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In the comments to this question, there was considerable doubt placed as to the subject of if the so called "9th planet" really exists. That wasn't really the intent of the previous question, so I ask this. Why hasn't this "9th planet" been detected before now, if it even exists?

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naively, because it is very small and far away. – njzk2 Jan 25 at 4:37
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Last year, i started cataloging all the pebbles in my garden. I've found a nice shiny one by the apple tree roots. My question is - why wasn't this pebble found before now? – Max Williams Jan 25 at 9:15
up vote 28 down vote accepted

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 seconds per hour - so standard surveys may not notice the movement and fail to recognize it as a solar system object.

Eris, which is the most distant confirmed object still known in the solar system, moves at a speed of 1.5 arcseconds per hour, which is so slow that it was missed the first time around. Most surveys of the outer solar system would not be able to find Planet Nine, even if it were quite bright, as they would just think it is a stationary star.

  • If the planet is near aphelion, it might be an order of magnitude further away than any major or minor planet we've found so far (excluding exoplanets, which are found by methods that don't apply in this case). The authors suggest an aphelion between 500 and 1200 AU. For comparison, Pluto is at 30-50 AU, while Eris at around 100 AU wasn't discovered until 2005. The potential 9th planet would be far larger than Eris, but is also likely to be much further away, and thereby fainter.

  • The WISE survey eliminated Saturn-sized planets within 10,000 AU, and Jupiter-sized planets within 26,000 AU. But the potential 9th planet is far smaller than those. WISE has also done a more sensitive search, which would pick up Neptune-sized objects, but that search has so far covered only a limited part of the sky.

  • The planet will be far harder to spot if it has the Milky Way in the background - there are too many stars potentially drowning out a faint object.

Here's the authors' summary:

Estimated 9th planet orbit

Estimated orbit for the putative 9th planet. The horizontal axis is the right ascension. The colored segments are regions where it should have been found by existing surveys.
Illustration by Brown and Batygin, assuming fair use applies.

The biggest unexplored territory is where, statistically, it is most likely to be: near aphelion. Sadly, aphelion is also very close to the Milky Way galaxy. Ugh.

So where is it? Probably distant. 500 AU+. Probably fainter than 22nd magnitude. Very possibly in the middle of the Milky Way galaxy.

Now go find planet nine.

More details on the authors' webpage: http://www.findplanetnine.com/p/blog-page.html


Finally, the gravitational dominance of the Sun reaches halfway to the nearest star. There's still plenty of unexplored territory for planets smaller than Saturn to hide in.

Solar system distances out to the Oort cloud and beyond
Note the log axis. We have a good map for the inner 50 AU, and are starting to find objects around 100 AU, but solar system objects might exist all the way to the outer edges of the Oort cloud.
Illustration from wikipedia.

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The current dominance of the Sun might extend half-way to the nearest star now, but statistically we come close to other stars frequently, maybe a light year or closer, which could easily mean that any far out object would no longer be orbiting, depending on how close of a pass. – PearsonArtPhoto Jan 23 at 23:13
    
Good point. I'm assuming that if the Oort cloud can be stable, it should be possible for larger bodies to remain stable as well. But I'm a hobbyist, not a professional astronomer, so I'll leave it for someone more knowledgeable. – j-g-faustus Jan 23 at 23:42
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Wow, nice job finding that page! +1 for that alone. But seriously, excellent answer. Nice job picking up on the parallax angle (pun absolutely intended). – HDE 226868 Jan 23 at 23:45
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@PearsonArtPhoto - more precisely, the "gravitational dominance" of the Sun (assuming that we're talking about 2 stars) would extend to the center of mass between them. Add more bodies and it gets much more complicated (as well as begging a definition of "gravitational dominance"). – Comintern Jan 24 at 17:59

This graph from XKCD says a lot about why that is the case

enter image description here

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 observation. Most likely this hypothetical planet is a long ways away, possibly as far as 1200 AU, and not particularly large, making it difficult to see. WISE was able to rule out the existence of any Saturn sized object up to 10,000 AU. This hypothetical object is about 1/5th to 1/10th the mass of Saturn. It is also far more likely to be more compact, as Saturn is rather low density. Bottom line, if it is visible in WISE, it would be on the very edge.

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Who is 'me' here? Earth is probably not that far from me! – MycrofD Jan 23 at 4:08
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Me is Randal, and he is measuring from the center of the planet. – hildred Jan 23 at 4:49
    
Shouldn’t the region “planets we would see during the day” include the moon? – Peter LeFanu Lumsdaine Jan 23 at 22:43
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@Peter: I think the point is that the "visible at night" / "visible in daytime" line seems to be drawn at around 0.1 solar diameters, i.e. the size of a small red dwarf star. Basically, anything bigger than that would have to be a proper star, capable of fusing hydrogen and emitting its own light, which would make it more visible. But yes, to properly match the description, the line really should turn down on the left enough to include the moon (and, for that matter, the earth, and some birds and airplanes) in the range of daytime-visible objects. – Ilmari Karonen Jan 24 at 3:09
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@MycrofD Randall Munroe is measuring distance between his location on the surface of the Earth and the center of any depicted object. I'm a little surprised he didn't include the sun on this chart. – WBT Jan 25 at 3:44

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 significantly with respect to Earth, and the hypothetical planet is too far away from the Sun to have much of an impact.
  • Transit. This is obviously impossible, as the planet never passes between Earth and the Sun.
  • Gravitational microlensing. Again, the planet does not pass between us and the Sun.
  • Direct imaging. This is difficult to begin with. The planet is also far away and would have a low absolute magnitude.

As to why indirect evidence for the planet wasn't accumulated earlier - well, objects in the Kuiper Belt and beyond (including the perturbed Trans-Neptunian Objects (TNOs)) were only first observed in 1992, and we've only built up significant data in recent years.

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Possible reasons that the planet has not been picked up previously:

  1. It's not there.
  2. Photographic proper motion surveys cover the whoke sky. To avoid these planet 9 would have to be fainter than about 18th or 19th magnitude. This puts a lower bound in the size/albedo/distance combination for any planet 9. The proposed planet could easily be fainter than 20th magnitude.
  3. Targeted searches for trans-Neptunian objects (TNOs) use big telescopes with wide fields of view. Nevertheless they do not cover anywhere near the whole sky and are generally limited to perhaps magnitude 22-23.
  4. Deep searches can go much fainter, but are "pencil beam" surveys and so have little chance of blindly picking up a particular object, or are single epoch and so cannot detect motion due to parallax and orbital motion.
  5. Infrared surveys (WISE and 2MASS) covered the whole sky. They were sensitive enough to pick up gas giants at the proposed distance of planet 9, because giants are kept hot by gravitational contraction. However, if planet 9 were rocky/icy and had no internal source of heat then it could have been too cold to detect.
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enter image description here

The clipping above from the Albuquerque Journal doesn't really answer your question, but astronomers have been hunting for a largeish planet beyond Pluto for quite some time now. See also:

http://www.zetatalk.com/theword/tword26b.htm

http://www.bibliotecapleyades.net/hercolobus/esp_hercolobus_2_02.htm (under "United States Naval Observatory Calculations")

http://www.bibliotecapleyades.net/hercolobus/esp_hercolobus_3d.htm

EDIT: Just to beat this to death, Percival Lowell may have been the first person to detect the 10th planet. Quoting https://en.wikipedia.org/wiki/Percival_Lowell:

Pluto's mass could not be determined until 1978, when its satellite Charon was discovered. This confirmed what had been increasingly suspected: Pluto's gravitational influence on Uranus and Neptune is negligible, certainly not nearly enough to account for the discrepancies in their orbits.[21] In 2006, Pluto was reclassified as a dwarf planet by the International Astronomical Union.

Of course, the same page continues to state:

In addition, it is now known that the discrepancies between the predicted and observed positions of Uranus and Neptune were not caused by the gravity of an unknown planet. Rather, they were due to an erroneous value for the mass of Neptune. Voyager 2's 1989 encounter with Neptune yielded a more precise value of its mass, and the discrepancies disappear when using this value.[22]

If you're willing to selectively believe Wikipedia, it's possible that Lowell did detect a 9th planet and that it wasn't Pluto.

One reason to disbelieve Wikipedia (and the given source) and to credit Lowell here is that this 9th planet does have an effect on Uranus' and Neptune's orbits, so the discrepancies Lowell noted wouldn't have disappeared, they would have simply been reduced.

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