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Trans-Neptunian Objects Under the Spotlight

When the James Web Space Telescope (JWST) was launched almost two years ago now—time flies—one of the key observations I was looking forward to would be those of the trans-Neptunian objects (TNOs). These are celestial bodies that orbit the Sun at a greater distance than Neptune, and of which we know so little given their remote location.

Common types of trans-Neptunian objects include:

  • Plutinos: Objects that share an orbital resonance with Neptune, such as Pluto.

  • Cubewanos: Objects that have a more classical and less eccentric orbit beyond Neptune. These are also known as Classical Kuiper belt objects.

  • Scattered Disc Objects: TNOs with more eccentric and elongated orbits, often influenced by gravitational interactions with Neptune.

  • Detached Objects: TNOs with orbits that are not significantly influenced by Neptune.

Some TNOs are large enough to be classified as dwarf planets, which, according to the IAU, is an object that orbits the sun, has sufficient mass for a nearly round shape, but—and here comes the key distinction with a planet—has not cleared its orbit of other debris and celestial bodies. Eris, Sedna, Makemake, Haumea, and Pluto are some examples of TNOs which are big enough to be classified as dwarf planets, with estimations ranging from 100 to 200 total dwarf planets sitting outside of Neptune’s orbit.

So, what do we know of these dwarf planets? Apart from Pluto which was visited by New Horizons in 2015—and is still, one of the most remarkable visits ever made—we know very little. In short, TNOs are a population of very cold objects dating back from the formation of our solar system, and whose surfaces contain a mixture of water, methane, and nitrogen ices with tholins. Pluto and Charon are a great example of this.

Recently though, JWST examined with its NIRSpec instrument three dwarf planets: Sedna, Gonggong, and Quaoar. So, what have we discovered?

As explained in the latest paper published on these observations, “Their surfaces are covered with complex organic molecules that are the products of irradiation of methane (CH4).

  • Sedna's spectrum shows a large number of absorption features from ethane (C2H6), as well as acetylene (C2H2), ethylene (C2H4), H2O, and possibly minor CO2.

  • Gonggong's spectrum reveals fewer and weaker ethane features, along with stronger and cleaner H2O features and CO2 linked with other molecules.

  • Quaoara’ spectrum shows even fewer and weaker ethane features, the deepest and cleanest H2O features, possibly due to hydrogen cyanide (HCN), and CO2 ice. There are also several bands of ethane and methane.

The differences in the apparent abundances of irradiation products are likely due to their distinctive orbits, which lead to different timescales of methane retention and different charged particle irradiation environments.”

Such results indicate that there must be a regular supply of methane to these surfaces, suggesting internal melting and geochemical evolutions comparable in complexity to what we see on Pluto (or Triton, a captured TNO). Put differently, these faraway worlds ought to exhibit the same level of uniqueness and diversity found on Pluto. With JWST expected to operate for many years to come, regular observations of these distant and fascinating objects will take place. We live in remarkable times.

Image credit: Artist's impression of noontime on Sedna. Credit: NASA, ESA and Adolf Schaller


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