Science, Maths & Technology

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An introduction to exoplanets

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# Interactive orbits

You are going to return to the interactive application you have already used, which shows the mathematical model of the motion of a star and planet orbiting around their centre of mass. You are now going to look at it from a different perspective.

## Activity _unit4.4.2 Activity 4  Interactive orbits

Active content not displayed. This content requires JavaScript to be enabled.
Interactive feature not available in single page view (see it in standard view).

You have already seen a different view of this interactive application. When it appeared in Section 3.5 you saw a two-dimensional (2D) view looking down on the orbits from above. Now you are seeing a three-dimensional (3D) view, as if you were looking down on the orbit from an inclined view. Consequently the orbital path of the planet appears foreshortened, just as any circle does when you view it like this.

You can change your viewpoint of the orbital plane – the pale red grid in which the orbits lie – by clicking anywhere on it, holding down the mouse button and moving your mouse around (by ‘clicking and dragging’).

Set the mass of the star to 0.3 MSun, the mass of the planet to about 10 MJ and the distance of the planet from the centre of mass to about 4.5 AU.

1. Take the fourth slider, which controls simulated time, to the value 0, and then move it slowly to the value 1. As before, you can also use the arrow keys on the keyboard to amend the values. Watch the planet.

Describe the motion of the planet.

The planet starts off on the axis that is initially pointing towards the right-hand side of the application, and moves anticlockwise all the way around the foreshortened circle, returning to its original position on the axis when t = 1.

1. Take the fourth slider back to the value 0, and then move it slowly to the value 1 again. Watch the star this time.

Describe the motion of the star.