You are on the surface of an object, and you have a fairly clear view out into space. Unfortunately, you are also very squashed. The light you observe from distant objects is slightly blueshifted. The surface of the object is composed primarily of carbon and oxygen, and the distance to the horizon seems about the same as that on Earth. By observing the sky for a few weeks, you realize that there are several planets orbiting your object. Where are you?

You are on the surface of a white dwarf.

Explanation:

When a star has exhausted their nuclear fuel they send off much of their outermost parts to form a planetary nebula that consists of only the hot core which consist of electron-degenerate matter. The star is then known as a white dwarf or degenerate dwarf.

As stated, being squashed is due to the large mass of the white dwarf which is comparable to the mass of the sun and the observed distance to the horizon is due to the volume of the white dwarf being similar to the volume of Earth.

The carbon and oxygen composition is due to the fusion reaction of helium in the core at a previous the stage of the stars life , while the orbiting planets are from the star's solar system.

The blueshifted light observed is due to the increased gravitational attraction of objects towards the star.

3. dark matter

empty space could also be a simple vacuum