system with respect to another is pretty much random, the chance of seeing that planet transit its star is only about 1/200. This low probability is the driving reason why the Kepler mission is looking at more than 100,000 stars.
I must admit, that i have never seen any observational documentation for the claim
I have seen several sources that refer to a common inclination of the stellar orbits with respect to the galactic plane.
But whether it is a 20/80 relationship or a more significant number, I am not sure ..?

Anyway; If the orientation was completely random, a 1:200 succes-rate for planetary eclipses wouldn't even begin to describe it ;O)

/Henrik

 Quote by Kometkaj Hi Andre. Well actually it is a well established theory, that there was a type II supernova prior to the creation of the Sun. This is quite normal for starcreations; When heavy stars that usually only lives <1mio years are born, they explode and tricker a secondary wave of starformations in the surrounding interstellar clouds. The new hypothesis is that there also was a type Ia supernova somewhere around the creation. This theory is based on findings of some characteristic isotopes in meteorites. Se http://news.uchicago.edu/news.php?asset_id=2087 This will not effect smaller scale collapses, such as individual star- and planetary creations. The interesting thing about this theory is in my opinion, that it makes Earth special, and could thus give us something to look for, when we are trying to find extra-terrestial life. /Henrik
Another minor point of interest:
These precursor stars are absolutely necessary for the formation of the elements aside from hydrogen, helium, and lithium so without them, we would not exist.

Mentor
 Quote by Kometkaj I must admit, that i have never seen any observational documentation for the claim I have seen several sources that refer to a common inclination of the stellar orbits with respect to the galactic plane.
Yeah, and they're all wrong. The internet is full of nuts. You can find lots of sites that say that the ecliptic is only inclined at an angle of 5 degrees with respect to the ecliptic. I don't comprehend why people bother publishing information that is obviously false. That's the internet for you.

 Anyway; If the orientation was completely random, a 1:200 succes-rate for planetary eclipses wouldn't even begin to describe it ;O)
Have you done the math to prove this assertion? The Kepler mission people did. The 0.5% figure assumes zero correlation between the orbital plane of the exoplanets about some other star and the Earth's equatorial plane. That is one of the reasons it has to look at so many stars.

 Quote by Kometkaj Anyway; If the orientation was completely random, a 1:200 succes-rate for planetary eclipses wouldn't even begin to describe it ;O) /Henrik
It certainly would be difficult to look for Earth-like planets, but there are a handful of factors that influence the probability of whether a planet gets found via the transit method. Of primary importance is the semi-major axis of the orbit. Imagine the very hypothetical scenario of a (doomed) planet whose orbital radius is the same as the radius of the star it orbits, i.e. they are touching. In this case, the only point of view from which we would NOT see transits would be when looking at this system along the axis of revolution, a very low probability. If you look at the statistics of planets found via the transit method, you'll find the vast majority have a semi-major axis less than 1/20 of an AU. http://exoplanet.eu/catalog-transit.php

Obviously the larger a planet is, the more likely that some portion of it will eclipse the star if it happens to pass around front. Also the bigger the planet the more light of the star it will block, and thus the easier it will be to pick out of the statistical noise. A quick look at the stats reveals the peak of the radius distribution of planets found via transits is just over one Jupiter radius. This effect is less strong than for orbital radius.

One might also initially think one would be more likely to catch a planet passing in front of a star with a bigger radius - not only is the stellar disk bigger but the planet is less likely to be passing along the stellar limb and more towards the stellar center, where the star is brighter. But also the bigger the star the less of a percentage of its surface area is blocked by the planet, making it harder to spot in statistical noise. But then again the larger a star is the more likely it is more luminous (easily said for main sequence stars), and the more luminous (at a given distance) the cleaner the data. I haven't had time to peruse the properties of the stars around which planets have been seen transiting, but if I get a chance I will.

 Quote by D H Have you done the math to prove this assertion? The Kepler mission people did. The 0.5% figure assumes zero correlation between the orbital plane...
Yes; Assuming that we can track and monitor an entire orbital revolution, then the apparent diameter of the planet from the stars perspective, should cover 360'/200 = 1,8degrees, if the probability for an alignment that allows us to see an eclipse should be 1:200 or 0,5%

Most of the exoplanets found so far have been closely orbiting Jupiters, but still; Whitout having looked into it, 1,8degrees sounds bigger or closer than the average of the observed exoplanets i have read about.

There might be a bias in my perception, since i have not done any quantitative statstics, but... Am i mistaken here?

/Henirk
 Besides all the formation mechanisms, note that practically anything that flies will rotate (unless it's designed not to), it's just natural. Non-rotation is a special, rare case of of zero angular speed. A ball that is thrown or that has just collided will also rotate.
 Mentor Transits are not really eclipses. The planet is a tiny speck that blocks a tiny amount of the light coming from the star. For example, an Earth-like planet in an Earth-like orbit about a Sun-like star will reduce the amount of light coming from the star by 80 or so parts per million during a transit. To see a transit the transit has to occur. Imagine a planet that is orbiting a star such that the orbital angular momentum vector is pointing straight at our solar system. We will never that planet transit its star. The planet never passes in front of its parent star from our perspective. Now imagine that you are a god and you can grab onto the orbital angular momentum vector and tilt the planet's orbital plane downward. As you tilt the orbit more and more, the orbit will at some point clip the top of the star (from our perspective). Tilt it a just bit more and the planet will pass in front of the star, but only for a short while. Kepler can't see these short duration transits that only block a small amount of the star's output. The transit duration will increase as you tilt the orbital plane ever more until you reach the point where we are seeing the orbit edge-on. For Kepler to reliably see an Earth-like planet in an Earth-like orbit the orbit has to be close to edge-on.

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