mass extinction fossil record gradual decline last fossil

Not every planet orbits a star. Some worlds travel through the galaxy on their own, possibly after being ejected from planetary systems or forming independently. Astronomers can sometimes detect these rogue planets when their gravity briefly magnifies light from a background star. Their total number remains uncertain, but they may be extremely common throughout the Milky Way.

For more than a century, the kilogram was defined by a particular metal cylinder kept in France. Since 2019, the SI system has instead defined it through a fixed value of the Planck constant. Other base units are also linked to fundamental constants or physical phenomena. Measurement standards no longer depend on preserving a single object that might change over time.