It is unlikely a 150 will be after your planets unless it's good cause it's gonna cost them millions just to invade them. If they do kill you, it's most likely not continuous - xp gained from it ain't that great. Unless of cause you somehow angered them.
To defend against such high levels try - get a bigger legion to defend your planets, level up as quickly as you can while maintaining average planets to sustain your journey.
If a 150% level was used, it would be unfair if you are the "higher level" and a low level took your planet. Only to discover this loss when you level, it might make it impossible to take back your planet.
If you got a good suggestion (other than the 150% I think which cannot be implemented yet) on how to make lower ranks fight back those high levels, feel free to post them in Suggestions place. It will be a great help to those lower levels. Note that low levels taking higher levels planet causes them huge loses. So it works both ways now. Both sides can suffer.
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If not I don't intend to spend another penny on the game that I can lose basically everything without any ability to fight back.
Note that you buy Galaxy points in this game. This is used to change race, spin the galaxy wheel, and buying artifact. Most artifacts are temporary boosts, some permanent. People can take away your artifacts but not the permanent boosts. (extremely rare case)
So, it is very unlikely (almost impossible) to lose all your investment in this game when people take your planets. Unless of course you spend the galaxy points on planet artifacts. Ship boosts (permanent) CANNOT be stolen from you. So even if your planets is at stake, YOU cannot lose all your investment in this game that easily. If you are lower rank, do use Galaxy points to boost your rank level and ship stats. That way you will be able to defend your planets more effectively using less time than those who do not buy galaxy points (as you can reach higher rank faster)
Do check the "buy artifacts" button in trade and 'Redeem points' link beside the Galaxy points (under the 3 bars -energy/hull/shield) to see which boost seems worthy and whether it is worth to buy galaxy points.
EDIT: There is a limit to the number of planets you can own. A high rank won't just come and take away all your 5 planets or so in a day. + there is a timer to limit number of invades you can do in a day.