According to planetary scientists in the latter half of the 20th century, planets around other stars should be...

• A. Common
• B. Rare
• C. Nonexistent
• D. Unknown

Answer: (A)

Planets form naturally from the disk of gas and dust that surrounds a young star. The process of dust grains sticking together, growing into planetesimals, and accreting into full-fledged planets is a robust outcome of standard planet formation theories. Since many stars form with such disks, the expectation in the latter half of the 20th century was that planets should be common around other stars, not rare or nonexistent. The solar system is viewed as a typical example, so the factors that produced planets here were thought to operate widely elsewhere. Observational limits at the time made them hard to detect, but scientifically it made the most sense to expect many planetary systems, rather than few.