The Exoplanet theme does have arrows for next and previous image. See our demo to see it in action.
In your site’s source code there is this code data-rel="prettyPhoto[product-gallery]" added to the product gallery images, which is not present in the Exoplanet theme.
So, it looks like you have a plugin installed which is overriding the product gallery lightbox functionality.
Could you disable that plugin please and let us know if it fixes your issue.