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

  • 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?

    naively, because it is very small and far away.

    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?

  • j-g-faustus

    j-g-faustus Correct answer

    6 years ago

    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.

    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.

    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.

    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).

    @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").

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Content dated before 7/24/2021 11:53 AM