ASTRONOMERS HAVE CONFIRMED the existence of more than 5,000 exoplanets to date. The vast majority of these — approximately 95% — orbit stars in an evolutionary stage akin to that of our Sun, a stage known as the main sequence. Main-sequence stars actively fuse hydrogen into helium in their dense, hot cores. Stars spend the vast majority of their nuclear-burning lives in this stage: In the case of a Sun-like star, the main sequence comprises 90% of the total stellar life cycle.
But what about the 5% of known exoplanets found around aging stars? These stars bear exotic names like subgiants, red giants and white dwarfs, and they have evolved to have vastly different sizes, temperatures and luminosities than they did when they were young. Any planets around these stars have therefore experienced a dramatic change in circumstances, which in many cases leads to their destruction.
With so few examples of these mature planetary systems in hand, it's difficult for us to know how late-stage planetary evolution unfolds. Nevertheless, the systems we do have serve as critical benchmarks in our quest to trace the complete planetary life cycle. These systems are teaching us that the evolutionary process is in some ways a symbiotic one: Not only do aging stars affect their planets, but the planets also affect their stars.
Red and puffy
A star's birth mass largely determines how its evolution proceeds; there isn't a ‘one size fits all’ pattern. But of stars confirmed to have at least one exoplanet, the average mass is essentially that of the Sun, and nearly
