The variety of TNO orbits provides valuable insight into the early evolution of the solar system, particularly the outward migration of Uranus and Neptune. However, it is NASA's James Webb Space Telescope (Webb) that has significantly advanced our understanding by examining the surface compositions of these distant objects. Bryan Holler and John Stansberry of the Space Telescope Science Institute (STScI) in Baltimore highlight how Webb is revolutionizing our knowledge of TNOs.
Pluto was the first TNO identified, discovered in 1930 by Clyde Tombaugh at the Lowell Observatory. The second TNO, 1992 QB1 (now named Albion), was not discovered until 1992 by Dave Jewitt and Jane Luu. Since then, over 5,000 TNOs have been cataloged. Their orbits preserve a historical record of how the early solar system's giant planets - Jupiter, Saturn, Uranus, and Neptune - evolved. Computational models suggest that Uranus and Neptune, during their migration, displaced many TNOs while trapping others into their current orbits.
Present-day TNO orbits are categorized based on their distances from the Sun, eccentricity (ellipticity), and inclination (tilt relative to the plane of planetary orbits). Of particular importance are the "cold-classical" objects, which have low eccentricities and inclinations, suggesting they have remained in their primordial orbits. These TNOs are considered the untouched remnants of the early solar system, and Arrokoth, studied up close by the New Horizons mission in 2019, is a prime example.
For TNOs that experienced orbital perturbations during planetary migrations, determining their original formation locations is challenging. However, analyzing their surface compositions offers clues about the makeup of the early solar system. These distant objects, at temperatures below minus 280 degrees Fahrenheit (minus 170 degrees Celsius), retain the chemical signatures of the original protoplanetary disk. Webb's large primary mirror and advanced instruments, particularly the Near Infrared Spectrograph (NIRSpec), provide an unprecedented look into TNO compositions.
Webb's NIRSpec captures spectra by dispersing light across wavelengths from 1 to 5 microns, revealing the molecular makeup of observed objects. Due to their formation in the cold outer solar system, TNOs were expected to exhibit surface compositions dominated by volatile ices such as water (H2O), carbon dioxide (CO2), nitrogen (N2), and methane (CH4). Over time, solar and cosmic radiation transformed these materials into complex hydrocarbons like methanol (CH3OH), acetylene (C2H2), and ethane (C2H6). Webb's observations have confirmed these expectations while also revealing unexpected compositional variations.
During its first two years of operation, Webb has obtained high-resolution spectra for more than 75 TNOs, including nearly 60 objects examined under the Large Cycle 1 program "DiSCo-TNOs" (program ID #2418, PI: Noemi Pinilla-Alonso). This dataset has led to the discovery of three distinct spectral classifications, an entirely unforeseen development. The classifications, based on spectral features in the 2.5-4 micron range, include:
- Bowl-type spectra, characterized by dominant water ice absorption features, carbon dioxide ice, and silicate-rich dust.
- Double-dip spectra, containing complex organic molecules, carbon dioxide, and carbon monoxide ices, with pronounced reflectance peaks at the 4.27-micron band - never observed outside laboratory conditions.
- Cliff spectra, exhibiting even more complex organics, high concentrations of carbon dioxide, and methanol (CH3OH) signatures.
These spectral types also correlate with the visible light color of TNOs, with bowls being the least red, double-dips displaying intermediate redness, and cliffs appearing the most red. Researchers hypothesize that these differences stem from formation temperatures, with Bowl-type TNOs forming closer to the Sun, where heat drove off volatile ices, while Double-dip and Cliff-type objects formed in colder regions, preserving these compounds. Notably, all TNOs on stable, cold-classical orbits fall into the Cliff category, while dynamically scattered TNOs exhibit all three types, supporting the theory of planetary migration redistributing these objects.
Looking ahead, Webb will continue extensive TNO observations. In its third cycle of research, it will investigate TNO satellites, conduct spectral analyses of extreme TNOs that venture into interstellar space, and revisit previously studied objects for deeper insights. Additional programs will explore TNO binary systems to uncover whether their moons formed through collisions or gravitational interactions.
With each new observation, Webb enhances our understanding of the outer solar system, offering invaluable clues about the early solar system's formation and evolution. The coming years promise even more revelations about these distant and ancient objects.
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