The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation

Abstract

Haumea—one of the four known trans-Neptunian dwarf planets—is a very elongated and rapidly rotating body1,2,3. In contrast to other dwarf planets4,5,6, its size, shape, albedo and density are not well constrained. The Centaur Chariklo was the first body other than a giant planet known to have a ring system7, and the Centaur Chiron was later found to possess something similar to Chariklo’s rings8,9. Here we report observations from multiple Earth-based observatories of Haumea passing in front of a distant star (a multi-chord stellar occultation). Secondary events observed around the main body of Haumea are consistent with the presence of a ring with an opacity of 0.5, width of 70 kilometres and radius of about 2,287 kilometres. The ring is coplanar with both Haumea’s equator and the orbit of its satellite Hi’iaka. The radius of the ring places it close to the 3:1 mean-motion resonance with Haumea’s spin period—that is, Haumea rotates three times on its axis in the time that a ring particle completes one revolution. The occultation by the main body provides an instantaneous elliptical projected shape with axes of about 1,704 kilometres and 1,138 kilometres. Combined with rotational light curves, the occultation constrains the three-dimensional orientation of Haumea and its triaxial shape, which is inconsistent with a homogeneous body in hydrostatic equilibrium. Haumea’s largest axis is at least 2,322 kilometres, larger than previously thought, implying an upper limit for its density of 1,885 kilograms per cubic metre and a geometric albedo of 0.51, both smaller than previous estimates1,10,11. In addition, this estimate of the density of Haumea is closer to that of Pluto than are previous estimates, in line with expectations. No global nitrogen- or methane-dominated atmosphere was detected.

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Figure 1: Light curves of the occultation.
Figure 2: Haumea’s projected shape.
Figure 3: Haumea’s ring geometry.

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Acknowledgements

These results were based on observations made with the 2-m telescope at Wendelstein Observatory, which is operated by the Universitäts-Sternwarte München, the 1.8-m telescope at Asiago Observatory, operated by Padova Observatory, a member of the National Institute for Astrophysics, the 1.3-m telescope at Skalnate Pleso Observatory, operated by the Astronomical Institute of the Slovak Academy of Science, the 1-m telescope at Konkoly observatory, operated by Astrophysical Institute of the Hungarian Academy of Sciences, the 0.65-m telescope at Ondrejov Observatory, operated by the Astronomical Institute of the Czech Academy of Sciences, the 1.5-m telescope at Sierra Nevada Observatory, operated by the Instituto de Astrofisica de Andalucia-CSIC, the 1.23-m telescope at Calar Alto Observatory, jointly operated by the Max Planck Institute für Astronomie and the IAA-CSIC, the Roque de los Muchachos Observatory 2-m Liverpool telescope, operated by the Astrophysics Research Institute of Liverpool John Moores University, the Roque de los Muchachos Observatory 2.5-m NOT telescope, operated by the Nordic Optical Telescope Scientific Association, the 1-m telescope at Pic du Midi Observatory, operated by the Observatoire Midi Pyrénées, and the La Hita 0.77-m telescope, which is jointly operated by Astrohita and the IAA-CSIC. J.L.O. acknowledges funding from Spanish and Andalusian grants MINECO AYA-2014-56637-C2-1-P and J. A. 2012-FQM1776 as well as FEDER funds. Part of the research leading to these results received funding from the European Union’s Horizon 2020 Research and Innovation Programme, under grant agreement no. 687378. B.S. acknowledges support from the French grants ‘Beyond Neptune’ ANR-08-BLAN-0177 and ‘Beyond Neptune II’ ANR-11-IS56-0002. Part of the research leading to these results has received funding from the European Research Council under the European Community’s H2020 (2014-2020/ERC grant agreement no. 669416 ‘Lucky Star’). A.P. and R.S. have been supported by the grant LP2012-31 of the Hungarian Academy of Sciences. All of the Hungarian contributors acknowledge the partial support from K-125015 grant of the National Research, Development and Innovation Office (NKFIH). G.B.-R., F.B.-R., F.L.R., R.V.-M., J.I.B.C., M.A., A.R.G.-J. and B.E.M. acknowledge support from CAPES, CNPq and FAPERJ. J.C.G. acknowledges funding from AYA2015-63939-C2-2-P and from the Generalitat Valenciana PROMETEOII/2014/057. K.H. and P.P. were supported by the project RVO:67985815. The Astronomical Observatory of the Autonomous Region of the Aosta Valley acknowledges a Shoemaker NEO Grant 2013 from The Planetary Society. We acknowledge funds from a 2016 ‘Research and Education’ grant from Fondazione CRT. We also acknowledge the Slovakian project ITMS no. 26220120029.

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J.L.O. planned the campaign, analysed data for the prediction, made the prediction, participated in the observations, obtained and analysed data, interpreted the data and wrote the paper. P.S.-S. helped to plan the campaign, analysed data, helped to interpret the data and helped to write the paper. B.S. helped to plan the campaign, analysed data, interpreted data, and wrote part of the paper. G.B.-R. and D.B. helped to plan the campaign, participated in the observations, and analysed and interpreted data. All other authors participated in the planning of the campaign and/or the observations and/or the interpretations. All authors were given the opportunity to review the results and comment on the manuscript.

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Correspondence to J. L. Ortiz.

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Reviewer Information Nature thanks A. Sickafoose and A. Verbiscer for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Figure 1 Declination residuals of Haumea astrometry.

The plus symbols show the declination residuals of the observed position of the photocentre of Haumea’s system with respect to the theoretical position in declination, from JPL#81 ephemerides. The residuals are shown versus the date of observation. All of the observations were obtained with the La Hita 0.77-m telescope, as explained in Methods. The thin solid line represents a sinusoidal fit to the residuals, with the period determined from a periodogram analysis that is coincident with the orbital period of the moon Hi’iaka. Outlier values have not been removed. In right ascension we did not detect an oscillating behaviour of the residuals because the orbit Hi’iaka does not extend as much as in declination and the quality of the data was not good enough to show the periodicity.

Extended Data Figure 2 Map of Earth showing the locations of the observatories that recorded the occultation (green dots).

The solid lines mark the limits of the shadow path. Mount Agliale is indicated in blue because the occultation by the main body was not positive there, but the occultation by the ring was detected. The dashed line denotes the centre of the shadow path. Note that Munich corresponds to the location of the Bavarian Public Observatory. The complete names of the observatories can be found in Table 1. The red marks at Trebur and Valle D’Aosta observatories indicate the two closest sites to the shadow path that recorded a negative occultation. The coordinates of Trebur observatory are 49° 55′ 31.5″ N, 8° 24′ 40.6″ E and the coordinates of Valle D’Aosta observatory are 45° 47′ 22″ N and 7° 28′ 42″ E. The shadow motion is from the bottom to the top of the figure.

Extended Data Figure 3 Upper limit on Haumea’s N2 atmosphere.

The black filled circles show the normalized flux from the star plus Haumea, as observed from the Asiago station, which is the one that provided the highest signal-to-noise ratio and enough time resolution to look for a faint atmosphere. They combine ingress and egress data and are plotted against the distance perpendicular to the local Haumea limb, as given by the solution shown in Fig. 2. The horizontal bars associated with each data point indicate the distance interval corresponding to the integration times of each point. The red line shows an example light curve obtained with an isothermal N2 atmosphere at T = 40 K and with surface pressure psurf = 15 nbar (at 3σ-level upper limit for better illustration, because the 1σ-level of 3 nbar would be difficult to notice). The red open circles show the expected flux at each data point after convolution with the finite integration point.

Extended Data Figure 4 Upper part of the ring.

An expanded view of Fig. 3 showing in more detail the events along the upper part of the ring. The best fitting mean ring radius is drawn as a solid curve. The grey area shows the full extension of a semi-transparent 70-km-wide ring that is consistent with the twelve secondary events shown in Fig. 3. The lengths of the red segments indicate the uncertainties stemming from the error bars on the ring timings.

Extended Data Figure 5 Photometric models.

a, Absolute V-band magnitude of the Haumea system as a function of time. Diamonds represent observations and lines are models. The cyan curve represents a model without a ring, the black curve is a model with a 70-km-wide ring and with a reflectivity of I/F = 0.09, similar to that of Chariklo’s main ring. The ring in this model contributes approximately 2.5% of the total flux of Haumea plus Hi’iaka in 2017. The dark blue curve corresponds to a model with a wider (140 km) and brighter (I/F = 0.36) ring, which contributes 20% of the total brightness in 2017. This model can be discarded because it would produce a change in the amplitude of the light curve that is too rapid to be compatible with the observations (see b). b, Amplitude of the rotational light curve determined from the ground for the same three models as in a (using the same colour coding). The diamonds represent observations from the literature1,17 and from this work (for 2017). See Methods for further explanations. Error bars show the errors of the measurements from refs 1 and 17 and from the determination in 2017 from this work, shown in Extended Data Fig. 6.

Extended Data Figure 6 Rotational light curve of Haumea.

The relative magnitude versus rotational phase obtained two days after the occultation with the Valle D’Aosta 0.81-m telescope with no filters is shown. The rotational zero phase was established at the time of the occultation and the rotation period used was 3.915341 h. Superimposed is a fit to the observational data. As can be seen, the absolute maximum in magnitude (absolute brightness minimum) is reached at the time of the occultation (arbitrarily located at a phase of 0 here), which means that the projected area of Haumea was also at its minimum. The continuous line is a fit to the data. The peak-to-peak amplitude of the light curve is 0.25 ± 0.02 mag. Error bars are 1σ.

Extended Data Figure 7 Profiles of the trails of the occultation star in two images.

a, Profile along a central line in the trail of the occultation star (blended with Haumea) in a drifted image taken from Crni Vrh observatory before the main occultation. On the y axis we show the light intensity along the line. The line starts 40 pixels before the beginning of the trail and ends 40 pixels after the end of the trail to show the background level and that the transition from trail to background is not easy to identify. The horizontal line marks the mean intensity of the trail. The thick line represents a profile smoothed with a 10-pixel boxcar to filter the high-frequency noise. The x axis has been translated from pixels to time using the drift speed of 40 arcsec per minute, given the known pixel scale of the telescope. The vertical dashed-dotted lines at 0 s and 300 s mark the start and end of the integration, respectively. The ut at start of exposure was 02:59:19.50. The intensity is basically constant with time. Before 0 s and after 300 s, the line profiles decay to the background level of the image because the pixels there were outside the trail. Hence, before 0 s and after 300 s, the plot does not represent the intensity of the source but the intensity of the background. b, Same as a, but from the image at the time of the occultation. The ut at the start of the exposure was 03:04:50.11. The dashed vertical line at 185 s marks the approximate moment at which the occultation begins. The smoothed curve shows that a clear drop in the signal is produced and lasts until the end of the 300-s integration. ADU, analog-to-digital unit.

Extended Data Table 1 Data on the occulted star
Extended Data Table 2 Timing of the secondary brief occultation events from the different observing sites on 21 January 2017

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Ortiz, J., Santos-Sanz, P., Sicardy, B. et al. The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation. Nature 550, 219–223 (2017). https://doi.org/10.1038/nature24051

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