▁galax y ▁is ▁a ▁factor ▁in ▁the ▁period icity ▁of ▁mass ▁ext in ctions ▁observed ▁in ▁the ▁Earth ' s ▁foss il ▁record . ▁One ▁hypothesis ▁supp oses ▁that ▁vertical ▁oscill ations ▁made ▁by ▁the ▁Sun ▁as ▁it ▁or bits ▁the ▁Gal actic ▁Centre ▁cause ▁it ▁to ▁regularly ▁pass ▁through ▁the ▁gal actic ▁plane . ▁When ▁the ▁Sun ' s ▁orbit ▁takes ▁it ▁outside ▁the ▁gal actic ▁disc , ▁the ▁influence ▁of ▁the ▁gal actic ▁t ide ▁is ▁we aker ; ▁as ▁it ▁re - ent ers ▁the ▁gal actic ▁disc , ▁as ▁it ▁does ▁every ▁ 2 0 – 2 5   mill ion ▁years , ▁it ▁comes ▁under ▁the ▁influence ▁of ▁the ▁far ▁stronger ▁" dis c ▁t ides ", ▁which , ▁according ▁to ▁mathematical ▁models , ▁increase ▁the ▁flux ▁of ▁O ort ▁cloud ▁com ets ▁into ▁the ▁Sol ar ▁System ▁by ▁a ▁factor ▁of ▁ 4 , ▁leading ▁to ▁a ▁massive ▁increase ▁in ▁the ▁lik elihood ▁of ▁a ▁dev ast ating ▁impact . ▁ ▁However , ▁others ▁argue ▁that ▁the ▁Sun ▁is ▁currently ▁close ▁to ▁the ▁gal actic ▁plane , ▁and ▁yet ▁the ▁last ▁great ▁ext inction ▁event ▁was ▁ 1 5 ▁million ▁years ▁ago . ▁Therefore , ▁the ▁Sun ' s ▁vertical ▁position ▁cannot ▁alone ▁explain ▁such ▁periodic ▁ext in ctions , ▁and ▁that ▁ext in ctions ▁instead ▁occur ▁when ▁the ▁Sun ▁passes ▁through ▁the ▁galax y ' s ▁spir al ▁arms . ▁Sp iral ▁arms ▁are ▁home ▁not ▁only ▁to ▁larger ▁numbers ▁of ▁mole cular ▁clouds , ▁whose