Isn't there meant to be a popup when your planet is under attack because they are not showing up for me

well popups only appear when your energy or shield regen ticks occur, or when you use energy or gain/lose credits (like buying/selling minerals)... anything client-side that affects your stats bar... when the stats bar refreshes, outstanding popups appear
so if you're at full energy and full shields, you'll rarely see your popups

didn't have full energy or shields at the time

does the "______ is under attack" message appear in your newsfeed at all?

yeah

45 mins ago From: __________
_________ has unleashed a deadly virus on Taidhont - the outbreak will last for a full day, and prevent popluation growth

53 mins ago
Taidhont is under attack!

53 mins ago From: _______
A strong ion storm has been unleashed on Taidhont - reducing the planet's defenses by 15% for an hour

53 mins ago From: _______
An Orbital mass-disruption has been unleashed on Taidhont - reducing the planet's attack by 30% for 2 hours