Sharpest image of Pluto ever taken

By Phil Plait | October 12, 2007 10:33 am

Astronomers using the monster twin Keck 10 meter telescopes have produced the sharpest image of the Pluto system ever taken! Now, mind you, these are the largest optical/IR telescopes on the planet, capable of incredibly high-resolution images. If you ever were wondering just how small and far away Pluto is, then keep that in mind when you look at this picture:

Pluto is small, and far away.

Pluto is the larger of the circular blurs, and its big moon Charon is the somewhat smaller blur (the sizes of the dots are not because of the sizes of the objects, but rather their brightnesses: brighter objects appear bigger). You can also see the dinkier moons Nyx and Hydra above and to the right.

These images are a little frightening, since they really bring home — literally — the scale of the solar system. Pluto is 2300 km across (smaller than the Earth’s Moon) and was about 4.8 billion kilometers away when that image was taken.

More images will be taken so that the astronomers can track the moons as they orbit Pluto. Better observations means the orbits can be measured more precisely, which in turn will yield better measurements of Pluto’s mass (the mass of the planet asteroid dwarf iceball thingy determines its gravity, which in turn drives the orbits of the moons, so by studying the moons you get Pluto’s mass).

Getting the mass of Pluto precisely is a little bit difficult since it’s so hard to observe — it’s really far away. When it was first discovered and its distance determined, it was thought to be a giant planet because it was so bright for that distance. More observations showed that it was actually small, but very reflective (its surface is very white), so it fooled astronomers into thinking it was bigger. Over time, the mass/size measurements got smaller and smaller, until Charon was discovered and Pluto’s size and mass were found with some accuracy. There was a joke that if we kept observing Pluto, it would disappear entirely. :-)

It may not be considered a full planet anymore, but it’s still a pretty interesting place. It’ll be years before the New Horizons probe gets there, so until then we’ll have to be satisfied with it being a blurry dot.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (56)

  1. I can’t wait for New Horizons to give us sharp, clear, digital pictures of Thingy Pluto up close.

  2. John H

    Pluto may be a dwarf planet but at least it has three moons which is more than four of the major planets have.

  3. Dan

    Pluto will always be a planet in my mind.

  4. Doc

    “That’s no moon…”

  5. This newly released Titan radar imaging is really cool. In case you need something with more visual punch on a Friday.

  6. > “That’s no moon…”

    “It’s a… thingy.”

    *cue dramatic music*

  7. Navneeth

    Hey, BA, I see a black disk (barely visible at a cursory glance) behind Pluto and Charon. What’s that?

  8. Sili


    Figure 4: The Pluto system. The region around Pluto and Charon was reduced in brightness so that all four objects could be shown individually in a single image. Photo by David Tholen.

    From the linked page with the images.

  9. Navneeth

    Thank you, Sili.

  10. Nice photo! Everything is just so far out there it is unreal.

  11. Duane

    The fact that the Imperial starships streaking across the image has not been brought up in the news means only one thing: Conspiracy!!! 😉

  12. Michelle

    Awesome. I can’t wait until we’re in 2015! W00t!

  13. Figure 4: The Pluto system. The region around Pluto and Charon was reduced in brightness so that all four objects could be shown individually in a single image. Photo by David Tholen.

    How long will it be before that explanation is counteracted on the woo sites as being a cover for Planet X passing in the background behind Pluto?

    I’m mostly surprised at the length of the star trails. I wouldn’t have thought that Pluto moved that much visually.

  14. Magnus Björk

    Hey, why not aim Hubble at Pluto? As I understand it, Hubble runs in circles around any telescope on earth (being in orbit, you know), so it should be able to get better pics, right?

    Anyway, no matter how small it turns out to be, Pluto will probably always be a planet in my heart.

  15. tacitus

    What’s really remarkable is that we may have the technology to resolve the disks of nearby extrasolar Earth-link planets to the same extent within a couple of decades (assuming the will and the money remains forthcoming).

    It may be hundreds of years before we reach another solar system, but the prospect of seeing other worlds that might be able to harbor complex lifeforms is tantalizingly close.

  16. Ed

    Magnus Björk: Not really. Hubble has some advantages but with the improvement of ground based observatories (e.g., addition of adaptive optics) since Hubble was launched the difference is not all that huge.

  17. I’m puzzled – the release says these are sharper than anything Hubble can do, but these images have been around for a while and are pretty widely reproduced. Are they just not processing the Keck data to show surface detail?

  18. PK

    There was a joke that if we kept observing Pluto, it would disappear entirely.

    And it did, as a planet…

  19. Matt

    Thanks for the link, Duane. But I’m puzzled– for all Hubble’s fabulousness, how come Pluto’s still just a dot? H does much better, it seems, with stuff that’s much farther away?

  20. Carey

    Nix “Nyx”, it’s “Nix”.

  21. Matt: Nebulae are HUGELY bigger in the sky than Pluto. While Pluto is a few thousand kilometers across, the nebulae and galaxies it usually gets pics of are light years across – bigger by enormous orders of magnatude.

  22. andy

    The old “planet” problem… I reckon the IAU went completely the wrong way about it. What’s a planet anyway? Depends what you’re doing.

    If you want to do a comparison of terrestrial planets in our solar system, you’d probably want to include our Moon and the jovian satellite Io (possibly also Europa) in your set of “planets”, despite them not orbiting the Sun.

    On the other hand, if you’re studying ice planets, numerous outer solar system moons, large asteroids such as Ceres, and EK-belt objects such as Pluto and Eris are examples… after all, our solar system doesn’t contain highly-dynamically-significant ice worlds in solar orbit (ok, there are Uranus and Neptune, but they have hydrogen envelopes which results in significant differences from the smaller ice planets)

    If you’re interested in the overall solar system dynamics, worlds like Pluto and Ceres probably don’t figure all that strongly, so you wouldn’t treat them as planets.

    Given Pluto, Ceres and Eris are members of larger populations, in a general sense of the word “planet” as a significant object in the solar system (which presumably is close to the cultural meaning of the term) I personally don’t think they make the grade, but applying a rigid definition the way the IAU has is not helpful.

    On a completely different subject, it is very fortunate that Charon was discovered before the series of eclipses in the 1980s, otherwise Pluto would still be just a dot and we wouldn’t have the maps of the surface at all.

  23. Thorin

    That picture kind of confuses me. Is Pluto’s orbital path on a “tilt” compared to Earth’s? Does Pluto have a much large axial tilt than Earth? The way the picture is taken I kind of picture them orbiting like the face of a clock (so a pole must be facing us?)

  24. Pluto

    I can haz plantary statuz???

  25. Kullat Nunu

    Recent stellar occultation suggest that Pluto is slightly smaller than believed (its atmosphere makes measuring the diameter difficult because stars don’t disappear or appear instantly). Hopefully there’s something left when New Horizons reaches its destination.

  26. Katie Berryhill

    How is that sharper than the Hubble image that showed details (albeit low-resolution) on the surface of Pluto?

  27. andy

    Katie Berryhill: as I understand it, the Hubble “images” are generated by measuring the brightness of Pluto as it rotates and computing a model of the surface based on that. They are not actually generated from a resolved image of the surface.

  28. slang

    Thorin, yes. Pluto’s orbit is way out of line with (the rest of) the planets in the solar system. Check or use to search for “pluto orbit”.

  29. Anonymous Freak

    The black ‘disc’ comes from the fact that in order to see Nix and Hydra, the exposure was so great that Pluto/Charon were completely overblown. So the Pluto/Charon part of the picture is taken from a lesser exposure overlain onto the Nix/Hydra picture. (See the actual link, and you’ll see all the pictures.)

    The official caption to this pictures is:
    “The Pluto system. The region around Pluto and Charon was reduced in brightness so that all four objects could be shown individually in a single image. Photo by David Tholen.”

    And is the picture of Nix/Hydra that shows how horribly overblown Pluto/Charon are.

  30. Troy

    I’d like to add to the BA’s introduction that Pluto was also believed to be bigger because in the lower resolution optics of the day Pluto and Charon were both part of the same disk. Charon was discovered in an image where they are fused but obviously not completely round.

  31. Troy

    (Reply to Thorin)
    Yes the Pluto system completely faces us, its rotation axis is very much like Uranus giving the moons (and any rings if they were to be found) the appearance of a clock face.

  32. Eric TF Bat

    You just wait until the high-res photos show up and we see the Face on Pluto. And the big caterpillar/worm thing that the scientific community will try to hush up by saying it’s actually a valley. The truth will come out!

  33. Matt

    Phil wrote that Pluto was 4.8 billion kilometers away. That’s nifty and all but I think it sails right over most peoples heads. I wish astronomers would talk about lightspeed more often. I think it’s the key to opening the public’s mind to the vast distances that surround them.

    How far away is pluto?

  34. andy

    You just wait until the high-res photos show up and we see the Face on Pluto. And the big caterpillar/worm thing that the scientific community will try to hush up by saying it’s actually a valley.

    Actually, I’m waiting for the discovery of the Stonehenge replica at Pluto’s north pole.

  35. Reminds me of Father Ted explaining the concept of size vs. distance to Dougal:

    Ted: (Holds up toy plastic cow) “OK, now this one is SMALL, right? And those (points out of window) are FAR AWAY…”

  36. Nigel Depledge

    To add a bit more detail to the Keck vs. Hubble point:

    The Keck telescopes are about 10 m (diameter of objective mirror) compared with Hubble’s 2.3 or 2.4 m (I think – someone plese correct this if I’m wrong). Thus, the resolving power of each Keck telescope is theoretically about four times that of Hubble. Linking the two telescopes gives even better resolution. Additionally, with about 16 times the area (roughly, for one 10 m Keck versus Hubble), the Keck telescope can gather 16x as much light as Hubble and thus record objects that are that much fainter.

    This is complicated by Keck’s being sited on the Earth’s surface and Hubble’s being in orbit.

    Keck’s resolution is degraded by atmospheric turbulence. This is, to a large extent, compensated for by the use of adaptive optics, but Keck will still not be as good a scope as if it were in orbit. So, its resolution will be between 1 and 4 times as good as Hubble, depending on the extent to which the adaptive optics can compensate for atmospheric turbulence (and, presumably, on the seeing conditions).

    Hubble does not have this limitation, but it is a smaller telescope.

    Then there’s the time over which light is gathered: depending on the exact mechanics of the Keck telescope, its maximum exposure time will be limited to the amount of time the telescope can record the same piece of sky. Because the Earth rotates about 15° per hour, this will be no more than (say) about 8 – 12 hours for something near the ecliptic. (This also gives a hint about the star trails in the image: if the exposure was several hours, the Pluto system would have moved against the background stars. With the scope tracking Pluto, this results in the stars appearing as streaks rather than points).

    However, there are times when Hubble’s orbital plane is approximately parallel to the direction of the Earth’s orbit. At these times, Hubble can face away from the sun and record exposures for many many hours (I believe the current longest exposure taken by Hubble was around 30 hours, for one of the Hubble Deep Field images). This cannot be done over the whole sky, because there are some directions in which the Earth would always obscure Hubble’s view for a part of each orbit. I think that, as long as Hubble is facing a direction that is approximately perpendicular to its orbital plane, it can take long exposures without the Earth getting in the way. This is not done all that often, because time on Hubble is very valuable.

    So, you have the Keck telescopes with better resolution than Hubble and a larger area over which to collect light, but with physical limits to the exposure time; and, on the other hand, you have Hubble with a smaller objective but (in principle) longer exposure times available.

    Thus, for a large object with faint features (such as a nebula, that may occupy several degrees across the sky), Hubble will be able to take excellent observations, whereas for smaller objects (such as Pluto), Keck is the better instrument.

    I hope this is useful, and I also hope that, if I have my facts wrong, someone else will come along and correct them.

  37. Nigel, that all makes sense, with the addition that the Keck photos are at 1.6 microns in the infra-red, while the Hubble photos were, as I recall, in blue light. The shorter wavelength allows higher resolution.

    I think the thing that comes off a bit odd is the press release shouting ‘Highest resolution evah!! We pwnd Hubble’ when a moment’s googling can get higher res Hubble images. Bad PR, IMO, and they should anticipate it.

  38. Troy

    I liked that Hubble vs. Keck comparison. An even bigger Keck type telescope is being built the “30 meter telescope” expected 2015.

  39. Alex

    One last thing on the Keck vs Hubble: One of the biggest problem inherent in observing from earth, is that the atmosphere absorbs large chunks of the electromagnetic spectrum before it can reach the ground.

    The net effect of this is that, while you can get really good resolution on earth by virtue of being able to build huuuuuge telescopes like the Keck observatory, you’re limited to a relatively narrow window of frequencies. As it happens, the Hubble mostly observes frequencies you could see on earth, but its lesser known companions in NASA’s Great Observatories program, observe gamma rays, x-rays in ways that fundamentally cannot be duplicated from within the earth’s atmosphere.

  40. Old news. There’s already an even sharper picture taken of Pluto.

  41. Nigel Depledge

    David, that’s a good point. I was assuming they used the same wavelength.

    Alex, you are quite right. Also, the forthcoming James Webb space telescope will observe at several IR wavelengths and will be able to detect very faint sources by virtue of being kept colder than can be achieved on Earth.

  42. Ahh… but wait until they zoom in and reveal the unnameable horror of the Fungi from Yuggoth!!!

  43. Rystefn

    All this talk about Pluto reminds me of Jonathan Coulton’s “I’m Your Moon”

  44. Here’s the deal:

    Tholen wasn’t after surface details but the dynamics of the satellite system, so hasn’t done the requisite processing for the former.

    Hat tip to

  45. Lurchgs

    All I can say is that it’s a good thing that Mother Thing took care of the alien base on pluto!

  46. Trent

    Pluto doesn’t just appear bigger because its brighter; it’s actually resolved in this image (Charon is also, barely, resolved). Pluto is about 10 pixels across here (a point source would appear about 5 pixels across).

  47. Trent

    The SpacEurope website gets the main points across very well.

    One other thing, on the Keck v. Hubble point. These are probably the two best telescopes *ever*, so it seems silly to try to pit the two against each other. In fact, the specifics of the science program in question will mainly determine which telescope you want to use.

    – An individual Hubble exposure will always be 1 micron). However, Keck currently cannot apply adaptive optics corrections at shorter wavelengths at all.

    – Hubble cameras have pretty fat pixels. E.g., the Nix+Hydra Hubble images have the finest pixels of any Hubble imaging camera (ACS-HRC, which is now sadly deceased), and they are about 3x bigger than the Keck camera. The smaller the pixels, the more precisely the positions of the objects can be determined, which is the primary measurement to be made here.

    Another interesting historical note: If Hubble, which has been around quite a while now, discovered these new satellites Nix and Hydra, then why weren’t these discovered *last* decade? . . . because the time allocation committee of Hubble didn’t feel it was worth a try. (And this was back when Pluto was a “planet”.)

  48. ..what’ s sharp about that??I can’ t see anything except black…
    But i’M interested in studying about the solar system and celestial bodies.

  49. Plutonium being from Pluto

    Home sweet home! 😉

    Great picture albiet from a vast distance. Only now stumbled on it.


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