NASA's Hubble and James Webb Space Telescopes have uncovered surprising clues about the earliest days of our solar system by examining some of the smallest and faintest objects ever observed beyond Neptune. The findings challenge expectations about how planets form and suggest that these distant, icy bodies have preserved evidence of their origins for billions of years. For the first time, astronomers combined observations from Hubble and Webb to investigate trans-Neptunian objects (TNOs), tiny bodies that travel around the Sun far beyond Neptune's orbit.
Researchers discovered 27 previously unknown objects, but their findings revealed an unexpected puzzle. There were fewer small objects than some theories predicted, and their colors closely resembled those of much larger objects in the same distant regions. These frozen remnants offer a rare opportunity to study the materials that eventually became planets.
Most are extraordinarily difficult to detect, shining more than 100 million times more faintly than objects visible to the naked eye. Two related studies published in The Astronomical Journal examined the newly identified objects to learn more about their sizes, compositions, and ancient history. Ancient Building Blocks of the Solar System Scientists believe planets began forming when tiny particles of dust and pebbles orbiting the young Sun gradually accumulated into larger bodies.
Over time, these materials formed objects roughly the size of cities, known as "planetesimals." These early building blocks eventually combined to create the planets we see today. However, that final stage of growth did not occur for many objects beyond Neptune. Instead of developing into full-sized planets, countless smaller bodies remained, preserving evidence of the solar system's earliest period of formation.
To investigate these distant survivors, research teams led by PhD candidates at the University of Victoria in Canada, working under the guidance of the National Research Council of Canada, and Northern Arizona University in Flagstaff conducted the deepest survey of trans-Neptunian objects to date. Both space telescopes observed the same region of sky simultaneously, with Hubble collecting visible light and Webb capturing infrared light. By combining these observations, astronomers could examine the objects' colors, estimate their sizes, and determine how they orbit the Sun.
Their colors are especially valuable because they provide clues about the chemical makeup of their surfaces, much like fingerprints that reveal an object's identity and history. The researchers focused on two distinct populations of trans-Neptunian objects, known as dynamically "cold" and dynamically "hot" TNOs. These descriptions refer to their orbital behavior rather than their temperatures.
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