What would happen if a large impactor hit our oceans? And more about the protoplanetary disk around HD 163296

What would happen if a large impactor hit our oceans? And more about the protoplanetary disk around HD 163296
Mars has a 3 times greater chance of being hit by a half mile-wide impactor & the new neighbor, Proxima b.
How do scientists determine the characteristics of an exoplanet and made its illustration if they never have visited it?