Catching Makemake’s Shadow: A New Look at Its Mysterious Moon

In this paper, Daniel Bamberger takes a fresh look at archival images of the dwarf planet Makemake taken with the Hubble Space Telescope (HST) between 2015 and 2019. These images not only confirm the existence of a small moon, called S/2015 (136472) 1 (nicknamed MK2), but also allow astronomers to estimate its orbit. Although MK2 was first announced in 2016, very little work has been published since. Bamberger’s goal is to bring attention back to these observations, especially because Makemake and its moon may soon begin a series of “mutual events”,  such as eclipses and transits, like when one object passes in front of the other. Catching such events could provide new details about both bodies.

Observations and Measurements

The Hubble images used here come from 13 different days between 2015 and 2019. MK2 is visible in twelve of them, though on one occasion it was hidden too close to Makemake itself. Each image set was carefully processed to reduce Makemake’s glare and highlight the faint moon. Bamberger then measured MK2’s position relative to Makemake, converted those distances into kilometers, and used them to fit a circular orbit. Longer exposures were not included in this analysis because they would be more useful for brightness studies, rather than orbit measurements.

Results

From this work, the best-fit orbit for MK2 has a period of about 18 days and a semi-major axis of about 22,000 kilometers. The orbit is tilted at nearly 84° relative to our line of sight, meaning we see it almost edge-on. This geometry is especially important: it raises the likelihood that mutual events are happening right now or will begin soon. Each event could last around 140 minutes and would occur twice during each orbit.

By combining Makemake’s estimated size with this orbital solution, Bamberger calculates the system’s total mass and density. The result is slightly less dense than earlier work by Alex Parker and colleagues, but still consistent with a largely icy world. The orbit’s orientation (whether the moon travels in the same direction as Makemake spins, or the opposite) remains unclear.

Why It Matters

The last time something like this was observed was with Pluto and its moon Charon in the 1980s. Those events allowed astronomers to map out Pluto’s surface and refine its size. A similar opportunity may now exist for Makemake. If telescopes like the James Webb Space Telescope are used, they could reveal more about MK2’s orbit and help determine whether mutual events are indeed observable.

Conclusion

Bamberger emphasizes that these results are preliminary but highlight the untapped potential of Hubble’s data. With careful planning, astronomers might still catch Makemake and its moon in the act of passing in front of one another, unlocking new details about one of the most mysterious dwarf planets in our Solar System.

Source: Bamberger