Re: Is the sun one of the foci of all the planets? Have astronomers

Yes, the sun is at one of the foci of the elliptical orbits of all the 
planets.  There is nothing at the other focus of each planetary orbit.  
Because the shapes and orientations of each planetary orbit are different,  
the second focus for each planetary orbit is at a different location in 
space.  The reason that the sun is at the focus of the orbits of all the 
planets is the fact that the orbit is controlled by the mutual 
gravitational attraction between the sun and each planet (This force is an 
inverse square law, meaning that the force drops off as the square of the 
distance between the sun and the planet).  It can be shown that the orbit 
that is produced by such a force has to be an ellipse (or a parabola or a 
hyperbola) with the sun being at one of the foci.

Reference: any introductory astronomy book, look under Kepler's laws of 
planetary motions. For example:
Foundations of Astronomy, 4th ed, by Michael Seeds, p71-73, 87-88