Twenty-three years after Purple Rain catapulted Prince to superstardom, the purple one continues to engage his listening audience on his own terms. Prince has recorded such an overwhelming amount of music over the years that it has been near impossible for the casual fan, never mind his record company, to keep up. But with Planet Earth, it’s as if Prince decided to offer up a Cliff’s Notes version of his career, circa 2007. The ten tracks he chose for the final album exhibit an intense precision and highlight all the elements — expert funk, aggressive guitar, cheeky, playful vocals — that made him a star in the first place.  “Guitar” is an aptly titled rocker. The title track critiques modern times. “Somewhere Here on Earth” hints at jazz and smooth R&B. “Mr. Goodnight” nods towards hip-hop. “Chelsea Rodgers” goes for funk. “Lion of Judah” commands hard rock.  “All the Midnights in the World” sits down at the piano. That might sound like a variety show, but with Prince writing, playing and producing, the final effect is a smooth, consistent blend where the author’s voice is never put to question.