What primarily causes the tail formation in comets?

The correct answer highlights that the tail formation in comets is primarily caused by the solar wind and radiation pressure. When a comet approaches the sun, its nucleus, which is composed of ice, gas, and dust, begins to heat up. This heating causes the ice to sublimate, turning directly from solid to gas, and releasing gas and dust into space.

As this gas escapes the comet, it interacts with the solar wind, a stream of charged particles released from the sun. The solar wind exerts pressure on the gas and dust released from the comet, causing it to form a tail that points away from the sun. The intensity of the tail formation is a result of both the solar wind and the radiation from the sun, which further pushes the particles away from the nucleus of the comet, streamlining the tail's formation.

In contrast, gravitational pull from nearby planets does not play a direct role in tail formation, nor does collision with celestial objects primarily influence how the tail develops. While the orbital speed of the comet affects its path around the sun, it is the interaction between the comet's released material and solar energy that influences the tail’s configuration and direction.