Wheee good times! Now you do some combination of:
1) Build some Earth-defending PDCs to hold them off while you build up a fleet
2) Build some slow, armored, powerful ships/bases to park on and defend the Sol side of the jump point
- you really at least want to park something there to see if they're coming through
3) Build a bunch of FAC/ftr to defend earth
4) Build up some fast and/or stealthy, high-sensor scouts to learn the characteristics of the enemy
- be careful about leading them to the jump point to Sol, anything going in is expendable
5) Build a fleet you think can take them and go kick some alien ass
- since you know they have a bunch of 800 tonners, make sure you have sensors/fire controls to target this size ship...
6) Keep exploring in other directions if you can
Depends how much you want to RP "what would Earth Command do if this actually happened" and where exactly you think your tech is compared to these guys. This is the fun part of the game - enjoy
Don't be in a huge rush - it may be 10-20+ years before you can go deal with these guys, maybe longer. My usual "RP solution" is to use the bunch of size-24 old ICBM PDCs as the nucleus of a missile defense of Earth, with newly designed MIRV'd size-24 missiles + crash-priority built missile defense PDCs and FACS (I usually can't afford fighter tech this early 'cause of the huge research cost) and other small cheap ships.