This isn't just about when a dying star gobbles up its life-supporting planets, as will happen with our own Sun five billion years from now. A star's internal chemistry can doom a planet's life long before the star itself dies.
The search for potentially habitable planets means a lot of discussion of what's sometimes referred to as the Goldilocks Zone, the relatively thin band in a solar system in which planets are neither too hot nor too cold to support life. But a star's habitable zone won't stay in the same place forever — it can contract, expand, or otherwise move about in response to the star's own continuing evolution.
We can see this on display if we return to the Sun's final demise and expansion into a red giant billions of years hence. This process will swallow up Mercury, Venus, Earth, and possibly Mars, which pretty obviously means that, in that phase of the sun's life, our current position in the solar system won't be inside the Sun's habitable zone. Indeed, we'll actually be inside the Sun, which is generally considered the least habitable part of any solar system.
But more subtle differences in a star's natural evolution can have huge effects on a planet's chances of habitability. As ScienceNOW reports, researchers at the have identified the elements carbon, sodium, magnesium, and silicon as particularly important for a given solar system's habitability. The greater abundance there are of these four elements in a star, the longer it will take for the star to evolve and for the location of the habitable zone to change significantly.
As it so often is when it comes to life, oxygen is crucially important — if there had been less oxygen in the Sun's chemical makeup, Earth likely would have been pushed out of the Sun's habitable zone about a billion years ago. Considering the first multicellular organisms only arose 1.2 billion years ago, and we don't even get to protozoa until 750 million years ago, such a move would have likely destroyed any chance of complex life taking hold on Earth.
Check out the original paper at arXiv. Image via NASA.