A study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to how hospitable a star system could be. Researchers looked at the X-ray brightness from 24 stars with masses similar to the Sun or less, each at least one billion years old. The artist's illustration depicts one of these older Sun-like stars with a planet in orbit around it, which researchers found to be relatively calm compared to younger stars. The large dark area is a "coronal hole", a phenomenon associated with low levels of magnetic activity.