A science blog with Eric Berger

Hubble telescope finds another moon around Pluto


It turns out Pluto has quite the little planetary system for being just a dwarf planet.

Astronomers using the Hubble Space Telescope discovered a fourth moon around Pluto, which they have temporarily designated P4. They found the fourth moon while searching for rings around Pluto.

Two Hubble images taken a week apart. (NASA, ESA, and M. Showalter (SETI Institute))

With the addition of P4, estimated to be 8 to 21 miles in diameter, Pluto now has Charon (648 miles in diameter), Nix and Hydra (about 20 to 70 miles across) in its system.

In a news release, astronomer Mark Showalter said of the discovery:


I find it remarkable too.

And I can hardly wait until 2015 to see what the New Horizons mission finds at the solar system’s former ninth planet.

Eric Berger

33 Responses

  1. Avery Gordon says:

    It’s news like this that should take precedence over much of the mess we’re bombarded with everyday. What astounding things we can do with that technology and that telescope! I’ve hoped since the space program began that discoveries like this would bring up the quality of our education, our ethics, and our embracing a larger sense of “our world.” But ignorance has bonded with technology instead, and we’re missing a huge link, like the info presented in this article. Thanks so much for posting it, AND the great photos!

  2. Dave says:

    Can we now give Pluto planet status back please?

    • jason says:

      Dave, I was going to suggest the same thing. Pluto may be small but it is working hard–4 moons is respectable. Of course if the moon has a diameter of 20 miles, you could drive around its equator in about one hour. This P4 must be about the size of a football stadium.

    • JohnD says:

      If you don’t like how Pluto is classified, all you have to do is become a member of the IAU and bring it up as a resolution.

      That or do what most planetologists do and ignore the IAU definition (which is biased toward observational astronomy) in favor of the simpler “Must orbit a star and be dominated by hydrostatic forces”.

    • Precious Roy says:

      NO! The Science Gods have spoken.

    • Eric Berger says:

      Now that’s a darn reasonable definition, John. Wonder why the IAU didn’t go for that.

    • lil ol me says:

      Eric said: “Now that’s a darn reasonable definition, John. Wonder why the IAU didn’t go for that.”

      Now, Eric, we all know there’s no grant money, press releases or Nobel Prizes in maintaining the status quo…..

    • Justin Kugler says:

      Sorry, I have to agree with Neil deGrasse Tyson on this one. Pluto behaves and is composed more like the other Kuiper Belt Objects than either the terrestrial planets, the gas giants, or the ice giants. Pluto being a KBO in no way detracts from its interest or importance in scientific inquiry.

    • Eric Berger says:

      But how many KBOs have moons? Or an atmosphere? I love the debate about the status of Pluto because both sides have a very defensible case to make. New Horizons is only going to make it better.

    • JohnD says:

      The definition that planetologists use is pretty close to the one that the committee submitted to the IAU at the start of deliberations back in 2006. This is not surprising as the committee was mostly composed of planetologists, with a historian and an astronomer thrown in for balance.

      As to why it wasn’t selected, do you want the practical reason or the political one (or the deeply cynical one)?

      The practical reason is that doing so would return Ceres and Vesta to planetary status, and would open up the problem of what about the Moon in 3 billion years along with the question of “double planets” such as Pluto-Charon. (The moon is expected to escape the Earth’s orbit and co-orbit the Sun in about that amount of time, according to some astrophysicists. Thus, you would have a moon becoming a planet.) It was also derided for being heliocentric and so does not apply outside of the Solar System. I note in passing that the version that was adopted still has these problems.

      The political one is that the IAU is mostly made up of astronomers, with very few planetologists, who tend to prefer the AGU. As a result, there was considerable consternation by the rank-and-file that astronomical prinicples were so little used in the definition, and hence led to the inclusion of a metric that only an astronomer could determine (cleared the neighborhood”).

    • JohnD says:

      Justin is right that Pluto does not belong in any of the established sub-categories of planet. It is not dense/rocky enough to be a terrestrial planet. And it is not big enough to be a jovian planet. And, when you plot the various bodies of the Solar System with density (or mass) on one axis and diameter on the other, Pluto clearly lands in a non-traditional cluster.

      What planetologists do at this point is say “Aha! Pluto, Eris, Titan, Ceres, and all of those others are a new type of planet!” (Some would go further and classify Mars, Io, and Vesta as yet another class of planet.) From this type of classification, we can deduce much about the interior of the planets and their geological evolution.

      And that’s the main problem with the IAU definition – it tells you nothing that is truly scientifically interesting about the things that it calls “planets”. It provides no springboard for further exploration and no reason to examine the idea in greater detail.

    • James says:

      Personally, I think that the fact that pluto’s orbit is strongly inclined relative to the planetary plane is sufficent to for it to belong to a different class of objects. Clearly it’s formation and history must have been very different from the others. Grouping it with Eris as a Plutoid makes sense since they have much more in common. I’d think that having an entire class of stellar objects named after it more then makes up for being kicked out of the planet club.

    • JohnD says:

      James, the categories “planet” and “KBO” are not mutually exclusive, just as the classification of something as an asteroid does not prevent it from also being called a comet (e.g., 60558 Echeclus, Chiron).

  3. Dr. Dave says:

    Amazing. And, who says there is nothing new to discover in our solar system.

  4. bnrtn says:

    Restore Pluto’s planetary rights immediately! It’s earned them!

  5. so far away says:

    Roughly, the 8 mile diameter is the same as the 610 loop and the 20 mile diameter corresponds with the Sam Houston Tollway, to bring it down to earth.

    I have no idea what 3 billion miles compares with.

    • JohnD says:

      I have no idea what 3 billion miles compares with.

      Consider it as either the distance covered by 6,300 Apollo flights or the separatoin between politican spin and reality.

    • Eric Berger says:

      If you got in your car, put the cruise on 65 miles per hour, and drove continuously you would go 3 billion miles in about 5,600 years.

      So pack a lunch.

    • so far away says:

      with a lot of potato chips…

  6. flash says:

    What about Planet X? Hmm?

  7. COWBOY says:

    A moon around Pluto?……Must be Goofy pulling down his pants.

  8. Brian L says:

    Now that we’ve found Persephone, maybe we can ask her to fix global warming!

  9. James Stall says:

    Pluto having an odd orbit or not, as far as I am concerned the solar system has NINE planets (and probably more).

  10. Jethro David Hightower says:

    Just because Pluto has a posse doesn’t make it a planet any more than an NFL player with a posse makes him a superstar

  11. Bill in Houston says:

    Nix? Really? After the guy who writes that spy show (Burn Notice) on USA?

    How about more Greek and Roman names. I can’t believe we’ve gone through the pantheon of gods and characters. There must be a centaur out there who needs a moon named after him.

  12. craig says:

    I just knew that dog could hunt. OK so Pluto is a lot smaller than even Mercury, orbits in an elipse instead of the move conventional circular orbit of the other planets, still it orbits our sun and is itself orbited by a number of moons. Reinstate Pluto. Size and orbit isn’t everything. Having moons should count for something.

  13. Mark says:

    I think Pluto should be renamed hydra, because I bet that most of the orbiting objects are really artifacts spawned by a collision resulting in the 5 headed monster we now see

  14. aaron says:

    Hey Eric,

    What are the streaks in the image. Are those just imaging artifacts are they comets in the background?

  15. [...] now has four moons, but it’s still not considered to be a planet. Go [...]