The James Webb Space Telescope has found that distant dwarf planets might be methane factories.

Makemake and its moon artwork
This artist's illustration depicts the distant dwarf planet Makemake, which lies beyond Neptune's orbit.
NASA / ESA / Alex Parker

Big new telescopes discover surprises anywhere they point, and the space between Neptune and the nearest stars is no exception. Unfailingly, the new telescopes answer some questions (or, at least, settle some arguments) but generate lots more questions. It’s career security for space scientists!

Eris and Makemake are two of the largest trans-Neptunian objects (TNOs), denizens of the space beyond Neptune. Most TNOs have extremely dark surfaces, the result of being weathered by solar and cosmic radiation for billions of years. However, the biggest TNOs — Eris and Makemake along with Pluto, Charon, Haumea, Sedna, and the former TNO Triton — are much brighter. That brightness suggests some kind of activity refreshes the surface, but it isn’t proof, and doesn’t suggest what kind of activity that might be. We’ve seen Triton, Pluto, and Charon up close, and they’re all different; so it’s anybody’s guess what’s going on with the worlds we haven’t seen.

Side-by-side comparison of Eris and Makemake surfaces and sizes
The icy dwarf planets Eris and Makemake are shown in an artistic comparison of their likely surfaces as well as their sizes. Located in the Kuiper Belt, a vast donut-shaped region of icy bodies beyond the orbit of Neptune at the edge of the solar system, Eris and Makemake are comparable in size to Pluto and its moon Charon.
Southwest Research Institute

Enter the James Webb Space Telescope (JWST), which is barging into all kinds of space puzzles these days. We knew already that both Eris and Makemake had methane ice on their surfaces. Now, a team of scientists has pointed JWST at them to measure isotopic ratios of hydrogen and carbon in their methane ice. The results appear in Icarus (paper 1, paper 2).

Why are isotopes important? When Eris and Makemake formed, they formed with at least some hydrogen in their water and rocks. That hydrogen would have had the same proportion of deuterium (heavy hydrogen) as other primordial objects, such as comets. Any methane that accreted along with the water and rocks would have the same proportion of deuterium.

But JWST did not find comet-like amounts of heavy hydrogen on Eris and Makemake. Instead, Eris and Makemake’s methane has much less deuterium than expected. What does that mean? In short: Hot chemistry has made “new” methane on both worlds.

There are two main ways to make new methane (without invoking space cows or any other kind of life). If you heat up rocks that contain organic (carbon-rich) materials and also water-containing minerals, the heat will break apart the organic molecules, and loose carbons can bond with hydrogen to make methane. This process doesn’t even need liquid water to work and can happen anywhere with rocks of the necessary composition above 150°C (300°F). This newly formed methane would then find its way out through cracks and pores to freeze onto the surface.

The second way to make new methane without life — that is, abiotically — is to run hot water through rocks that contain small carbon molecules, such as carbon monoxide and carbon dioxide. These molecules are typically in ice form at trans-Neptunian temperatures, and would have been abundant when Eris and Makemake were first forming. If the two worlds became hot enough in the distant past to partially melt and form internal oceans, then the water contacting the rocks would’ve made all kinds of chemical reactions happen — including ones that generate methane gas abiotically. Depending on how hot the worlds became, there could have been some pretty spectacular methane-driven volcanism, driven by the buildup of gas within the deep ocean.

Diagrams depicting the creation of "new" methane
Scientists used data from the James Webb Space Telescope to model the subsurface geothermal processes that could explain how "new" methane ended up on the surfaces of Eris and Makemake, two dwarf planets in the distant Kuiper Belt. The illustration points to three possibilities, including the potential that liquid water exists within these icy bodies at the edge of the solar system, far from the heat of the Sun.
Southwest Research Institute

So JWST has uncovered evidence that Pluto and Triton are not alone in having youthful surfaces that have been refreshed by geologic processes: Eris and Makemake have probably also experienced at least some geology and, possibly, explosive methane volcanism. If Makemake were volcanic in the past, it might not be at present because it’s not as big; the Uranian moon Titania might be what Makemake looks like today. However, Eris is much bigger — significantly more massive than Pluto, actually — so it’s very likely that Eris remains, like Pluto, active today.

At the end of the paper, the authors muse that “visits to Eris and Makemake with spacecraft are intriguing to ponder.” They sure are.


Image of Mark Holm

Mark Holm

February 29, 2024 at 8:16 am

This article does not explain either hiw different deuterium-hydrogen ratios came to be or how they imply active chemistry in TNOs. If these bodies formed in part of the solar nebula with a particular D/H then how did they come to have parts of themselves with different D/H? Chemistry famously does not make new isotopes, though it may contribute tomsorting existing isotopes. What is thenproposed mechanism here and why does it imply methane producing chemistry?

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Andrew James

March 1, 2024 at 5:18 pm

This might answer your question. One possibility is that these objects formed in regions of the outer solar system where the ratios of deuterium to hydrogen were different from those closer to the Sun. Another possibility is that TNOs experienced interactions with other bodies or were affected by processes such as radiation bombardment, which could have altered their deuterium-hydrogen ratios over time. Differences in deuterium-hydrogen ratios could indicate the presence of volatile compounds on the surface of TNOs that have reacted with solar radiation or other external factors. Study of deuterium-hydrogen ratios in TNOs is a valuable tool for understanding the chemical processes and environmental conditions.

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Monica Young

March 28, 2024 at 11:03 am

Thank you for your comment. While it is true that chemical reactions cannot change the isotopes of the elements involved, chemical reactions *can* favor lighter over heavier isotopes. I hope that helps!

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