The Outer Worlds

17 February 2024

Illustration of the icy dwarf planets Eris and Makemake. Southwest Research Institute
Illustration of the icy dwarf planets Eris and Makemake.

Whether or not you agree that Pluto isn’t a planet, in many ways, Pluto is quite different from the classical planets. It’s smaller than the Moon, has an elliptical orbit that brings it closer to the Sun than Neptune at times, and is part of a collection of icy bodies on the edge of our solar system. It was also thought to be a cold dead world until the flyby of New Horizons proved otherwise. The plucky little spacecraft showed us that Pluto was geologically active, with a thin atmosphere and mountains that rise above icy plains. Geologically, Pluto is more similar to Earth than the Moon, a fact that has led some to reconsider Pluto’s designation as a dwarf planet.

Astronomers still aren’t sure how Pluto has remained geologically active. Perhaps the gravitational interactions with its moon Charon, or perhaps interior radioactive decay. But regardless of the cause, the general thought has been that Pluto is an exception, not a rule. Other outer worlds of similar size and composition are likely dead worlds. But a new study shows that isn’t the case for at least two dwarf planets, Eris and Makemake.1

Telescopic images of Makemake (left) and Eris (right). NASA/ESA
Telescopic images of Makemake (left) and Eris (right).

This new study doesn’t rely on high-resolution images like we have for Pluto. Our current observations of Eris and Makemake show them only as small, blurry dots. But we do have spectral observations of these worlds, which is where this study comes in.

The team looked at the spectral lines of molecules on the surface of these worlds, most specifically that of methane. Methane, or CH4 has two important variants. One is composed of standard hydrogen atoms, while the other contains one or more atoms of a type of hydrogen known as deuterium. Deuterium has a nucleus containing a proton and neutron rather than just a proton, and this skews the spectrum of methane a bit. From the spectral observations, the team could measure the D/H ratio for methane on both worlds.

How D/H ratios compare to possible origins. Glein, et al
How D/H ratios compare to possible origins.

This ratio is determined by the source of the methane. If Eris and Makemake are dead worlds, then the methane they have stems from their origin more than 4 billion years ago, and the D/H level should be on the higher end. On the other hand, if the surface methane was generated through an interior process and vented through active geological processes, then the D/H ratio should be lower. The team found that the ratio is most consistent with thermogenic and abiotic mechanisms, suggesting that both Eris and Makemake are active worlds, or at least were active in geologically recent times.

Eris is about the same size as Pluto, so it isn’t too surprising that it’s a geologically active world given what we now know about Pluto. But Makemake is much smaller, about 60% the size of Pluto. If Makemake is an active world, then it is likely that other dwarf planets such as Haumea are as well. If that’s the case, then most if not all dwarf planets are geologically active. As the authors suggest, it might be worth sending a probe or two to the outer worlds for more study.

  1. Glein, Christopher R., et al. “Moderate D/H ratios in methane ice on Eris and Makemake as evidence of hydrothermal or metamorphic processes in their interiors: Geochemical analysis.” Icarus (2024): 115999. ↩︎