Avian, what's up with this new Earthlike planet?

[Avian(Admin)]

Originally posted by Bob Lentil
Yes, that's why it has to go so fast around its orbit. Assuming it keeps the same mass but slows down, its orbit would decay and it would spiral into the sun. Likewise, if it maintains the same speed and gets heavier, the gravitational force between the sun and the planet increase, and you get the same result.

Technically, there is no such thing as centrifigal force. The sun exerts a centripital force on the planet due to gravity which causes the planet to curve around the sun in its orbit. Otherwise, the planet would just travel in a straight line.

Heh - it's easy to get confused about these types of things. There are many common misconceptions about astrophysics. Maybe I can help clear up some things.

The masses of the planets do not significantly determine how fast they go around the sun. If Jupiter were in the exact place where Mercury is, it would take the same time to go aorund the sun as Mercury - about 88 earth days. If Mercury, a tiny pebble compared to Jupiter, were placed here Juptier is, it would take the same time as Jupiter to go around the sun - about 12 earth years. The gravitational force exterted by the mass of the planet on the sun has little effect on their overall motion on the sun. The sun's massive gravity when compared to the planets is the overwhelming dominating factor here.

The acceleration that an object experiences due to gravity (or any other force) is indepenant of mass of the object. Drop a feather and bowling ball at the same time in a vacuum, and they hit the ground at the same time (neglecting the gravity generated by the items, which is insignificant when compared to the gravity of the earth). The same thing when you talk about the planets and how fast they go around the sun. The planet's mass really doesn't matter - it's their distance from the sun. Kepler's third law states that the time the planet goes around the sun is proportional to its distance from the sun (to elaborate, it states that Time^2=Distance^3). Notice that the mass of the planet does not enter into the law.

By the way, if the sun's mass were to increase, that would change the gravitational acceleration the planets experience, and therefore their periods would shorten from their current times. But Kepler's law would still hold.

The gravity of the planets have a much dramatic effect on each other and other solar system objects over long periods of time. Jupiter, for instance, is known to have a great effect on comets, changing their orbits as they orbit around the sun, and sometimes causing them to crash in thesun, or even giant planet itself. This is what happened in 1994 with comet Shoemaker-Levy 9 impacted Jupiter.

Hope that helps clear things up!