Unstable Planetary Systems

A new study looking at the formation of planetary systems suggests habitable planets like Earth may be rare.

Planets form by coalescing within disks of gas and dust orbiting a young star. The study argues that if a second such disk interacts with the first during that formative phase, an unstable planetary system can result in which planets follow odd orbits, hot Jupiters dominate, and worlds the size of Earth are thrown into deep space. Since most stars form in multiple star systems, the chances of such disk interactions are fairly high, which might mean other Earths are rare.

Of course, rare is relative. The galaxy is so huge that even if Earths are rare in percentage terms, in absolute numbers there could still be thousands or millions of them.