In 2367, a Borg cube arrived and began assimilating the planet, killing millions in the process. However, the assimilation was halted when the Borg vessel was attacked and destroyed by Delcara and her "planet killer".
The planet was heavily damaged in the process, with about 95% of the surface destroyed. With the drastic change in mass, the climate changed drastically within a few months, and the planet became a "frozen snowball." (TNG novel: Vendetta)