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Occultation Helps Scientists Study Large Plutino Ixion and Learn More About a Star

Plutino Ixion

Images from the 4.3-meter Lowell Discovery Channel Telescope (LDT) showing the disappearance of the target star during the occultation. (left) The full field of view, with white circles indicating the occultation star (“Occ”), two comparison stars (“Comp A” and “Comp B”), and two sky regions (“Sky 1” and “Sky2”). Each of these regions were used in the data analyses. (right) Zoomed portion of the field taken during the occultation, when Ixion blocked the light from the star. Background stars that are not apparent on the left are visible. The exposure time was only 0.33 seconds; therefore, Ixion itself is too faint to see in this image. (Credit:  modified Fig. 3 from Levine et al. 2021)

PSI’s Amanda Sickafoose is a coauthor on a paper that discusses Ixion, a large Plutino which is a class of minor bodies located in the 3:2 orbital resonance with Neptune. Ixion was predicted to pass in front of a bright star (Gaia magnitude 10.3) on Oct. 13, 2020. The shadow path, expected to be less than 1,000 kilometers wide, passed over parts of nighttime Arizona and California. Successful observations were made from two telescopes in Arizona.

The occultation itself lasted just under 45 seconds. Ixion is roughly four thousand times fainter than the star it occulted.  As shown in the image above, the star was completely visible beforehand and completely blocked out during the occultation. By observing this event, the research team was able to place a lower limit on Ixion’s diameter of 709.6 kilometers (assuming a spherical shape). This size confirms Ixion as being in the top four largest Plutinos and a possible dwarf planet. The work also placed an upper limit of 2 microbars for the pressure of any global atmosphere on Ixion. The lack of atmosphere on Ixion can be compared to Pluto’s tenuous atmosphere of approximately 10 microbars.  These are the most accurate measurements of these characteristics for Ixion to date.

Although the goal of the observations was to learn more about Ixion, the star was found to be bigger than expected. Going into and out of the occultation, gradually fading and reappearing starlight was an indication that the star was large with respect to the spatial resolution of the images. Analyses determined that the star’s radius is 130 Solar radii. Only a small number of this type of star, M5 III, has directly measured sizes. Spectral data show that the star is likely a mid-M red giant, which is consistent with the large size.

The publication is led by Stephen Levine of Lowell Observatory and is entitled “Occultation of a Large Star by the Large Plutino (28978) Ixion on 2020 October 13 UTC.” It is available through open access in the Astronomical Journal at

May 9, 2021
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