Pluto’s Moons: Five and Counting

Within the past two weeks, astronomers used the Hubble Space Telescope to identify a fifth moon circling Pluto.

It's hard to believe, but the arrival of NASA's New Horizons spacecraft at Pluto is just three years away. The logistics of the high-speed flyby, already challenging, just got more complicated: Pluto turns out to have a fifth moon.

Pluto and its five moons
Three sets of 3-minute-long exposures, taken on July 7th with Hubble's Wide Field Camera 3, reveal the presence of "P5," Pluto's fifth known satellite. A dark vertical band was used to reduce the brightness of Pluto and Charon, and faint horizontal stripes are imaging artifacts.
M. Showalter & others / NASA / STScI
Although for now its official designation is S/2012 (134340) 1 — "134340" being the minor-planet number assigned to Pluto — the new find has been nicknamed "P5". (Easier to remember, don't you agree?) Its existence was announced last night by the IAU's Central Bureau for Astronomical Telegrams.

Long-time Pluto-watcher Mark Showalter (SETI Institute) led the nine-member discovery team. They took advantage of Pluto's opposition on June 29th, when this little world was a mere 31¼ astronomical units (2.9 billion miles) away, to image the system 14 times from June 26th to July 9th with the Hubble Space Telescope and its Wide Field Camera 3.

"Here's an interesting stat," Showalter notes. "P5 is 1 arcsecond from Pluto and fainter by a factor of 100,000. I continue to be amazed at what Hubble can do with fine-tuned observations."

Yet even with HST's powerful optics, P5 shows up as barely a blip. Its magnitude is just 27, which puts its diameter somewhere between 6 and 15 miles (10 and 25 km), depending on the reflectivity of its surface. The orbit is still uncertain, though the tiny moonlet appears to be circling in the same plane as Pluto's other satellites and roughly 26,000 miles (42,000 km) out. That puts P5 nearer to Pluto than Nix, Hydra, and the not-yet-named P4 (discovered last year) though not nearly as close as Charon.

Orbits of Pluto's moons
The orbits of Pluto's five known moons. Just-discovered "P5," the smallest of all, might be in a resonance that carries it completely around Pluto once for every three circuits made by Charon. As now planned, NASA's New Horizons spacecraft will pass just 6,200 miles (10,000 km) from Pluto in July 2015.
Source: NASA / ESA / A. Feild (STScI)
It's no coincidence that all these moons orbit in the same plane as Pluto's equator. Most likely they formed from debris tossed out when a renegade object struck Pluto long ago. Collisions in this distant region of the solar system are typically so slow that most of the resulting fragments couldn't have reached escape velocity, which is a bit under 1 mile per second for Pluto. So most of it would have stuck around.

But that doesn't automatically lead to satellite formation. Ballistically speaking, any stuff that lingered should just have just fallen back onto Pluto itself. However, tidal interactions among the most massive chunks could have allowed enough of them to remain in orbit to form Charon and the other moons.

What's driving the search for Pluto's extended family is the possibility that such small objects or even rings might pose a danger to New Horizons as it zooms through the system at 32,000 miles per hour (14.3 km per second) on July 14, 2015. Plans now call for the spacecraft to pass well inside Charon's, at a point about 6,000 miles from Pluto.

"We have been searching for hazards because this is the last chance to observe Pluto before the team has to settle on the backup flyby trajectory," explains Showalter. "Searching for new, interesting targets is just a fringe benefit."

Here's the NASA/STScI press release describing the discovery of P5.

29 thoughts on “Pluto’s Moons: Five and Counting

  1. Paul Vondra

    I think the new moon of Pluto ought to be named "Goofy" in honor of the IAU bureaucrats who decreed that a perfectly round world in an independent orbit of the Sun that is well over 1,000 miles in diameter, has a sensible atmosphere and a set of five neatly coplanar moons is somehow not a planet.

  2. john crawford

    Paul has an excellent point-several in fact. But in 50-100 years how many other Kuiper belt objects will meet the same definition? 10? 50? 100’s? When the first four asteroids were discovered they were counted as planets, then there just became too many of them. So when the true nature of the objects became apparent the first asteroid belt was discovered. By chance of Pluto being large and close (relatively speaking) it was considered a planet. Now with the realization that the second asteroid belt contains probably more bodies than the first, with perhaps many, many as large or larger than Pluto the definition of a ‘planet’ has become unwieldy.

  3. Paul Vondra

    Replying to John, I don’t think the number of items in a classification should affect the definitions that qualify for that classification. That’s totally unscientific. There are something like 200 billion stars in our galaxy but that doesn’t stop a self-luminous mass of gas powered by nuclear fusion from being a star. Why should planets be any different? And as many others pointed out, a "dwarf star" is still a star. A "dwarf galaxy" is still a galaxy. A "dwarf person" is till a person. Why would a "Dwarf planet" not be a planet? The whole thing is "Goofy."

  4. john crawford

    Nothing like having the keyboard freeze up. So if 1000 miles is the criteria that seems arbitrary too? Why not 500 miles? 1000 kilometers? Having moons? Asteroids smaller than 100 mile diameter have moons. Stars-what are Brown dwarfs? Super Jupiters? The problem is that eventually there will be no quick, easy and absolute criteria of what makes up a planet as opposed to a dwarf planet or asteroid (planetoid? Minor Planet?) What is the cut-off between meteoroids and asteroids-one foot, ten feet, a hundred feet? What is the cut-off between a Super-Jupiter and a brown dwarf? Size? Temperature? Eventually there is just going to be a long range of objects from micro-meteroids on up to stars and super-giant stars. The idea of lumping Pluto with Sedna, Eris, Makemake and the rest seems to be to not give it any special status as a Kuiper belt object. For me that works for now.

  5. Paul Vondra

    So John, your main problem with Pluto’s planethood has nothing to do with Pluto itself, it has to do with "if we let one in we have to let them all in." Show me another dwarf planet in an independent orbit of the Sun, perfectly round, over 1,000 miles in diameter, with an atmosphere and a set of coplanar moons, and I agree, we’d have to let it in too. But the hypothetical exiatence of such a world is no reason to exclude Pluto now.

  6. john crawford

    Well Paul you could fit almost nine Plutos inside of Mercury (smallest official planet). To apply the Hulk’s comment in the Avengers movie to Pluto-"Puny planet."

  7. Phil

    OK, there is a continuum of solid object sizes, from dust to Jupiters. They don’t have natural gaps in the range to self-define groups. The dividing line between "dwarf" and "normal" planets is always going to be someone arbitrary. It needs to include size, shape, moon count, composition, and influence on its neighborhood. I can’t help but note that if Pluto were an AU or two from the Sun, it would have lost most of its atmosphere and volatiles, and would be considered just a large asteroid. It might not even be able to hold on to its moons. Anyway, there will probably never be a definition of "full fledged" planet that will please everyone.

    As @John pointed out, what we consider "minor planets" (the asteroids) has changed. At first they were counted as normal planets, but at 7 or so (IIRC) it was realized that things were getting out of hand, and they were demoted to minor planets. Pluto was initially thought to be Earth sized — if its true size was known from the start, it probably would never been called a planet.

  8. John Tashjian

    In keeping with the precedent that the IAU set, I would recommend that Pluto’s new moons might be named Hecate and Thanatos.

    Given that, in Greek mythology, Pluto (Hades) was ruler of the Underworld, it would cetainly be in keeping with that theme to name the new moons with those names, wouldn’t you say?

  9. Bruce

    I agree with much of what Phil just said. For earthly examples, how do we define the difference between a rock and a bolder, or a town and a city? But Phil I question the last three items you listed in your suggested list of planetary inclusion factors; “moon count, composition, and influence on its neighborhood.” Venus and Mercury are moonless, but I doubt anyone would suggest that they’re not planets. Composition shouldn’t matter either, so long as it’s formed naturally and made of regular baryonic matter. Your last criterion harkens back to the “cleared its region of space” phrase that the potentates of astrophysical nomenclature came down from on high with when they demoted Pluto to dwarf-dom. I can well imagine days of debate just like what we’ve just read between Paul Vondra and John Crawford, with each of them making equally valid points and counter points. My problem with the last factor is that we are bound to find cases of planet sized objects in shared or crossing orbits. In my totally unimportant opinion: If the body in question 1) is in hydrostatic equilibrium (spheroid), 2) is not in orbit of another planet (which would make it a moon), 3) is at least as large as Pluto and 4) its not so large that deuterium fusion could have ever taken place when it was formed, then it’s a planet. Descriptive adjectives can be added to subdivide the class, i.e. nomad if it doesn’t orbit a star or stars, minor or dwarf if it’s small, and so forth.

  10. john crawford

    Bruce-your criteria for ‘planethood’ sounds logical-except for ‘at least as large as Pluto.’ Why not as least as large as Ceres? As Vesta? Why not set the bar a little higher-at least as large as Mercury? Paul said that dwarf stars and dwarf galaxies are still stars and galaxies, so fine, dwarf planets are still planets, but like stars and galaxies that meet that ‘dwarf’ criteria those planets have that little (pun intended) extra adjective to help indicate their sizes.

  11. David Oesper

    There are good scientific reasons for not considering Pluto to be a planet. However, there are many arbitrary/historical aspects of this wonderful endeavor we call astronomy. For example, why do we not now call a "planetary nebula" an "ejection nebula" instead? Pluto was a planet for 76 years. Based on historical considerations, then, one could simply define a planet in our solar system to be "any object orbiting the Sun and not another planet that has a diameter at least as large as Pluto." Since Pluto’s diameter is 2306 +/- 20 km and Eris’ diameter 2326 +/- 12 km, our solar system now had 10 known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, and Eris. I doubt we will discover more than a handful of new objects in our solar system having a diameter larger than 2300 km.

  12. Bruce

    Thank you for that ringing 3/4 endorsement, John, and for your historic defense of the remainder, David. Actually John, you did poke my proposal at its weakest point. Even while I was writing it I debated myself as to which body to set the lower limit at. I discounted the larger asteroids because aren’t they nothing but the largest chunks of a failed planet? Mercury would work as a substitute choice, but I’ve already ticked off the asteroid fans, I don’t want to alienate the Plutonians as well. So David’s suggestion for anything with a diameter over 2300 km might work for now. However, what if the New Horizons wiz-by of Pluto somehow provides evidence that it was once part of Neptune’s family of moons, as has long been suggested? The great debate may have to wait until the returns from the outer precincts come in.

  13. Paul Vondra

    So John, just how many Earths could you fit inside Jupiter? (And where on Jupiter would you STAND to watch the clouds go by?) How many Mercurys could you fit inside Earth? Is the Sun a star? How many Suns could you fit inside Antares? Antares is a red supergiant. But it’s also a star. The Sun is a yellow dwarf. But it’s also a star. Jupiter is a gas giant. But it’s also a planet. Earth and Mercury are rock worlds. But they are also planets. Pluto, Eris and Makemake are ice worlds. But they are also planets. Ceres and Vesta I withhold judgment on. They’re under 1,000 miles in diameter. That, roundness, and an independent solar orbit would be MY criteria. And before you go jumping on 1,000 miles as being arbitrary, at least it’s arbitrary and EXACT. "Clears its neighborhood" is arbitrary and VAGUE (e.g., what about Trojans?)

  14. Russ Palmer

    Pluto is already a member of the class of planets, in the sub-class "dwarf planet". Jupiter et al are in the sub-class "giant planet". The Mercury et al are in the sub-class "rocky planet". Perhaps we should just formalize the other sub-classes and put an end to all of this useless argument. Pluto is still a planet, as are Mercury and Venus – even though Pluto has moons and they do not. Are we to strip away Mercury’s and Venus’ planetary status because they, alone of the ‘planets’ have no moon? A formal ‘sub-class’ approach would agree with other branches of science and stop this nonsense permanently. Let’s get on with enjoying the discoveries!

  15. Bruce

    Didn’t you intend to write, "Great comment Russ!" John? If so, I second the sentiment. (Please take no offence, Phil.
    Your comment was good, but Russ topped both of us, don’t you agree?) "Blessed are the peacemakers …" Matthew 5:9

  16. Phil

    Ah, bummer! You had my hopes up for a moment! 🙁

    Anyway, my list of criteria was not meant to be either exhaustive or exclusive — I was listing the many criteria that were suggested in the original Pluto debate a few years ago. Perhaps I should have made that clearer. Certainly, Mercury and Venus are planets, even with no noticeable natural moons. A planet doesn’t have to have moons of its own, but cannot be the moon of something already called a planet (where do double planets like Earth/Moon and Pluto/Charon fit in?).

    No one is going to be happy with all these criteria. For one, I could point out that 2300km or 1000mi is purely arbitrary, and based on human-created units rather than something intrinsic to stellar systems. It’s a circular argument, too: we’re determined to call Pluto a planet, dammit, so let’s set the minimum just a tad smaller. A lot of comet comas meet the size criterion — does that make them planets? The only thing I think we could agree on, size-wise, is that it be too small for any kind of fusion.

    Planet definitions need to work for other stellar systems, too. Most of the argument has been over the planet/brown dwarf split, but the definition needs to be clear on the low end, too. As I like to say, an automated alien survey ship passing through our Solar System would likely report 4 planets, plus a bunch of debris.

  17. Bruce

    Nice comment Phil. Granted, any lower limit diameter that might be selected would be arbitrary and bound to displease some. Also, your comet coma comment commits conundrum complication. There will also be questions on the upper diameter limit as well, since puffed up hot Jupiters are likely to be even larger than some red dwarfs, and certainly white dwarfs. And neutron stars are shrunk to rocky planet size, but they’re not planets of course. Some smallish black holes are likely too tiny to matter, if diameter is the important parameter. Therefore I propose a massive rethink of the planetary criterion question: Dump diameter and use mass instead. Pluto’s mass is about 1.27E22 kg. We could round down to an even 1E22 kg for the lower limit. What do you think Phil?7

  18. Phil

    Again, using Pluto as the benchmark for a minimal "full" planet is purely arbitrary. You want Pluto to be a full planet, so set the minimum mass (or diameter) to just let it in. A TNO just under that limit — sorry, you’re a dwarf; above it, welcome to the club. There’s nothing magical about 1E22 kg or 2300 km — bodies do not radically change characteristics at that point. Good point though, about hot Jupiters and neutron stars and even black holes. Ironically, black holes would probably be clearly classed (on mass) by origin: a black hole formed from a stellar collapse versus a primeval one from the Big Bang versus something that grew directly from gas and dust in a galactic core.

    The IAU attempted (and didn’t quite succeed) in defining planets in a non-arbitrary manner, that is, not based on our Solar System’s existing known planets.

  19. Mike W. Herberich

    IF there, at all, should and/ or WILL be lower and/ or upper limits to anything from diameter, mass, density, etc., why not use truly universal values/ constants like e (Euler, my favorite, not only because he comes from the same little town in Switzerland, namely Basel, but rather for it’s omnipresence in all that has to do with nature), or pi (or 4*pi, for that matter) or G (gravitational constant), etc.? Lambda could be a bit premature though ;-D. Kepler’s constant (3rd law)? Etc. …

  20. Charles Peterson

    First of all my compliments to Kelly Beatty for his timely, comprehensive, and lucid article on Pluto’s newly discovered 5th satellite.
    I must take issue with Paul Vondra’s (facetious?) suggestion, apparently motivated by his disagreement with Pluto’s 2006 demotion to dwarf planet, to label a natural satellite so as to reflect his opinion on Pluto’s status change. I would recommend instead that Paul apply the adjective "goofy" to the then-serious proposal before the August 2006 IAU Assembly in Prague that called for 12 major planets that would have included not only Pluto, but also Ceres, Eris (then called Xena), and even Pluto’s satellite Charon! This proposal was submitted in utter disregard of astronomical history (circa 1847) by respectable scientists who should have known better.
    The quotes that follow are from page 115 of the book "Is Pluto a Planet?" by David A. Weintraub. At its discovery in 1846, Neptune was counted as the thirteenth planet. But the next year in "1847 William Herschel’s decades-old suggestion that the trans-Martian objects are minor planets, or asteriods, and not major planets, took hold. In the minds of many, 1847 was a watershed year during which Ceres, Pallas, Juno, Vesta, and Astraea were reclassified as asteroids." Thus did Neptune become generally accepted as the eighth planet of our Solar System, but not without some holdout scientists of the day protesting the demotion of 5 "planets" to "asteroids".
    Let us heed George Santayana’s sage admonition from 1905 that "Those who cannot remember the past are condemned to repeat it". Can Paul imagine any respected scientist today counting 200,000 “planets”? Barring some future discovery, there are and remain 8 major planets in our Solar System. That said, there are millions of minor bodies, including the dwarf planets, that deserve proper respect for preserving the history of our solar system as well as for performing some important transport functions during its early evolution.

  21. Charles Peterson

    [Compensating for the lack of space above, allow me now to acknowledge Phil’s earlier (and excellent) post referring to the circumstances leading to the classification of asteroids in the mid 19th century.] Because many of Paul Vondra’s assertions echo sentiments currently shared by many of us interested in planetary astronomy, I believe that such statements merit a direct response. Paul errs in stating that he doesn’t "think the number of items in a classification should affect the definitions that qualify for that classification. That’s totally unscientific." Quite to the contrary, I claim that no scientific classification system can remain unchanged for long during an active period of new discoveries. That includes both the discovery of more new "items" and the discovery of more information about known "items". Before 1847 we had "planets" and "comets" orbiting our sun. Now we have "major planets", "dwarf planets", and "minor planets" (or asteroids) which have several subclasses of their own (e.g. Trojans, Centaurs, TNOs, etc), As the number of known asteroids becomes more numerous, so will the number of their subcategories. An excellent example of this redefinition/reclassification process is the discovery of about 100 natural satellites during the past 20 years. Before about 1990 our solar system had "rings" and "satellites" orbiting planets. After sensitive CCD detectors (and in situ spacecraft photos) became available, we now have "rings", "ring satellites", "regular satellites", "retrograde irregular satellites", and "prograde irregular satellites". Scientific classification systems must always be subordinate to new discoveries, never the other way around.

  22. Charles Peterson

    Regarding a newly proposed dynamical requirement for major planet status, Paul Vondra properly objects to it on the grounds that " ‘Clears its neighborhood’ is arbitrary and VAGUE (e.g., what about Trojans?)". Therefore, I would like to take the opportunity to raise this discussion of planetary status to a higher level by introducing a concept that has not yet been mentioned and that is not yet appreciated even by many planetary scientists. I believe that Paul’s objection here does indeed have merit, deserves an informative response, and can be most simply satisfied by generalizing the concept/definition of "planetary satellite" to also include small bodies captured in COMMENSURABILITY RESONANCES. At this point I strongly urge the curious reader to google "Resonant trans-Neptunian object" and invest a half hour reading the well-written and illustrated Wikipedia article. In summary, Pluto (and about 100 other TNOs) was long ago captured by Neptune into a 3:2 commensurability resonance such that Pluto must orbit the sun EXACTLY 2 times (on average) for every 3 orbits of Neptune. Like the Trojans of Jupiter, which are all captured into one type of a 1:1 commensurability resonance, the captive bodies in these 3:2 resonant orbits are not only in long term stable dynamical configurations, but they are also prevented by this resonance from suffering close encounters with Neptune. Therefore, in addition to the "vague" requirement that a major planet must "clear its neighborhood", I would also propose the more precise disqualification that (in addition to not orbiting another planet) no major planet can be captured by another into a commensurability resonance. Note that even if Pluto’s mass were equal to that of the Earth (instead of 450 times less), it would still not qualify as a major planet under this criterion because it would nonetheless be regarded as a "captive" of Neptune, which is about 17 times more massive.

  23. John F. Tashjian

    Well, in keeping with the IAU’s tradition for the naming of moons for places like Pluto, I would like to send some suggestions the IAU’s way (for Pluto’s two new moons): Hecate, Persephone, Thanatos, Cerberus, and/or Chronos.

    I hop that these might help. Enjoy. 😉

  24. John F. Tashjian

    Well, in keeping with the IAU’s tradition for the naming of moons for places like Pluto, I would like to send some suggestions the IAU’s way (for Pluto’s two new moons): Hecate, Persephone, Thanatos, Cerberus, and/or Chronos.

    I hop that these might help. Enjoy. 😉

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