The dense cores of past planets might survive a star’s expansion period into its white dwarf phase. (Credit: CfA/Mark A. Garlick)

In five or six billion years, our sun will expand into a red giant star hundreds of times larger than it is now. It will envelop Mercury and Venus, and possibly Earth as well, and then slowly puff away its outer layers. The hot, dense core left behind is called a white dwarf. And it will glow for billions of years.

It’s an open question what happens to the pla