It Was There All Along

Hubble Space Telescope Images Reveal Neptune’s Moon

This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, 4.8 billion kilometers from Earth. The moon is so small (no more than 12 miles across) and dim, it was missed by NASA’s Voyager 2 spacecraft cameras when the probe flew by Neptune in 1989. Several other moons that were discovered by Voyager appear in this 2009 image, along with a circumplanetary structure known as ring arcs. The black-and-white image was taken in 2009 with Hubble’s Wide Field Camera 3 in visible light. Hubble took the color inset of Neptune on August 19, 2009. Credit: NASA, ESA, and M. Showalter (SETI Institute).

Neptune is the most distant planet in our solar system, which makes its systems of moons and rings a challenge to observe from Earth. These things are small and dim, even in Hubble Space Telescope images and data.

On July 1, Dr. Mark Showalter, a scientist at the SETI Institute in California, spotted something in Hubble images that hadn’t been seen before circling around the giant planet. Or least, it hadn’t been seen by human eyes. It was a small moon orbiting the planet about once every 23 hours. He was actually studying faint arcs (segments) of rings. “The moons and arcs orbit very quickly, so we had to devise a way to follow their motion in order to bring out the details of the system,” he said. “It’s the same reason a sports photographer tracks a running athlete—the athlete stays in focus, but the background blurs.”

It turns out that this little moon, now dubbed S/2004 N1, was in images taken by Hubble between 2004 and 2009, but it’s extremely small and dim. It’s likely no one was looking directly for that moon, so it remained hidden in the data archives, waiting for Dr. Showalter to find it.

This finding is a great example of archival discoveries—that is, findings made as astronomers go back through earlier observations. They do this for a variety of reasons. Sometimes it’s to check and see if the object ever appeared to observers before. Or, as in the case of a supernova explosion, to see what the object that exploded looked like in earlier observations. Or, sometimes, they do it to chart change over time (for example, changes on the surface of Mars).

Prior observations of astronomical objects through the world’s panoply of telescopes (both ground-based, orbiting, and in situ (at planets) give us a good feel for how the planets, moons, rings, and distant objects like stars, nebulae, and galaxies change over time. The universe is a constantly evolving place, and it will always provide us with new objects and events to discover and study.


  1. Neptune is not the solar system’s most distant planet. It is the solar system’s most distant gas giant. Please do not report one point of view as fact when that viewpoint represents just one side in an ongoing debate. To many planetary scientists, the solar system’s most distant planet is either Eris or Sedna.

  2. C.C. Petersen

    Laurel, thank you for writing. You are correct that this is a matter of opinion. Right now, Pluto, Sedna, and Eris are classified as dwarf planets and that is what I will call them until I hear differently. As much as I do not like the IAU’s decision-making process, I do somewhat agree with the classification of dwarf planets as they stand in most respects. I don’t agree with all respects. However, in the interests of keeping the discussion clear, they are currently classified as a dwarf planets. This article is not about Pluto, so let’s stay focused on what it IS about. It is about Neptune. My next entry is about Pluto, which I’m sure will also elicit some discussion. We shall have to agree to disagree on this; and I do appreciate your point of view on this.

  3. How about counting dwarf planets as a subclass of planets? That was the intention of Dr. Alan Stern when he coined the term, and that is how many astronomers view dwarf planets in spite of the IAU vote. I think it’s important to emphasize that a vote by the IAU does not make something “fact” or “true.” Another option is to either note that the subject is a matter of ongoing debate or to sidestep it entirely by referring to Neptune as the outermost gas giant in the solar system. Thank you for welcoming my comments and for your respectful response.

  4. C.C. Petersen

    Laurel, I believe that in my previous comments I called them dwarf planets, which implies they ARE a subclass of planets. That’s the part I agree with. Other aspects of WHY they are dwarf planets (inherent in the definition), are not so clear-cut, in my opinion.

  5. I’m glad you agree with the idea that dwarf planets are a subclass of planets. When we include them under the broad “umbrella” of planets, Neptune is no longer the furthest of all planets from the sun (and neither is Pluto).

    Regarding Neptune, I would like to see orbiters like Galileo and Cassini sent to the Uranus and Neptune systems. It makes no sense to cut funding for planetary science when we have come so far in exploring the solar system. These types of missions are what excite people about science.

  6. C.C. Petersen

    When the IAU’s action first came to light, declaring Pluto a dwarf planet, I felt it was a correct application of what Stern and others had suggested all along. So, in that sense, I agree with it. I do NOT agree with the process, nor do I think the definition is set in stone. It needs to be revisited, as with any other scientific definition.

    Missions to the outer planets should have continued after Voyager and Cassini. Of course, we’re “lucky” to get New Horizons, but to my way of thinking, we should have had about a dozen of them! 😉

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