There seems to be some confusion out there about what the sun should be like in a billion years compared to how it is in the Ninth World. This is the passage that's causing the confusion:
The people of the Ninth World don’t realize it, but at its current age, the sun’s luminosity should have increased to a point where life on Earth (as we know it) is impossible. And yet it continues. Something happened millions of years in the past to prevent life from disappearing. Most planets in the solar system remain, although their orbits have altered somewhat, but the planet we call Mercury is long gone. (Ninth Worlders don’t know it ever existed, so they don’t wonder why it’s absent.) [Numenera Corebook, page 130]
A lot of people seem to think this is referring to the sun's future evolution into a red giant and expansion to consume Mercury. However, this is not the case.
The sun will evolve into a red giant when it has consumed all its hydrogen and begins fusing helium.
This will not happen for 5 billion years, far far beyond the time of the Ninth World. At this point the sun's outer layers will begin expanding, consuming Mercury, Venus and even Earth.
Earth will be consumed in approximately 7.6 billion years.
Here is a nice diagram of the life cycle of our sun.
What the corebook is referring to is that in a billion years the sun should have grown hot and bright enough that it would have burned the atmosphere off of Earth, making it uninhabitable to human life. This is what one of the prior 8 great civilizations managed to prevent or forestall.
To learn more about this, check out
episode 8 of Cosmos: A Spacetime Odyssey - Sisters of the Sun. Or better yet, watch the whole season. You'll be glad you did.
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