This is going to end badly, unless another, friendlier alien nation shows up nearby that Earth can become a protectorate of.
Ignore the aliens, we'll be fine
This is a game not a simulator.
It is a simulation within a game
Anyway, what are you going to replace to prevent starvation? Who needs science anyway.
Replace nothing, Stellaris starvation mechanics are nonexistent. The game assumes that even if you wipe out all food production buildings, you haven't really wiped out all food or food production - only reduced its availability. I've gone decades without food and starvation hasn't even triggered, so I'm guessing not colonizing or exploring at all may have bugged it out somewhat. At any rate, even with the happiness penalty, it is so small that utopian economics eliminates the penalty twice over. Road to Utopia cannot be compromised
2239:03:19: Alejandro Perez re-elected, maintaining all of Earth's planetary defence units at their current level. Reports come in from the observatories that the Mandasura are technologically inferior to the wonders of Earth, and thus are nothing to fear for the time soon. However, proposals are made to fortify the solar system. [Should I compromise the no-spaceport, no-spaceship, no militiarized space rule by fortifying planet Earth with orbital defences? Of note is that Earth cannot actually afford to defend itself without either ending social welfare or opening more boreholes, which I think may ruin the idealism a tad bit].
2241:06:17: Hypercomms forum research completed allowing users to one day theoretically upvote in 5 dimensions in a multi-tiered media campaign to heighten the planetary unity of all customers.
2244:06:12: Yesinia Qesim takes over from previous Earth governor as governor of Earth, continuing the infrastructure projects and production targets of expansion and refinement. When it comes to laboratories, MORE MORE MORE!
2244:12:14 Research into droids by CyberIA commences, CyberIA promising that the new droids would be able to much more convincingly interact with humankind in a non-disturbing way. At the very least, they would be able to walk up and down stairs good.
2245:01:04: At great expense, planetary capital project is begun, pooling all of Earth's resources together to form one true megacity capital of ALL planetkind. Owing to Governor Qesim's resourcefulness, the cost of the capital was much lower than expected, completed ahead of schedule with no compromise on quality - the capital even increasing the productivity of the borehole nation nearby, paying back its investment in minerals.
2248:05:25: The first droid units enter the fray, entering service in all good industries and boring ones too. Droids call for tech support upon malfunctioning, the droid unit on the other end of the datauplink is a droid, and the technician sent to perform rudimentary diagnostics is a droid.
2248:09:08: Research to increase the longevity of leaders is begun, Alejandro Perez arguing that having Earth's leaders live longer would increase the stability and progress of the planet, and would certainly be worth the cost to the state. It would practically pay itself back.
2249:06:16: Charles Paquett becomes President, replacing Alejandro Perez as President of Earth. President Paquett personally oversees much of the infrastructure development of Earth, being an architect at heart. Though some of his buildings displayed a rather dull sense of imagination, they nevertheless were functional and were completed with rapidity that was unusual even by the high standards of the United Nation.
Location of us, location of them and all the stars surrounding us. Hopefully nothing in Omicron Persei VIII.
Important crossroads, besides us are a race of genocidal plants. They clearly hate us and want Earth to burn. Their technology is inferior to us but their fleet is superior to our fleet by an unknown quantity vs our 0. I am guessing they are not actually one of the starting civs since they would have found us a lot sooner otherwise, probably being instead a civilization in the atomic age that just recently attained FTL travel. Furthermore, if they were strong enough to confidently wipe us out they would've done so already, so it seems we've got some more years still before they potentially feel confident in setting Earth on fire. If there is another alien space faction out there, they will undoubtedly view our neighbour more unfavourably than us, as our neighbour gets a gargantuan -1000 relations to everything that moves, so it is possible we'll survive long enough without any space defences to gain the protection of another alien species.
Alternatively we'll have to break the rules and have a military space defence budget. And that is bad.