As we have just seen, planetlike bodies have been observed around bodies that have too little mass to be classified as stars, while planets have been observed orbiting the neutron star remnants of stars that were some 20 M_⊙ before they exploded. Indeed the more than 1800 exoplanets that have been observed in orbit around stars of masses ranging from 0.08 M_⊙ to over 20 M_⊙.

While some planets are orbiting stars similar to the Sun, many planets orbit burned-out remnants of stars and even orbit pairs or trios of stars. Most of the planets we have found are in orbit around stars located in the disk of the Milky Way Galaxy (which is where our solar system resides), but a few have been found in large clumps of stars outside our Galaxy’s disk. (The word galaxy when used alone is capitalized only when referring to our Milky Way.) These clumps, called globular clusters, contain very old stars that formed shortly after the universe began. Because it is in a globular cluster, the oldest-known planet dates back to the cluster’s formation 13 billion years ago.

The closest-known exoplanet is orbiting the star Alpha Centauri B, just 4.3 light-years away. The most distant exoplanet known is located nearly 30,000 ly away.