# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: katieHaylor on 02/07/2018 15:38:31

Title: Could radar help determine the existence of "planet 9"?
Post by: katieHaylor on 02/07/2018 15:38:31

Can we use radar to find Planet 9?

What do you think?
Title: Re: Could radar help determine the existence of "planet 9"?
Post by: Janus on 02/07/2018 16:32:31
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.
Title: Re: Could radar help determine the existence of "planet 9"?
Post by: evan_au on 03/07/2018 12:24:01
Quote from: Carol
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.