Tonight's Sky
Sun
Sun
Moon
Moon
Mercury
Mercury
Venus
Venus
Mars
Mars
Jupiter
Jupiter
Saturn
Saturn

Tonight's Sky — Change location

OR

Searching...

Tonight's Sky — Select location

Tonight's Sky — Enter coordinates

° '
° '

Why are all planetary systems and most galaxies disk-shaped?

Jaison Solomon, Trivandrum, India
NGC3982
Face-on spiral galaxy NGC 3982. NASA/ESA/the Hubble Heritage Team (STScI/AURA)
The shapes of large astronomical objects reflect three natural laws. First, the force of gravity from a faraway object decreases with the square of the distance — i.e., 2 times as far, so (½)2 is ¼ the force — but does not depend on direction. Second, nature keeps things spinning. Spin can come from collisions or when galaxies are forming. Once an object gains some rotation, it retains this “angular momentum.” The only way to get rid of this momentum is to give the spin to some other object. Third, energy is conserved. The gravitational force meets this requirement because orbiting objects move faster near a mass and slower as they are farther out — the energy of motion converts into energy fighting against gravity.

The force of gravity makes some objects, such as the Sun, spherical. Every point on the surface of a sphere is the same distance from the center and thus at constant gravitational force.
 
Suppose a star rotates rapidly. The radial force of gravity isn’t the only influence pulling toward the center of the system; spin also affects it. Along the equator, rapid rotation reduces the pull of gravity and the object is more extended — it becomes a flattened sphere. This effect shapes Jupiter; its diameter at the equator is larger than between its poles.

Finally, consider a collapsing spinning gas cloud. Gas will need to keep its angular momentum constant, so the rate of rotation increases as it collapses under gravity. When the spin becomes close to the speed of a circular orbit, increases in rotational speed force the gas to move outward and get stuck in circular orbits. The originally somewhat-spherical gas cloud collapses with all of its material rotating in the same direction and in the same plane; spin plus gravity equals a rotating disk with a vertical thickness much less than its radius.
 

This process is widespread. Clouds that form stars make spinning circumstellar disks where planets are built; gas funneling into the centers of galaxies produces accretion disks around supermassive black holes; and the gas that makes galaxies gives us the thin, beautiful disks of spirals. — John S. (Jay) III Gallagher, University of Wisconsin-Madison
0

JOIN THE DISCUSSION

Read and share your comments on this article
Comment on this article
Want to leave a comment?
Only registered members of Astronomy.com are allowed to comment on this article. Registration is FREE and only takes a couple minutes.

Login or Register now.
0 comments
ADVERTISEMENT

FREE EMAIL NEWSLETTER

Receive news, sky-event information, observing tips, and more from Astronomy's weekly email newsletter. View our Privacy Policy.

ADVERTISEMENT
ADVERTISEMENT
Apollo_RightRail

Click here to receive a FREE e-Guide exclusively from Astronomy magazine.

Find us on Facebook