According to Prof. Daniel P. Whitmire from the University of Arkansas, the suspected Planet Nine triggers comet showers linked to mass extinctions on Earth at intervals of about 27 million years. Planet Nine is thought to be gaseous, similar to Uranus and Neptune; hypothetical lightning lights up the night side. Image credit: R. Hurt, IPAC / Caltech. Though astronomers have been looking for Planet Nine for over a century, the possibility that it’s...