<p>You need to consider which material the planets are formed of.</p>

<p>Terrestrial planets are formed from less material (way less), and with less momentum (since they were orbiting near from the Sun). Gaseous planets (which is another name for external planets, due to their material), also known as gaseous giants, are formed with a lot more of material with a lot more of angular momentum.</p>

<p>Angular momentum formula is $\vec L=\vec r\times m\vec v$.</p>

<p>Orbital speed formula is $\vec v=\vec\omega\times\vec r$.</p>

<p>Thus: $\vec L=\vec r\times m\vec\omega\times\vec r$</p>

<p>Since all vectors are perpendicular:</p>

<p>$L=r^2 m \omega$</p>

<p>Thus a higher mass $m$ makes a higher momentum $L$, and a higher distance to the Sun $r$ makes a quadratically higher momentum $L$.</p>

CC BY-SA 3.0
2014-02-04T20:53:40.633Z
What is the relation between less momentum and orbiting near the sun? My question is why terrestrial planets tend to rotate slower than their initial condition, and continue to slow down?
Yoda
CC BY-SA 3.0
2014-02-04T21:09:54.267Z
@Yoda Answer edited to answer your comment
Envite
CC BY-SA 3.0
2014-02-04T21:17:43.233Z
Thanks alot for your answer. But I still haven't understood why terrestrial planets slow down with time as they cool and freeze? Could tidal effects from other objects could alone be responsible for that?
Yoda
CC BY-SA 3.0
2014-02-04T22:51:05.177Z
Who said they slow down with time?
Envite
CC BY-SA 3.0
2014-02-04T23:38:48.230Z
Isn't our day slowing down with time?
Yoda
CC BY-SA 3.0
2014-02-04T23:41:05.370Z
Yes, but that has nothing to do with Earth's origin. It is caused by the friction between the tidal water bulge and the ocean's crust.
Envite
CC BY-SA 3.0
2014-02-04T23:44:48.483Z
So the planets today rotate faster than their original gases?
Yoda
CC BY-SA 3.0
2014-02-04T23:49:07.903Z
Original gases did not rotate. The original materials of the planets orbitated the Sun. The accretion of materials with different orbital angular momenta made the protoplanets start rotating.
Envite
CC BY-SA 3.0
2014-02-05T13:53:03.027Z
I remember reading that Earth's loss of rotation speed was due to tidal acceleration, @Envite. Quoting Wikipedia: 'The acceleration causes a gradual recession of [the Moon] in a prograde orbit away from [Earth], and a corresponding slowdown of [the planet's] rotation.'
OnoSendai
CC BY-SA 3.0
2014-02-05T14:37:49.697Z
@OnoSendai Yes, I said so in comment #6
Envite
CC BY-SA 3.0
2014-02-05T14:43:15.113Z
@Envite And I need coffee. ><
OnoSendai
CC BY-SA 3.0
2014-02-04T15:22:28.497Z
2014-02-04T21:09:27.750Z
2014-02-04T21:09:27.750Z
Envite
<p>We know that planets form as a result of gaseous material revolving around a center of mass that froze to form planets. However shouldn't the angular speed of that matter increase as they go closer to the center to maintain the angular momentum? </p>

<p>Why then the average rotational period of the external planets are faster than that for the terrestrial planets?</p>

CC BY-SA 3.0
2014-02-04T13:09:21.530Z
Wait, what? What does your last sentence have to do with the earlier ones?
Ilmari Karonen
CC BY-SA 3.0
2014-02-04T20:49:23.453Z
Typing error, I just edited - I meant external planets
Yoda
<orbit><planet><rotation><planetary-formation>
Rotational speeds of the material forming planets should increase, not decrease
Yoda
# Rotational speeds of the material forming planets should increase, not decrease

(Asked by: username_0 on 2014-02-04)

We know that planets form as a result of gaseous material revolving around a center of mass that froze to form planets. However shouldn't the angular speed of that matter increase as they go closer to the center to maintain the angular momentum?

Why then the average rotational period of the external planets are faster than that for the terrestrial planets?

**Tags:** orbit, planet, rotation, planetary-formation
> **Comments:**
> **username_1**: Wait, what? What does your last sentence have to do with the earlier ones?  
> **username_0**: Typing error, I just edited - I meant external planets  

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## 1 answers

### Answer by username_2 on 2014-02-04. Score: 1

You need to consider which material the planets are formed of.

Terrestrial planets are formed from less material (way less), and with less momentum (since they were orbiting near from the Sun). Gaseous planets (which is another name for external planets, due to their material), also known as gaseous giants, are formed with a lot more of material with a lot more of angular momentum.

Angular momentum formula is $\vec L=\vec r\times m\vec v$.

Orbital speed formula is $\vec v=\vec\omega\times\vec r$.

Thus: $\vec L=\vec r\times m\vec\omega\times\vec r$

Since all vectors are perpendicular:

$L=r^2 m \omega$

Thus a higher mass $m$ makes a higher momentum $L$, and a higher distance to the Sun $r$ makes a quadratically higher momentum $L$.

> **Comments:**
> **username_0**: What is the relation between less momentum and orbiting near the sun? My question is why terrestrial planets tend to rotate slower than their initial condition, and continue to slow down?  
> **username_2**: @Yoda Answer edited to answer your comment  
> **username_0**: Thanks alot for your answer. But I still haven't understood why terrestrial planets slow down with time as they cool and freeze? Could tidal effects from other objects could alone be responsible for that?  
> **username_2**: Who said they slow down with time?  
> **username_0**: Isn't our day slowing down with time?  
> **username_2**: Yes, but that has nothing to do with Earth's origin. It is caused by the friction between the tidal water bulge and the ocean's crust.  
> **username_0**: So the planets today rotate faster than their original gases?  
> **username_2**: Original gases did not rotate. The original materials of the planets orbitated the Sun. The accretion of materials with different orbital angular momenta made the protoplanets start rotating.  
> **username_3**: I remember reading that Earth's loss of rotation speed was due to tidal acceleration, @Envite. Quoting Wikipedia: 'The acceleration causes a gradual recession of [the Moon] in a prograde orbit away from [Earth], and a corresponding slowdown of [the planet's] rotation.'  
> **username_2**: @OnoSendai Yes, I said so in comment #6  
> **username_3**: @Envite And I need coffee. ><  

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