[katieHaylor (OP)]

Carol asks:

Can we use radar to find Planet 9?

What do you think?

[Janus]

Radar requires you to generate a signal, bounce it off a distance object, then record the echo.   The further the object is away, the weaker the echo.  To give you an idea of how distance effects signal return,  Venus, at its closest, is 107 time further away than than the Moon, yet the return signal from Venus would be 1/5,000,000 as strong as one from the Moon.   Pluto, at its closest, is some 139 times further away than Venus, so you can imagine just how weak a return signal from it would be. 
On top of that, there are a lot of radio sources out in space producing their own signals and such a weak return signal would be lost in all the radio noise. 
Then there is the signal propagation delay.   It would take something like 5 1/3 hours for a radar pulse to reach Pluto and another 5 1/3 hrs for the echo to return.   Thus you would have to point your radar at Pluto, send a pulse, and then wait 10 2/3 hours for it to return.  And this is for a planet for which we already know where to point the radar and how long we should wait for the return signal.   If searching for an unknown planet past Pluto, you would have to aim at a particular point in the sky, sent your pulse, then wait long enough to make sure a return signal would've have had time to get back, and then aim at a different point and repeat.   A very slow process.   And since you won't know exactly when to expect the return signal, you wouldn't know if a blip was a new planet or just radio noise.   You'd have to repeat the process a few times while aimed at the same point to see if it were consistent, which would slow things down even further.   

All in all, using radar to find trans-Plutonian planets is just not practical.

[evan_au]

Can we use radar to find Planet 9?

Janus described a scenario where you have a single dish alternately transmitting and receiving in a particular direction.

Other radar systems have separate transmitters and receivers.
See: https://en.wikipedia.org/wiki/Bistatic_radar

And we have a similar system within the Solar System: The Sun is continually emitting high power wideband electromagnetic radiation in all directions, and we have detectors on Earth that can pick it up - we call them telescopes.

Because the Sun is an omnidirectional transmitter, we need no synchronisation between transmitter and receiver.

It is very hard to detect a faint, distant planet against a background of equally faint stars and small, faint asteroids.
So the outermost planets were discovered by their gravitational effects on closer planets, which gave astronomers clues about where to point their telescopes.

Modern telescopes can take a survey of the entire sky every few days, and modern computer algorithms can search for spots of light which move in a predictable way between the surveys - some of these systems are searching for near-Earth asteroids.

If we ran the same algorithms on surveys taken months apart, we may find a lot more distant objects in the Kuiper belt, especially if the algorithms looked for objects with a large parallax shift.
See: https://en.wikipedia.org/wiki/Near-Earth_object#Projects_to_minimize_the_threat