Fifth Moon Found Around Pluto

This just in! Astronomers working with the Hubble Space Telescope have spotted a new moon around distant Pluto, bringing the known count up to 5. The image above was released by NASA just minutes ago, showing the Pluto system with its newest member, P5.

This news comes just a couple of weeks shy of the one-year anniversary of the announcement of Pluto’s 4th known moon, still currently named “P4”.

The news was shared this morning by an undoubtedly excited Alan Stern of the Southwest Research Institute (SwRI) on Twitter.

Astronomers estimate P5 to be between 6 and 15 miles (9.6 to 24 km) in diameter. It orbits Pluto in the same plane as the other moons — Charon, Nix, Hydra and P4.

“The moons form a series of neatly nested orbits, a bit like Russian dolls,” said team lead Mark Showalter of the SETI Institute.

A mini-abstract of an upcoming paper lists image sets acquired on 5 separate occasions in June and July. According to the abstract, P5 is 4% as bright as Nix and 50% as bright as P4.

The satellite’s mean magnitude is V = 27.0 +/- 0.3, making it 4 percent as bright as Pluto II (Nix) and half as bright as S/2011 (134340) 1. The diameter depends on the assumed geometric albedo: 10 km if p_v = 0.35, or 25 km if p_v =0.04. The motion is consistent with a body traveling on a near-circular orbit coplanar with the other satellites. The inferred mean motion is 17.8 +/- 0.1 degrees per day (P = 20.2 +/- 0.1 days), and the projected radial distance from Pluto is 42000 +/- 2000 km, placing P5 interior to Pluto II (Nix) and close to the 1:3 mean motion resonance with Pluto I (Charon).

The new detection will help scientists navigate NASA’s New Horizons spacecraft through the Pluto system in 2015, when it makes an historic and long-awaited high-speed flyby of the distant world.

See the news release from NASA here.

(H/T to Ray Sanders at

Top image: NASA, ESA and M. Showalter (SETI Institute)

24 Replies to “Fifth Moon Found Around Pluto”

  1. Wow. It is a dwarf planet, smaller than the Moon, but it has 5 moons. Haha. Pluto is a really unique member of our Solar System, I think.

    1. Unique? I really don’t think so.. there appear to be a great number of TNOs, KBOs etc. It may be just a common icy object like any other, just like how the Sun isn’t unique among its stellar brethren.

      1. But it have a name & history, couldn’t they leave it as “honorary” planet.

      2. Pluto is clearly a different class of object than the other planets are. It shouldn’t be lumped together with them using an arbitrary definition. But then, Earth is a different class of object than Neptune, and Neptune is quite obviously not the same type of object as Jupiter. For that matter, Pluto and Ceres are both currently “dwarf planets”, dispite the fact that they have vastly different compositions and formation histories.

        The current definition is kinda stupid, due to its “lumpy” nature.

        A better idea would be to scrap the scientific use of the word planet, and replace it with a branching spectrum definition of object. This branching definition would be defined exclusively by the physical atributes of the object in question.

        At the low mass end, it would start out with any object that was being held together more by gravity than by chemical bonds. At the upper mass end it would stop when the objects in question started being able to sustain a self supporting fusion reaction for a significant amount of time (although that is a bit fuzzy, isn’t it?).

        Along the mass spectrum, branches would come off. So, for instance, you could potentially have 3 planets: 1 large ice world, one terrestrial super-Earth, and one small gas “giant”. They’d all have the same mass, but because they have different physical characteristics they’d be on different branches.

        This spectrum definition (or a better thought out version of it) would eliminate a lot of the current silliness with the world “planet”.

        But one way or the other, Pluto is dinky. Whatever name we decide to call such worlds, they shouldn’t be in the same branch of objects as Jupiter:P. That’s silly.

      3. The current definition is more aligned with populations (planets, asteroids, Kuipers, Oorts) than with traits (terrestrials, neptunes, jupiters). Both are convenient.

        But already biologists have a problem with taxonomy (convenient labels like fish and mammals) vs populations (mammals _are_ fish, cladistically). There is no correct way to do convenient classifications, often you can or want to do make many versions in parallel.

        Ceres was a planet almost as long as Pluto was. But nobody seems to want to honor our first asteroid planet in that way, instead a Kuiper planet is all the rage. Go figure.

      4. If people want to have “fuzzy” definitions of various objects that they can use in everyday speech, by all means they should do so. Most people I deal with on a daily basis call tortoises turtles, seem to think all frogs are toads, and think that the words “solar system” and “galaxy” are synonyms (seriously). Whatever. That’s nothing but pop culture, and thus irrelevant. That’ll happen without any of us prompting it, and doesn’t need to be officially sanctioned by the IAU.

        What matters is the existence of a self-consistent USEFUL scientific definition. Right now there isn’t one. Lumping Earth and Jupiter together is useless, as is lumping Ceres and Pluto/Sedna together. They’re different types of objects and should be treated as such. A branching spectrum definition is the easiest, cleanest, least confusing way to go about creating a useful classification system for planets and planet-like objects.

        Maybe people disagree with me and think that another detailed classification system would be better. Ok, sure. If it works, I’ll use it.

        But this doesn’t work ===> “That big ball of gas over that is currently burning deuterium and is *just* short of hydrogen fusion mass. Let’s group it together with a Mars, a frozen desert world, and use a single word to describe both as if they were the same type of object! That will teach everyone not to mess with Astronomers!” That’s the current system, and it’s dumb:P.

  2. Thanks to Hubble Team for discovery…As, it is said, “small is better…”.

  3. We should name it Tyson, hopefully if we are really nice to Dr. Tyson he will let Pluto be a planet again.

    1. Please no, naming it Tyson would only make me think of Charon biting an ear off Pluto.

  4. how much would it cost to name p5 after oneself? like naming it zoutsteen (or salt lick)

    1. I am hoping/guessing/suspecting a couple of more even smaller moons, and possibly a few ringlets.
      It seems Pluto possibly experienced a collision of some form some (probably long) time ago. I dont know what statistics would imply about the number of collisions out there, but even if its likely very low now, it might have been significantly higher in the far past. Just speculating…

      1. I was just thinking of that: New Horizon was already being considered retargeting outside of the system, or at least in Charon’s wake, due to the system looking denser (P4). That is why they looked closer at the system in the first place.

        Now this.

      2. Couple of smaller moons are possible, even around Charon who knows. But I think ringlets are a bit of a stretch. Of course its anyone’s bet until NH actually gets there.

  5. Boy, this makes the Mercury-Venus-Earth trio look like exceptions rather than the rule as they have no tiny moons (and Luna seems to be the exception to the exception).

    I wonder how many other KBOs will have multitudes of moons when we can get close enough to see them.

    1. Actually, the Moon is also impact remains (same composition as Earth), but it reassembled in one large body as opposed to Mars and Pluto moons. (Though Charon is larger in relation!) The smaller remains would have deorbited fast.

      It is arguable that Mars moons are impact remains, but at least one impact scar seems to be a deorbited previous moon.

      If Uranus’ tilt (as well as Triton’s capture by Neptune, and retrograde moons) is evidence of the outer planets being subjected to a flow of impactors with enough force to affect rotation, so may Venus slow and retrograde motion be. But again it is arguable, because coupling to the Sun may be the better prediction. And where did the impact ejecta go?

      At this time it may be tempting to say that Mercury looks safe to be the odd man out. But what Messenger has found shows that it is both more reminding of other terrestrials (thicker and more diverse crust than believed) and less (funnily layered core with lots of sulfur). One proposed idea IIRC why it has a relatively thin crust would be impact stripping, with the stripped material ending up in the Sun instead of as moons.

      Many or most KBOs seems to have impact moons, like asteroids, see my link in another comment.

      1. Venus might even be a complete merger, the slow rotation would have a simple explanation if it was. Not discounting other possible explanations.
        To me, collisions in the past seems to have been vastly more common than statistics based on current configurations would indicate.

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