In an attempt to learn about past climates, scientists have drilled and extracted cylindrical cores from the Greenland and Antarctic ice sheets. Because of the great thickness of these sheets, the cores can have combined lengths of thousands of meters.

In 1930, astronomer Clyde Tombaugh discovered a faint point of light orbiting the sun beyond Neptune. The new world “Pluto” was considered the ninth planet for 76 years, but in 2006 the International Astronomical Union voted to reclassify Pluto as a “dwarf planet.” What prompted this change in nomenclature?

The Rise and Fall of the Ninth Planet