Namaka’s Shadow: Catching a Tiny Moon Eclipse a Distant Star
In a recent research note, Flavia L. Rommel and collaborators reported an exciting astronomical achievement: the first confirmed stellar occultation by Namaka, a small moon of the dwarf planet Haumea. Stellar occultations, where a distant solar system object passes in front of a star and briefly blocks its light, are powerful tools for measuring the sizes, shapes, and orbits of faraway worlds. These fleeting events help astronomers study objects that are otherwise too small or too distant to observe clearly. In this case, the occultation captured both Haumea and Namaka, offering new clues about the nature of this unusual binary system in the outer solar system.
Finding the Right Moment
The team began their work by predicting when and where the occultation would occur. Using a previously published orbit for Namaka and detailed positional data for Haumea, they calculated that both bodies would pass in front of the same star on March 16th, 2025. This alignment was predicted using data from the NIMA ephemeris and a specialized tool called sora. The researchers intentionally ignored a minor effect called Haumea’s barycentric wobble, assuming that the star’s path relative to Haumea would still be accurate. The prediction turned out to be impressively precise: both events happened within a second of the predicted time.
Catching the Shadow
To observe the event, the team used NASA’s Infrared Telescope Facility (IRTF) in Hawaii, equipped with a fast camera called MORIS. By taking images every 0.3 seconds, they recorded the brief dimming of the star as each object passed in front of it. The star’s light dropped by about 45% when Haumea occulted it and slightly during Namaka’s passage. They then analyzed the data using tools like astropy and praia, which allowed them to isolate and measure the light loss precisely. By comparing the light from the target star to nearby stars, they corrected for atmospheric effects and extracted clean light curves that showed exactly when the occultations began and ended.
Measuring Namaka
These measurements revealed that Namaka’s shadow stretched 83 ± 2 kilometers across—a distance known as a "chord," representing the part of Namaka’s silhouette that crossed Earth’s view. Because only one chord was observed, it only sets a lower limit on Namaka’s actual diameter, which could be larger depending on the object’s shape and how it was oriented. Assuming Namaka is spherical and using its estimated mass, the researchers calculated that its density must be less than or equal to 4000 kg/m³. For comparison, water has a density of 1000 kg/m³, so this result leaves room for both rocky and icy compositions.
Probing the Haumea System
This occultation also opens the door to deeper investigations of the Haumea system. Haumea is a rapidly rotating, egg-shaped dwarf planet with at least two moons, Hiʻiaka and Namaka. Previous occultations have already provided data on Hiʻiaka. With this new result from Namaka, astronomers can refine their models of the system's orbits and even begin to measure Haumea’s gravitational harmonics—numbers that describe how its mass is distributed. One key harmonic, J₂, tells us how "flattened" Haumea is due to its spin and internal structure. Depending on how differentiated Haumea is (i.e., whether it has a rocky core and icy outer layer), J₂ could range from about 0.16 to 0.24. Understanding these values helps scientists figure out what lies beneath Haumea’s surface.
Small Shadows, Big Discoveries
Although this study used only a single observation, it provides a critical step toward understanding how this remote and unique system formed. Namaka is part of the only known collisional family in the trans-Neptunian region—a group of objects believed to have originated from a massive impact involving Haumea. To fully unravel this history, more occultation data will be needed in the future, especially multi-chord events that capture the object from different angles.
The Power of Precision Astronomy
In all, Rommel and her team demonstrated the power of precise prediction, rapid observation, and careful data analysis in exploring the distant reaches of our solar system. Even a small moon like Namaka, casting a faint shadow across a star for just a few seconds, can reveal big secrets about the structure and history of a dwarf planet billions of kilometers away.
Source: Rommel
