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Expert Voices

Does Planet Nine really exist?

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of "Your Place in the Universe." Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights

For the past few years, the possibility of a new (and big!) planet hanging around in the outermost regions of the solar system has tantalized scientists and the public alike. But after years of searching, astronomers have found zero new planets in that realm. 

Is "Planet Nine" really out there, or not?

Related: The evidence for 'Planet Nine' in our solar system (gallery)

The deep dark

Artist's illustration of Planet Nine, a world about 10 times more massive than Earth that may lie undiscovered in the far outer solar system.

Artist's illustration of Planet Nine, a world about 10 times more massive than Earth that may lie undiscovered in the far outer solar system. (Image credit: Caltech/R. Hurt (IPAC))

We've only been studying the region of the solar system past the orbit of Neptune for a few decades now, and after a moment of introspection it's easy to see why: astronomy out here is kind of challenging, because the objects we're trying to hunt down are a) very, very small and b) very, very far away. That makes them hard to spot.

Besides Pluto, discovered by basically blind luck in 1930, our understanding of the outer solar system was completely absent until 1992, when astronomers found their first Kuiper Belt object, a frozen little remnant from the formation of the solar system, lazily circling the sun in near perfect darkness beyond Neptune.

Since then, we've found thousands more such objects, categorizing and subcategorizing them as we go (as astronomers are wont to do). For the rest of our story, we'll be focusing on a class of characters known as extreme trans-neptunian objects, or eTNOs. If you've never heard that jargon term before, don't be scared: it's astronomese for "really, really far past the orbit of Neptune."

In 2003, astronomers discovered perhaps the strangest eTNO yet, Sedna. Sedna is big, about half the size of Pluto, but sits in a truly ridiculous orbit. Over the course of 11,000 years (twice that of all of recorded human history), Sedna swings from 76 astronomical units (AU; one AU is the distance between the sun and Earth) to over 900 AU, then back again.

Sedna is weird.

The case for nine

The orbit of Sedna is so weird that it demands explanation. How can such a massive almost-planet reach such a huge, detached orbit without getting completely ejected from the solar system altogether?

Perhaps there's something else out there, keeping Sedna on a leash.

More recently, a couple teams of astronomers began to notice some other funky eTNOs. Namely, a group of half a dozen objects with similar orbits — they had roughly the same amount of ellipticity, and those ellipses were clustered together.

Imagine picking up a random flower from a field and looking at the petals. You'd normally expect the petals to be distributed evenly around the flower, but if you saw them all clustered together you might think something suspicious was going on.

And the same goes for these strange eTNOs: there was no reason to expect these kinds of orbits by random chance. The best explanation, the astronomers claimed, was that a new planet, Planet Nine (until we come up with a better name), was shaping and shepherding them in their orbits.

Related: Planet Nine likely to be found within a decade

But still eight remain

It's not a bad argument. The inability to explain the orbit of Uranus led to the detection of Neptune, so there is some historical antecedent to the strategy. And since then, more eTNOs have been found in the same strange, clustered orbits.

But in the years since the claim of a ninth planet made headlines, astronomers haven't snagged a picture of it. Which isn't too worrisome, at least not yet: if Planet Nine exists, it is very small (relatively) and very far away, making it hard to spot.

And in that same time, other astronomers have weighed in, arguing that the special eTNOs aren't so special after all. It could be that because of the way our surveys are designed and conducted, we're simply more likely to spot eTNOs with these funky orbits, and not any of their friends with more normal orbits. In other words, these eTNOs aren't shepherded by some mysterious entity in the outer solar system. There's simply nothing to explain — they only look different because we haven't finished looking yet.

What's more, it's hard to square the existence of a ninth planet with the formation of the solar system as we currently understand it. Astronomers can, of course, work to fold in a ninth planet (say, by arguing that it's an ejected failed core of a planet or a captured rogue exoplanet), but the more complicated the scenario gets, the harder it is to swallow.

Without a smoking-gun picture of the planet, the astronomical community isn't going to be fully swayed by the wayward motion of a handful of iceballs in the outer solar system. So for now the search for a new planet continues. 

Learn more by listening to the episode "Does Planet Nine Exist?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Clyde V., Scott M. Matthew A., The Manly Astronaut, Scott Monte, Michael H., and Eric C. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

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  • rod
    As this report stated "But in the years since the claim of a ninth planet made headlines, astronomers haven't snagged a picture of it. Which isn't too worrisome, at least not yet: if Planet Nine exists, it is very small (relatively) and very far away, making it hard to spot."

    Other past reports show planet nine could be some 10 earth masses and orbit about 600 AU from the Sun. That suggest a period close to 14700 years, https://www.sciencedaily.com/releases/2016/10/161019173023.htm
    "Brown and Batygin's discovery of evidence that the Sun is orbited by an as-yet-unseen planet that is about 10 times the size of Earth with an orbit that is about 20 times farther from the Sun on average than Neptune's changes the physics."

    It seems continued searches come up short confirming planet nine is out there and that is how good astronomy measurements and testing work.
    Reply
  • Helio
    rod said:
    It seems continued searches come up short confirming planet nine is out there and that is how good astronomy measurements and testing work.

    Yes. I would have liked to have seen something about the IR surveys. It was noted in the past, likely more than a year ago, that the IR surveys would have recorded such a large object, which would be glowing in IR. But, there was the chance that if the galactic plane happened to be in the background then it might go undetected. Are there any updates on this IR story?
    Reply
  • rod
    Helio, planet nine surveys are reported but results are not confirmed. https://astronomy.com/magazine/2020/01/in-pursuit-of-planet-nine, https://ui.adsabs.harvard.edu/abs/2019PhR...805....1B/abstracthttps://www.foxnews.com/science/planet-9-already-found-study
    Planet nine models I read about range quite a bit, e.g. 5 to 15 earth masses, distances 400 to 800 AU, eccentricity ranging 0.2 to 0.5, etc. Nothing conclusive I see, even at the NASA ADS site, https://ui.adsabs.harvard.edu/
    Reply
  • rod
    FYI, according to wikpedia, planet nine if it exists could be fainter than apparent magnitude +22, really faint target :), https://en.wikipedia.org/wiki/Planet_Nine
    Reply
  • rod
    FYI. I did a MS SQL query of exoplanets using this site, http://exoplanet.eu/ searching for 5 to 15 earth masses. 156 reported. The min semi-major axis is 0.0116 and max is 4 AU from the host star. Nothing like the searches for planet nine claims in solar system some 400 to 800 AU from the Sun.
    Reply
  • Helio
    rod said:
    FYI. I did a MS SQL query of exoplanets using this site, http://exoplanet.eu/ searching for 5 to 15 earth masses. 156 reported. The min semi-major axis is 0.0116 and max is 4 AU from the host star. Nothing like the searches for planet nine claims in solar system some 400 to 800 AU from the Sun.
    Yes, but that's would I would expect even if most every star has one of those. The transit method requires it to be in the plane of our view, but only closer objects will actually be seen to block starlight. A tiny fraction of a degree from our plane of view will make it unobservable. Worse yet is how long we would have to wait for it to cross the disk.

    Given this orbital period issue, the Doppler approach hasn't a chance either, and I doubt direct imaging would work since it would have very little illumination.

    Reply
  • Helio
    rod said:
    FYI, according to wikpedia, planet nine if it exists could be fainter than apparent magnitude +22, really faint target :), https://en.wikipedia.org/wiki/Planet_Nine
    That looks about right. I used 600 AU, 25,000km R (ice planet), 50% albedo (Geo) and got 20.6 ap. mag.

    The HST can see to about 31 ap. mag., JWST should be able to reach to about 34.
    Reply
  • rod
    FYI, here is a more detailed report on Planet Nine (P9) apparent magnitude calculations, the arxiv report is attached. https://ui.adsabs.harvard.edu/abs/2017Icar..282...19M/abstract "The magnitudes of P9 estimated in this study are given in Tables 10 – 12 where the planets Neptune, Jupiter and Pluto, respectively, are used for brightness models as described in the following paragraphs. In each case though the radius of P9 is scaled to 3.66 RE."

    The tables present a detailed view with V band magnitudes 21 to 22 or so and different distances from 1 AU to 1120 AU or perihelion distance. I like the space.com report wrap up, "What's more, it's hard to square the existence of a ninth planet with the formation of the solar system as we currently understand it. Astronomers can, of course, work to fold in a ninth planet (say, by arguing that it's an ejected failed core of a planet or a captured rogue exoplanet), but the more complicated the scenario gets, the harder it is to swallow.

    Without a smoking-gun picture of the planet, the astronomical community isn't going to be fully swayed by the wayward motion of a handful of iceballs in the outer solar system. So for now the search for a new planet continues."
    Reply
  • Helio
    Interesting paper. Thanks Rod.

    When I thought about it, their 3.66 Re for No. 9 estimate is kinda amusing for an object that may not even exist. It reminds me of the Beatles White album played backwards where we can hear "number nine" clearly, but it was never there. I hope the planet can be found.
    Reply