It would be a bit nasty to make people not be able to attack for 6 hours just to send it out to find a planet.
But the reason for it is right, players are sending out their MS to "save it" from retaliation, which is unfair. To solve this, instead of making players have to not attack for 6 hours. You can.
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1. Have the sending out process do this. "The Journey to search for a planet was a Dangerous one, with enemy ships on every turn, also having to convert all power to engines and tractor beam to pull a planet across the stars. The Journey although perilous and exhausting has been fruitful, you have aquired a new planet for your Empire. Though your vessel has been completely ravished, Repairs are needed."
Your MS has taken substantial losses, maybe 100% i.e (all volleys/shields/fleets) are gone. Maybe 100% is too harsh, but you have got a planet off of it, so lets say 50%.
"The Journey to search for a planet was a Dangerous one, with enemy ships on every turn, after hours of searching, no planets have been found which are not gaurded too heavily by other Empires, Exhausted. Damaged. Crippled. The vessel returns to the Empire, Defeated. Embarassed. Battered."
Your MS has taken substantial losses, but you saved a lot of damage by not having to lower defences to tow the planet home.
So instead of 100% losses say 50%
or If thats too harsh. May well be.
Instead of 50% losses, say 25%.
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This way "Searching for a Planet" will now be a expensive excursion if you have a fully trained MS. Although this is regretable. It does mean less players will be searching, = More + Better planets to find.
Also it will get rid of the "saving stats" bug of sending out a MS, or atleast you will save less stats. May very well pay more to stay and fight than run away.
Comments?
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2. Or to save from this losses for no reason to search for a planet. Possibly simply have a button appear when a MS is out searching for a planet. Whereby someone can then enter 1-15 AT and attack you, but it will simply be a MS vs MS battle, The option only appears when someone is searching for a planet. The battle carries out like normal, with the exception of only MS's being involved. Battling in deepspace, their are no armies involved, and no resources to take.
As no armies are involved niether attack / nor defence take damage / need repairs. Just MS's.
Obviosuly, If you have no MS out, i.e yours has been sent out. You wont be able to attack someone via this method. It will say "
You have to have a MS to chase another MS through space." Or something

The theory being, a MS which has left its armies and worlds behind to go out into deep space to find planets, can still be chased and shot at.
You can still build your MS in the armoury like usual. Even though it is millions of light years away, we just invented some teleport device to give resources to the MS.
This way if your MS is being massed you can still unbank naqudah and repair it's shields. You can do what you'd normally do, the only thing you cant do is use your MS to attack back, it is busy, it is searching for a planet. It will take no detours to chase a MS back to their world. It will take the hits of the other MS fire back and keep going full speed!.
This suggestion has the ability to stop "saving stats" without making costs mandatory on planet searching. If nobody wants to mass your MS like usual it simply wont be massed.