what ship can attack planet??

Research Planitary invasion. once done you can build a transport. Be warned they are unarmed. so are easy picking's make sure you have cleared the area of enemy ships first.

You first need to research Planetary invasion, then the transport class will become available, or if you're like me you can choose to build your own better transport in the ship builder adding on a troop module.

Always send an escort with your troop transports.

Once you research the planetary invasion tech, a troops module will also become available. I recommend designing your own troop transport instead of using the core one. I very rarely build any of the core ships since you can do alot better designing ships to custom fit a situation.

Yeah, planetary Invasion is the one you are after.

I never escort my transports. I use recon ships to spot trouble but keep my transports as transport only fleets.

My warships tend to move 4 or 6 speed but my transports are designed as 9 or 11 speed, so mixing them up for protection would slow them down too much.

Their "defence" is to spot danger in time and run away really fast.

I do the same, make my transports really fast. I clear out the area of enemy ships, then send my transports, it usually takes only 1-3 turns to get there.

Oops, missed your last question. Soldering is the strength of your troops once you invade. So if your Soldiering ability is at 25% your troops will have an advantage of 25 on the planet invasion. You can also see enemy Soldiering ability with espionage and decide how many troops to send against their planets. Or use trial and error, its your empire!

Research things like Orbital Bombardment and Space marines to increase your Soldiering ability. Some of these techs also give you addition Invasion options like Tidal Disruption. These all have pro's and con's to them and are described when you see the options.