Jacob is the only loyalty mission remaining but i cant land on that planet even after sending many probes.It just show the option for scanning no "land" option available.Help
As far as I'm aware, you need to scan for and probe the surface anomaly (which represents the ship Jacob is looking for), before the game will allow you to land.
Sending many probes doesn't do anything if you don't hit the target. While you are scanning, you should get the words "Anomaly Detected" on your element scanner, and on the planet in the scanning circle you should see a white bar showing the direction the signal is coming from. Keep moving the scanner in that direction until you see a dot instead. Hit the dot with a probe. That'll reveal the landing site info, and it should let you land once you do this.
A number of side missions in the game require you to do this before they let you land on the planet in question.