We have hardened cities, but that does need a buff.
aye, this is the most likely method. The big issue is the population damage, not the planet damage(key worlds can be protected from that by aux goverment).
Though having a single technology that applies to all worlds mitigate this damage much would be too strong(at least at the sort of % necessary to keep novalith from wrecking economies). There needs to be per-planet investment as is the case with shield generators.
Perhaps something like:
Hardened Cities: Each level of the Emergency Facilities planetary upgrade, in addition to increasing planet health also reduces population loss due to bombardment(including novalith cannon) by 8/16%% to a maximum of 24/48% for worlds at emergency facilities upgrade level 3.
or perhaps:
Hardened Cities: Adds an additional level to the emergency facilities upgrade for all planets. Rather then increasing planet HP further, this final upgrade rather then further increasing planet health instead reduces all population damage suffered by the planet by 65%
The point is there should be some per-planet cost, but the mitigation should be considerable. I chose to link it to the emergency facilities planetary upgrade because it was the best way I could think of to add a per-planet cost without adding a new structure.
I suppose something could also be done on the starbase front.
Regardless at present the current population damage of Novalith exceeds the population regenerated at any 3 planets combined during it's cooldown. To keep it from wreckign the advent's economy we're going to need a population loss mitigation that would be too strong to have applied to all planets with a single research(frankly a 15-20% reduction won't cut it IMO as a single novalith would still be able to keep 2 key-economic worlds bottomed out at all times).