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The seventh inner moon of Neptune


During its 1989 flyby, the Voyager 2 spacecraft imaged six small moons of Neptune, all with orbits well interior to that of the large, retrograde moon Triton1. Along with a set of nearby rings, these moons are probably younger than Neptune itself; they formed shortly after the capture of Triton and most of them have probably been fragmented multiple times by cometary impacts1,2,3. Here we report Hubble Space Telescope observations of a seventh inner moon, Hippocamp. It is smaller than the other six, with a mean radius of about 17 kilometres. We also observe Naiad, Neptune’s innermost moon, which was last seen in 1989, and provide astrometry, orbit determinations and size estimates for all the inner moons, using an analysis technique that involves distorting consecutive images to compensate for each moon’s orbital motion and that is potentially applicable to searches for other moons and exoplanets. Hippocamp orbits close to Proteus, the outermost and largest of these moons, and the orbital semimajor axes of the two moons differ by only ten per cent. Proteus has migrated outwards because of tidal interactions with Neptune. Our results suggest that Hippocamp is probably an ancient fragment of Proteus, providing further support for the hypothesis that the inner Neptune system has been shaped by numerous impacts.

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Fig. 1: Detections of Hippocamp in 2004–2016.
Fig. 2: Image processing steps leading to the discovery of Hippocamp.

Data availability

All source data used in this study are in the public domain and may be obtained from the STScI archive at The Voyager images referenced in this paper can be retrieved from NASA’s Planetary Data System at Data files for every image analysed in this investigation, at nearly every intermediate step in the analysis, are permanently archived at


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Support for this work was provided by NASA through grant numbers HST-GO-10398, -11656 and -14217 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. Additional support for M.R.S. and R.S.F. was provided by NASA’s Outer Planets Program through grant NNX14AO40G. We thank A. Roman of the Space Telescope Science Institute for extensive support during the planning of the HST observations. M. Brozovic of the Jet Propulsion Laboratory provided numerical integrations to help us identify detections of Naiad.

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Nature thanks T. Becker and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Authors and Affiliations



M.R.S., I.d.P. and J.J.L. are co-investigators in the HST programmes that led to the discovery of Hippocamp. M.R.S. and R.S.F. performed the data analysis and modelling. I.d.P. provided additional data analysis methods that contributed to our interpretation of the results. J.J.L. contributed the theoretical analysis and interpretation of the Neptune system’s long-term evolution and the origin of Hippocamp. All authors contributed to the final version of the manuscript.

Corresponding author

Correspondence to M. R. Showalter.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Recovery of Naiad.

a, b, Portions of an HST image after processing and co-adding as described in the text. The location of Naiad in each panel is indicated by a small square; close-ups are shown in the upper-right insets. The outline of Neptune’s disk is indicated by a blue ellipse. a, View from Visit 01, orbit 1 of HST programme GO-11656, obtained on 2009 August 19. The image shows the first unambiguous detection of Naiad since the 1989 Voyager flyby of Neptune. b, View from Visit 08, orbit 2 of programme GO-14217, taken on 2016 September 2.

Extended Data Fig. 2 Phase curves of Neptune’s inner moons.

ag, Measurements of disk-integrated reflectance D = I/F versus phase angle for each of Neptune’s inner moons, obtained through broad visual filters. Error bars are ±1σ. Colours indicate the instrument, filter and observing mode, as defined in the legend. Solid lines are least-squares linear fits to the data; dotted lines indicate the range of the uncertainty in the model, ±1σ, as derived from the covariance matrix of each fit. The values in Table 1 correspond to the mean and uncertainty extrapolated to phase angle α = 0.

Extended Data Fig. 3 Deep searches for small moons.

a, b, Multiple HST images co-added into a ‘map’ in which longitude increases from 0° to 360° along the horizontal axis and radial position is 0–400,000 km along the vertical axis. a, View derived from the five HST orbits of programme GO-11656, obtained on 2009 August 19. b, View from the two orbits of Visit 03 in HST programme GO-14217, taken on 2016 September 2.

Extended Data Fig. 4 Diagram of the Neptune system.

All of the known features of the Neptune system interior to Triton are shown to scale. (Triton orbits about three times farther out than Proteus.) Rings and arcs are shown in green. Moon shapes are indicated by red ellipses indicating their dimensions a × c, enlarged relative to their orbits by a factor of 20.

Extended Data Fig. 5 Image processing steps.

a, Image icwp01n7q_flt.fits, taken on 2016 August 31. b, The same image after hot pixels and cosmic-ray hits have been removed. c, The boolean mask, where white indicates pixels ignored in further analysis. d, The image after the mean of other images from the same HST visit have been averaged and subtracted. This step removes most of the glare. e, The image after an unsharp-masking process involving the subtraction of a median-filtered version of the same image. The outline of Neptune’s disk is indicated by a blue ellipse in each panel.

Extended Data Table 1 Measurements of Hippocamp obtained in this study
Extended Data Table 2 Measurements of Naiad obtained in this study
Extended Data Table 3 Comparison of projected mean longitudes at three epochs
Extended Data Table 4 Candidate Voyager images of Hippocamp

Supplementary information

Supplementary Data

This file contains Source Data for Table 1.

Video 1: Annotated video of Hippocamp with Proteus.

This six-frame video shows Hippocamp just to the left of Proteus on 2016 August 31 (Visit 01, orbit 2 of GO-14217). Images are full-size. Timing is sped up by a factor of 500. The proximity of Proteus, moving in the same direction and at nearly the same speed, guides the eye and makes it easier to see the smaller moon. The disk of Neptune is shown in blue and the orbits of the two moons are drawn in yellow. A red circle identifies Hippocamp both in the full frame and in the white square enlarged and inset at lower left.

Video 2: Un-annotated video of Hippocamp with Proteus.

This six-frame video shows Hippocamp just to the left of Proteus on 2016 August 31 (Visit 01, orbit 2 of GO-14217). Images are full-size. Timing is sped up by a factor of 500. The proximity of Proteus, moving in the same direction and at nearly the same speed, guides the eye and makes it easier to see the smaller moon. The area inside the white box is enlarged and inset at lower left. The disk of Neptune is shown in blue.

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Showalter, M.R., de Pater, I., Lissauer, J.J. et al. The seventh inner moon of Neptune. Nature 566, 350–353 (2019).

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