Distance of a planet to the star? QUESTION: 
Suppose we discover a planet orbiting a nearby star. 
The distance to the star is 3 pc. 
We observe the angular radius of the planet’s orbit to be 0.1 arc sec. 
How many AU from the star is the planet? 

**CONCERN: I am not sure what "The distance to the star is 3 pc" means. The earth's distance to the star? or the sun's distance to the star? or even better, the planet's distance to the star??!!

Any explanation of the question would be greatly appreciated!

My initial approach was some fixed position (say sun/earth's distance) to the star is 3pc, 
so I will have    0.1 arc sec = 3pc/distance, and solve for distance as the answer.
(I reckon this is a very simple question for astrophysics, but I need to get basics right)
Use this (http://www.ast.cam.ac.uk/~mjp/calc_parallax.html) but substitute the exoplanet for the Earth, you don't need to involve the Sun in your calculations, just assume the star is 3pc (parsecs) from Earth.




  From Figure 2, the distance between the Sun and the star is : 
  d = r / tan P 
  If P is 1 second of arc: 
  d = 150 000 000 / tan 1" = 30 million million km 
  This distance is called one parsec and is a basic unit for measuring astronomical distances. 
  Distance in parsecs = 1 / P in seconds of arc