YA Sci-Fi about three friends leaving a planet and one is an alien. Read 2019.

[Max]

This book was read by me in 2019 and was about these three characters -- two boys and one girl -- who are on a planet that they need to leave soon, and their parents are researchers (at least, the boys' parents are). The other details I can remember are that the girl is secretly an alien sent to watch what the humans are doing. One of the boys and the alien develop feelings while they are on a spaceship, but that is all I remember. They could have been on a new planet as well, instead of them having to leave an old planet. That part is a little fuzzy in my mind.
Thank you!