Yes you are adventuring in space, and yes you can explore planets, but according to producer Michael Gamble, Mass Effect Andromeda is not an ‘Open World Game’. In an interview with OXM (Official Xbox Magazine), he discussed some of the aspects of the new Mass Effect game saying that it would be an ‘exploration-based game’.
“I definitely wouldn’t call Andromeda an open-world game,” Gamble said. “We like to use the term ‘exploration-based game’. You still have the concept of tight story deliverance and all the great things you come to expect with Mass Effect. The layer on top of that is a layer of exploration. Sometimes that happens in open spaces, but not always. You can cruise around some of these planets in the Nomad, but it’s not the traditional sandbox-type game.”
This seems to be in line with the previous Mass Effect games, where, instead of having a big connected open world, you explored bigger levels and hopped from one to the next. Most of the combat takes place in tighter, smaller areas. Mako will be returning in Mass Effect Andromeda, so you can expect the locations to be bigger than before, though I do hope it handles better than in the first Mass Effect.
Bioware has also said earlier that they are focusing on delivering better side quests, and is taking inspirations from how Witcher 3 handled that content.