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Astronomers are inferring the existence of a "Planet Ten" (or actually the true "Planet Nine"?), a Mars-sized body in the Kuiper Belt, several times closer to the Sun than where the hypothetical Neptune-like Planet Nine is expected to be:
An unknown, unseen "planetary mass object" may lurk in the outer reaches of our solar system, according to new research on the orbits of minor planets to be published in the Astronomical Journal. This object would be different from — and much closer than — the so-called Planet Nine, a planet whose existence yet awaits confirmation.
In the paper, Kat Volk and Renu Malhotra of the University of Arizona's Lunar and Planetary Laboratory, or LPL, present compelling evidence of a yet-to-be-discovered planetary body with a mass somewhere between that of Mars and Earth. The mysterious mass, the authors show, has given away its presence — for now — only by controlling the orbital planes of a population of space rocks known as Kuiper Belt objects, or KBOs, in the icy outskirts of the solar system.
[...] According to the calculations, an object with the mass of Mars orbiting roughly 60 AU from the sun on an orbit tilted by about eight degrees (to the average plane of the known planets) has sufficient gravitational influence to warp the orbital plane of the distant KBOs within about 10 AU to either side.
Also at New Scientist.
The curiously warped mean plane of the Kuiper belt
We estimate this deviation from the expected mean plane to be statistically significant at the ∼97−99% confidence level. We discuss several possible explanations for this deviation, including the possibility that a relatively close-in (a≲100~au), unseen small planetary-mass object in the outer solar system is responsible for the warping.
(Score: 2) by Grishnakh on Monday June 26 2017, @03:07PM
A couple of reasons (IANAastronomer BTW):
1) Effectiveness at this distance.
2) Light.
3) Cost.
Launching some satellites in Earth orbit is only going to do so much. We already have a couple of space-based telescopes: the Hubble which has been operating for some time now, and the James Webb which is launching next year hopefully. But even with things like this, we can only see small bodies in the outer Solar system so well. Here's an article [planetary.org] that discusses this very issue: why can Hubble get such great pics of distant galaxies but Pluto and its moons just look like some faint blobs of light? Simple: galaxies are unfathomably enormous, Pluto and its moons are tiny (even compared to Earth and our moon). Remember, it took 10 years for New Horizons to get to Pluto from here, and it was the fastest object ever launched from the Earth. And Pluto isn't even that close to the edge of the Solar system; the Kuiper Belt and all the objects there are beyond it, which is why we've only recently found all the other dwarf planets out there (Eris, Makemake, Huamea, etc.).
The other big problem is light. We can see exoplanets because they're generally large, and they pass in front of their host star, so we see the difference in light when they do this. We can see other planets in our own system because they're reflecting light from the Sun back to us. But anything past Pluto is so far away from the Sun that it's not receiving much light in the first place to reflect back.
So to see any more planets way out past Pluto, we'll need either a more powerful telescope, or more likely, we'll need to put telescopes in orbit way out where Pluto is.
And of course, that brings us to cost. These space telescopes cost a fortune due to the enormous mirrors and precision optics needed, and the large size of the telescope. They generally also have problems, and need servicing: Hubble barely even worked when it first launched! It took multiple Space Shuttle missions to fix the problems it had (and later to upgrade it with newer technology for better images). We can service stuff in Earth orbit; we can't service anything out where Pluto is. The probes we launch, like New Horizons, don't have such large and sophisticated imaging equipment, more like the equivalent of a DSLR, since the idea there is to fly the probe relatively close to the planet/moon it's observing. So bottom line: it costs a lot of money to build and launch space telescopes, and it's just not very feasible to send them out to where Pluto is just to get a better view of some small planets/dwarf planets out there. We have more interest in other things anyway, like exoplanets within the habitable zone of their host star, galaxies, nebulas, etc., not some little cold, dead rock/ice balls way out in the boonies of our own system. They're mildly interesting, but not so much compared to other planets in other systems which might bear life.