It can be unlocked after completing Memo Task, and accessed either by interacting with the Starsea Voyager Vehicle, or by talking to Elsasani.
Overview
To head-out on a Starsea Voyage, Planet Tenders must have at least Lumia Battery ×1, which can be purchased from the Loomix Mart.
Planet Tenders can then select up to 2 Neighbors to assist them, with each Neighbor differing on which tasks they excel at, are good at, or are not strong at.
Upon setting out on the first Starsea Voyage, Planet Tenders will start in the Shallows of whichever Galaxy they selected (currently Hexia or Taffeh), with deeper Uncharted Bays becoming available upon unlocking the various Star Gates with Islet LucadropsIslet Lucadrops.
Other Galaxy's Starseas can be explored when departing from that particular Galaxy.
Islets are the main exploration content within the Starsea. They are instanced zones, typically focussing on a specific Creature type, Item, or will feature a puzzle that can be completed for extra rewards.
Rarely, Planet Tenders may also come across an "Encounter", which can either be a new Neighbor to be invited, or an already-invited Neighbor, who will offer a reward once spoken to.
Each Islet will initially be Unknown, notated with a " ? " icon. Upon exploring an Islet, its core type will be revealed, and can be returned to at a later date.
Wandering Star. These do not currently have a use outside of collection.
Lumia Battery was replaced with 
Star Anchor.