Are planetary nebulae surrounded by planets?

Answer 1

No, the term planetary nebula is very misleading and it has nothing to do with planets.

A Planetary Nebula forms when stars up to 8 times the mass of our sun reach the end of their lifes (so our sun will form one). Stars heavier than 8 times our sun explode when they die.

After several billion (10 for our Sun) years fusing hydrogen, the star runs out of hydrogen, and there is no longer enough energy flowing out from the core to support the outer layers of the star. The core is thus compressed and heats up. Currently the sun's core has a temperature of approximately 15 million K, but when it runs out of hydrogen, the compression of the core will cause the temperature to rise to about 100 million K. When this happens, the outer layers of the star expand enormously because of the very high temperature of the core, and become much cooler. The core continues to contract and heat up, and when its temperature reaches 100 million K, helium nuclei begin to fuse into carbon and oxygen. The resumption of fusion reactions stops the core's contraction. Helium burning soon forms an inert core of carbon and oxygen, with both a helium-burning shell and a hydrogen-burning shell surrounding it. In this last stage the star will observationally be a red giant.

Helium fusion reactions are extremely temperature sensitive and this means that just a 2% rise in temperature more than doubles the reaction rate. This makes the star very unstable - a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space. The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000K, there are enough ultraviolet photons being emitted to ionize the ejected atmosphere, making it glow. This glowing cloud is a planetary nebula. [Note there may have been planets orbiting the star, the innermost of these will have been destroyed in the reg giant phase but outer planets may still continue to orbit the dying star but these have nothing to do with the planetary nebular].