She's very interested in outerspace/planets/stars and asked when the sun would die. This has also added to her fears and she asks when people will learn space travel so that we can move to another planet if the sun dies.
This exact thing has been an ongoing concern of my DD's probably since about age 5. It still comes up. I have had to Google the estimated "time of death" of the sun a few times to show her that scientists agree that this is several billion years away. She still worries. She has long had other existential concerns about the universe as well.
So, your DD is not the only one. DS5 has other existential concerns as well. For both, I find that "being the adult" and saying that I promise that will never happen and being very firm that they do not need to worry about XYZ (as opposed to at other times when we are more curosity-focused--"what do you think?") is helpful).
