52 votes
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Why hasn't the "9th Planet" been detected already?

Brown and Batygin, the authors of the paper on the possible planet, have a webpage addressing this. A few reasons not already covered: It moves quite slowly - the authors estimate 0.2-0.6 arc ...
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  • 1,231
36 votes

Why hasn't the "9th Planet" been detected already?

This graph from XKCD says a lot about why that is the case The bottom line is, the 9th planet is too small to be detected through WISE, and too far/small to have been detected through visible ...
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31 votes
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9th planet location?

It's too dim to be seen during a normal survey during the majority of its orbit. Update: Scientists at the University of Bern have modeled a hypothetical 10 Earth mass planet in the proposed orbit ...
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28 votes
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How bright would the sun appear from the hypothetical Planet Nine proposed by Caltech?

Between $1/40,000$ and $1/8,000,000$ of the brightness as seen from Earth, depending on what the actual orbit would turn out to be, and where the planet is in its $15,000$ year orbit period. ...
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  • 1,231
23 votes

Why hasn't the "9th Planet" been detected already?

There are many normal methods that we use to detect exoplanets, but none of them work well in the case of the 9th planet. Here are some of the main ones. Radial velocity. The Sun is not moving ...
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17 votes
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Could the 9th planet be virtual?

The ninth planet can absolutely be a "virtual", in the way you describe it, meaning observed data indicating the gravitational influence by an object is not actually caused by such an object. A ...
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17 votes
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Can the paper narrowing Solar System's barycentre to within 100m help find Planet Nine?

Short answer Short answer is no, it can't prove or disprove the existence of Planet 9. The reason is because, even if there is a significant difference between the barycenter of the solar system with ...
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14 votes

Ninth planet - what else could it be?

What else could it be? Very simply, it could all be a coincidence... What they have done is noted that objects in the outer solar system have their orbits grouped in interesting ways. Someone ...
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  • 2,377
13 votes

Why hasn't the "9th Planet" been detected already?

Possible reasons that the planet has not been picked up previously: It's not there. Photographic proper motion surveys cover the whoke sky. To avoid these planet 9 would have to be fainter than about ...
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  • 114k
13 votes
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How long should it take for us to observationally determine if Caltech's Planet Nine is really there?

Those are model calculations, which hint to the existence of a possible body of about 10-times the mass of Earth. Calling this a discovery would clearly be premature. The confidence level is just a ...
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  • 11.2k
13 votes

Could gravity waves be used to confirm the existence of a ninth planet?

No, they can't. Gravity waves from a small, simple object moving slowly are very, very faint, to the point of being undetectable with current (or foreseeable) technology. The waves that have been ...
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  • 4,488
11 votes
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How was the hypothetical ninth planet kicked so far out of the Solar System?

It's hard to say much about this planet, given that most of its properties are unknown. It hasn't been directly observed; instead, its effects on Trans-Neptunian Objects (TNOs) have been simulated and ...
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  • 33.7k
11 votes
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Hypothetical trans-Neptunian planets orbiting the Sun

The recent possible discovery of Planet Nine by Batygin & Brown (2016) has caused quite a stir, in the astronomy community, Astronomy Stack Exchange, and the rest of the world. This is, of course, ...
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11 votes

How bright would the sun appear from the hypothetical Planet Nine proposed by Caltech?

If it exists, then the orbit has a perihelion of maybe 300 au and an aphelion of perhaps 2700 au. You can then just scale from the brightness $m=-26.7$ of the Sun at the Earth. It would be between 12....
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  • 114k
10 votes
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How would parallax affect an object at 200-1000 AU (for example, the 9th planet)?

The parallax is found from a triangle with the Earth's orbit at its base. The annual parallax here would then be between 1/200 and 1/1000 radians ($\sim$ 1000 to 200 arcseconds) which is indeed ...
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  • 114k
9 votes

What wavelength to best detect the "9th planet"?

The possible planet 9 is thought to be about 10 Earth masses and is unlikely to be a gas giant (it may be the core of an "interrupted" gas giant). As such, it will not be generating significant ...
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  • 114k
8 votes

9th planet location?

The position of the hypothetical object is not known with any certainty, so it's hard to know where to point your telescope. The paper proposes a wide range of orbital distances anywhere from 400 to ...
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8 votes

Is there a ninth planet?

According to Caltech there is possibly a planet 10 times the mass of earth in a 15-20 thousand year orbit at an odd angle. It hasn't been observed yet. There's a search for it ongoing.
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8 votes
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Is Planet Nine shepherding the Kuiper Belt?

Estimates of Planet 9's orbit currently suggest that it doesn't have any major effect on the Kuiper Belt, because its orbit is too far away and on a different plane than the Kuiper belt, and from the ...
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8 votes
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What type of planetary-mass object would Planet Nine be?

It is a planet - either a gas giant core, a mini-Neptune, or a super-Earth. First off, Mike Brown has stated outright “It is a planet—there’s virtually no doubt,” he said. “What we now call ...
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8 votes
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If "9 th planet" is ejected by Jupiter or Saturn, why does it have a very far perihelion?

This is certainly a problem for any hypothesis that form planet 9 by ejecting it from the inner solar system. The issue (for those wondering) is that if you simply scatter an object out of the inner ...
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  • 114k
8 votes
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Is Planet Nine observable in principle?

If they knew exactly where to point Mr. Hubble, then yes, it should be easily visible, though at that distance, it would still be blurry, not a clean image. The estimated apparent magnitude of Planet ...
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  • 22.9k
8 votes
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How does 2015 RR245 compare to the other KBOs predicting "Planet 9"?

It was the first thing I thought too, but I would say it won't count in the statistics. The earlier paper by Batygin & Brown was talking about bodies orbiting further out. 2015 RR245 orbits ...
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  • 2,377
7 votes
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What wavelength to best detect the "9th planet"?

Direct reflection of sunlight is the most likely scenario for a ninth planet discovery, however that does not hold if the object has a very low albedo. I assume you are interested in what wavelengths ...
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6 votes

What wavelength to best detect the "9th planet"?

There are two basic ways to detect such an object. First is to detect it through reflected sunlight. Second is from the heat that it produces. We already know that the reflected light of such an ...
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6 votes

Why hasn't the "9th Planet" been detected already?

Adding to the other excellent answers here, if there were an unobserved "Planet 9" the possibility of it being a difficult to observe primordial black hole has been raised. According to Phys....
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6 votes
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If thousands of exoplanets have been discovered, why haven't we discovered planet 9 yet?

There are two primary reasons Planet Nine (if it exists) is extremely hard to detect: it's very dim and its position is not well constrained. Brightness - We are able to see planets in our solar ...
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  • 3,046
6 votes
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Why does Wikipedia state that the Solar System has one unconfirmed planet?

That was a rather recent (16 January 2021) change to that wiki page made by a user named Mariofeds, who does not have a wikipedia user page and who has just one contribution to wikipedia. It may well ...
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5 votes
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If Planet Nine indeed exists, how large a impact event there should be to be seen from Earth?

We can compare the brightness of a comet hitting planet 9 with the 1994 impact of Shoemaker-Levy 9 / SL9 on Jupiter. It was seen by Hubble, and Galileo which was orbiting Jupiter at the time. View ...
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