That's before you get to the problem that space is ridiculously big, and sweeping it across the entirety of the surface of the imaginary sphere that might contain planet #9 would probably take millions of years (citation needed).
A very good laser has a beam divergence of tenths of milliradians, or 0.01 degrees. At the radius of planet 9, this is a very large beam, and we'd have a hard time putting out enough power to compete with the sun.
Furthermore, the sky has an area of 40,000 square degrees. Our little laser beam projects a circle with an area of 0.0000785 square degrees. You'd need to aim at 500 million spots to see the whole sky.
That's smaller than I expected. To convert that into human scale sqrt(500 million) = 22360, in millimeters that's around 22 meters (72 ft).
So if we had a square wall 22 meters by 22 meters we could paint a grid of 1 mm by 1 mm squares on it and end up with around 500 million squares.
Then we'd need to stand there in front of the wall with a laser pointer and shine the laser at each individual square and check the light bouncing back at us.
And there's uncertainty in whether your measurement is accurate.
And there's a background of other illuminated squares that can interfere with your readings.
And you're spinning while this is going on.
And the target square is continually moving. You might have just missed it.
1. Build a Dyson sphere around the sun.
2. Capture all the mass in the solar system to build a giant freaking laser.
3. Use the energy to from the Dyson sphere to power the laser.
4. Melt your entire civilization.
The scale isn't lost on me. I'm aware of how much ground would need to be covered when you get out that far, and I figured that might be a pretty damning factor. That said, you wouldn't need to sweep it across the entire surface, only enough of the surface to rule out any interfering debris between us and the object. Wouldn't take much. We would need VERY accurate models of the orbit based on effects on other bodies, but it's hard to totally rule it out in my head.
Edit - podcast link https://www.preposterousuniverse.com/podcast/2018/08/27/epis...
:)
In all seriousness, that would definitely kill any possibility. Sweeping an entire orbit at the resolution needed to hit something smaller than Uranus/Neptune would take eons.
I think someone should re-imagine all of Star Wars as hard Sci-fi set in the vicinity of a system like Alpha Centauri. That's still a vast setting.
Douglas Adams, The Hitchhiker's Guide to the Galaxy
The problem is that the power requirements go up sharply with distance. The amount that hits the object decreases with the square of the distance, as does the amount that’s returned. The end result is that your power requirements are proportional to the fourth power of the distance. Twice as far means you need sixteen times more power.
Now extrapolate that to something hundreds of times farther away than the closer planets....
As a quick back-of-the-envelope calculation: assuming you have a gigantic 100-meter diffraction-limited aperture, the smallest possible beam divergence for green light (550nm wavelength) is roughly 2 nanoradians. In the limit of large distances from both the earth and the sun, the area covered by that beam would already be receiving about 1GW of total energy from the sun. To outshine this you would need a laser of comparable power or more; the most powerful continuous laser I could find a reference to is only about 1MW, and was designed to destroy satellites and cruise missiles. [1]
I wouldn't rule out the possibility that you could detect a reflected laser beam, with sufficiently sensitive detectors, but it's hard to see what you would gain as opposed to just using sunlight.
Furthermore, lasers are by nature monochromatic, so you wouldn't really be able to do spectroscopy. At best, you could collect a few different points on the spectrum using lasers of different wavelengths.
Unless the pulses are long or frequent you get more light from an equally long exposure with sun light
Lasers are not the important part here- collimation is. The closer the illumination is to a perfect cylinder, the more visible it stays. Lasers often happen to be highly collimated, but it's not a defining feature. For instance laser diodes emit laser light in a cone 20+ degrees wide, and lenses are needed to produce a beam.
Bottom line: lasers aren't a panacea- generating a collimated beam isn't trivial, and just because we can make immensely powerful lasers that doesn't mean they are collimated.
> Unless the object was utterly black on its surface, I'd think we would be able to monitor wherever the beam traverses and look for tell-tale spectra coming back in our direction due to the laser being scattered by a solid object's surface.
There isn't really any tell-tale spectrum available- obviously you need something below X-rays, because those will be absorbed rather than reflected. In practice you also need something above millimeter waves, because those are also absorbed but more importantly the collimation of an EM source is inversely proportional to its wavelength. So millimeter waves are ~2000x less collimated than visible light, unless you use a massive antenna.
Just above millimeter waves are infrared waves, and just below X-rays are ultraviolet waves. Along that entire range, the sun is very active, so any light is going to be washed out against that background. You don't really get to rely on the spectra being unique.
> the necessary power needed for a laser strong enough to travel to such a distant point (it might not be physically possible to create a laser large enough to exceed the light we would already see reflected back from the already formidable output of the Sun)
The reflected light of the laser has to be brighter than the suns reflection over the entire area of the object you're trying to see. For an Earth-sized object at 1000 AU, that's 174 gigawatts. Not only do you have the impossible task of actually hitting the object, you need to have a hundred nuclear reactors powering a single source the entire time you do it.
https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment...
I'd also then guess that a radar setup instead of visible light might work better, but that's also got resolution issues and would need a gigantic and incredibly sensitive receiver to start picking things up.
Space based systems for doing all this would probably work better since they wouldn't have to contend with the rotation of the earth itself, but it'd still be hard.
The Sun on the other hand is irradiating all space at all times. Aside from eclipses, we know Planet X is currently reflecting sunlight. We just have to look at it, compare to background, look again, compare to background, and notice the movement.
Not forgotten, just glossed over. You'd need to have a receiver that is pointing at a given (admittedly massive) part of the sky over a given range of possible times that a return photon would take to traverse the gap back to earth and strike the receiver.
> We just have to look at it
You're right, but the problem is that something orbiting at that distance follows an EXTREMELY slow orbit, so it's difficult to use the method you are describing. It's quite effective for looking for asteroids/space junk, but at that distance it becomes pretty tough to spot.
Imagine looking up and seeing the feeble sunlight. Some of it gets reflected back and starts a long journey toward some telescope on Earth.
Now imagine seeing "extremely powerful laser" from earth. Some of it gets reflected ... (ditto).
For your method to work, the latter must be more powerful than the former.
In other words, if you point your laser at Mars, a Martian should suddenly find the Earth shining brighter than the sun.
I don't believe mankind possesses such power yet (thank god).
Again, I know I am oversimplifying a complex problem, and I could speculate on factors that might render this impractical, such as atmospheric effects or the necessary power needed for a laser strong enough to travel to such a distant point (it might not be physically possible to create a laser large enough to exceed the light we would already see reflected back from the already formidable output of the Sun), but I don't have the requisite knowledge of physics to really make solid assumptions about such things.