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https://www.newscientist.com/article/2100700-mystery-object-in-weird-orbit-beyond-neptune-cannot-be-explained/

What could be potential reasons for a) the object only now being discovered (200km in size expected) b) it spinning opposite ?

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  • $\begingroup$ New Scientists cites Arxiv, and there's Wikipedia. Was first seen and identified as "an object" in 2011, then "re-discovered" in 2016 with a better understanding of its orbital elements. I don't know the answer to your question but those may be helpful! $\endgroup$ – uhoh Aug 12 '16 at 6:30
  • $\begingroup$ With regard to your second question, it's better to use the term "orbiting" (or "revolving") rather than "spinning". Spinning typically refers to an object's rotation about an axis passing through its center of mass. $\endgroup$ – David Hammen Aug 12 '16 at 12:20
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    $\begingroup$ To answer that question, it's currently a mystery. Mysteries in science are a good thing. Those are the things that press science forward. (“The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka’ but ‘That's funny…’”) While scientists will eventually solve this mystery, you are asking us to conjecture. Asking for conjectures typically does not make for a good question for this site. $\endgroup$ – David Hammen Aug 12 '16 at 12:25
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The combination of size and distance is the reason it wasn't discovered earlier.

This XKCD graphic shows this pretty well:

Graphic of planet size vs. distance

Anything below and to the right of the areas "stuff we can see through telescopes" and "planets ruled out by the WISE survey" is too small to see from Earth. Improvements in telescopes and data processing are gradually moving those boundaries to the right.

To find a planet, you have to take photos of the sky at intervals and compare them. The stars will be in the same position in both photos, the planets will have moved. Doing such a survey of the whole sky is difficult. Doing it manually is a monumental task (it took 4 years to discover Pluto), these days computers can help with that.

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  • $\begingroup$ It took 4 years to discover Pluto, and even then its discovery was very lucky. $\endgroup$ – gerrit Aug 12 '16 at 10:04

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