Stock footage used in this tutorial is from the free Nebula pack on PremiumBeat. Their new Space Kit can be found here. However, I am fairly certain this stuff can be made using the Particle Designer, or easily enough irl using liquid in an aquarium or onto a sheet of glass....
Making Planets
2 Tracks. Track 1 is the background, Track 2 is the planet.
Fish Eye makes the planet look spherical. Choose your size to taste (I used 70), set Degree to 200. Use Mask Designer to mask the effect area into a circle.
Focus adds the sphere’s shadow. Break the Width and Height lock, and set to within 10 points of each other (41 and 50 in this tutorial example). This gets rid of the cat’s eye shape, rounding things out. Gradient Depth is 3, and Alpha Degree is 200-255. You want the outer ring shadow to be very dark for realism.
Use PiP to key-frame position, scale, and even rotation as desired.
Lens Flare is optional, but can add more depth to the planet when added to its layer (sphere highlights), or act as a mini sun. Flare 4 is very sun-like. Key-frame position if you want I nice “sunrise” or “sunset”.