# Determining stable orbits for other stars

1. Dec 7, 2003

### aau

Howdy,
first post, good to be here. I'm sure it will be a learning experience.

My question concerns planetary formation and planet orbits for other stars, specifically the star Vega. I'm having a "disagreement" with someone who claims that Vega's mass, diameter, and density prevents the formation of planets close to the star. I'm having trouble getting this person to define "close", nevertheless I maintain we don't know nearly enough about the physics of planet formation to know this for sure. He also says replace the sun with Vega and Mercury and Venus would not have stable orbits. When I asked the reason he cited the universal law of gravitation and calculated the force of Vega's gravity at Mercury's distance. I find this unconvincing. He also stated force of gravity depends on diameter and density as well as mass, which I dispute. So can anyone clue me in as to what I'm missing here? Thanks!

2. Dec 7, 2003

Staff Emeritus
Maybe your friend is talking about the Roche Limit ? If a smaller body orbits too close to a massive body it will suffer tidal stresses and be broken up?

3. Dec 7, 2003

### aau

I could calculate the Roche Limit for Vega if I knew its density and the density of protoplanets in the circumstellar disk. I'm sure the density of Vega is less than the sun's density of 1.41, and I suspect the density of condensed material close to the star must be similar to iron or rocky asteroids, so around 3-5. Plugging in some guesstimates i get a Roche Limit of around 3 million kilometers, pretty close to the star. This would seem to strengthen my case. I suspect this distance is actually too close to the star's feeding zone, however, so no planet could form as it would be starved for material. Still this is a start. Thanks.

4. Dec 7, 2003

### Janus

Staff Emeritus
You really don't need to know Vega's density, only it's mass and use the formula:

$$R_{Roche}= 1.52 \sqrt[3]{\frac{M}{\rho}}$$

Where

$$M$$ is the mass of Vega
and

$$\rho$$ is the density of the protoplanet.

5. Dec 7, 2003

### Phobos

Staff Emeritus
Welcome to Physics Forums, aau!