For the first time, scientists used the joint power of NASA’s Hubble and James Webb Space Telescopes to study some of the most far-flung bodies in our solar system, Trans-Neptunian Objects (TNOs). Some of these are the smallest and faintest ever directly seen. The researchers unexpectedly found fewer small TNOs than they expected, and that the colors of these bodies followed the same relationships as their larger family members.

These objects are typically small, faint, icy bodies orbiting the Sun beyond the orbit of Neptune. Most are more than 100 million times dimmer than objects visible to the unaided eye. In two complementary papers published Tuesday in The Astronomical Journal, teams analyzed the color, composition, and size distribution of 27 newly discovered tiny, dim TNOs. 

This class of small bodies offers the best view into an early stage of planet-building, when a disk of dust and pebbles in orbit around the Sun coalesced into city-sized “planetesimals” — the solid building blocks that clump together to form planets — but had not yet merged into full-sized worlds.  Beyond Neptune, this second stage never happened, leaving behind a frozen population of planetesimals.

In the deepest TNO survey to date, teams led by PhD candidates from the University of Victoria in Canada, under the guidance of the National Research Council of Canada, and Northern Arizona University in Flagstaff examined a patch of sky simultaneously with Hubble, observing the TNOs’ visible light, and Webb, observing their infrared light. The team of researchers measured the objects’ colors, which are like a fingerprint of the surface composition, as well as their sizes and determined their orbits. 

In the coordinated observations, the teams studied two different types of TNOs. The first, dynamically “cold” TNOs, are on their original, relatively circular orbits around the Sun in the plane of the solar system. The second type, dynamically “hot” TNOs, formed between the current locations of Uranus and Neptune but were pushed outward where they are today when the outer gas giants migrated early in the solar system’s history. Today they reside in highly elliptical orbits and move in and out of the plane of our solar system.

Prior to these observations, astronomers thought that small TNOs from both hot and cold populations would have undergone many collisions, changing their surfaces compared to larger TNOs. But that’s not what the observations showed. Instead, the small bodies look like their larger counterparts. This implies that collisions are not changing the surfaces significantly—perhaps because there are fewer collisions than expected, or because the TNOs somehow retain their primordial, pre-collision compositions. The teams are still trying to unravel this mystery.

“You could imagine a scenario where getting knocked around and fragmented would change the surface composition, and then you would see a different surface color for tiny TNOs compared to their larger siblings. So it’s really fascinating to see that the smallest objects are somehow ‘remembering’ and preserving the history of how they were made,” said Northern Arizona University PhD candidate Anastasia Morgan, who led the study of color and composition. 

“These dynamically ‘hot’ TNOs retain a signature of where they were born, even though they’ve been orbitally scrambled since then,” said co-author David Trilling of Northern Arizona University.

Both the “hot” and “cold” populations seem to keep the same colors as when they were formed, with little change since the birth of the solar system. 

The Webb data also allowed researchers to measure the number of objects of each size. They found that the overall size distributions for both populations were surprisingly similar.

“It’s very interesting that the process of planetesimal formation ends up producing the same distribution of sizes for both cold and hot populations, despite forming in different regions of the early solar system. The process seems to be insensitive to disk conditions, producing similar planetesimal sizes whether the disk is hot or cold, and dense or fluffy,” said University of Victoria PhD candidate Marielle Eduardo, who led the study on size distribution. 

Researchers also found fewer of these very small bodies than they expected based on some planet formation models. Webb discovered 27 new, remarkably dim TNOs, one so faint it is equivalent to standing on Earth and seeing a small swarm of fireflies on the Moon. The smallest one they observed has a diameter of about 3 miles (5 kilometers), which is about five times smaller than what is possible to detect with the most sensitive ground-based telescopes.

This project would not have been possible without Hubble and Webb working together to detect and characterize these TNOs. With Hubble’s sensitivity in visible light and Webb’s in infrared, the space telescopes provide more insights than either can on its own.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

To learn more about NASA’s space telescopes, visit:
https://science.nasa.gov/universe

This NASA/ESA Hubble Space Telescope image features a sprawling cosmic vista in the Large Magellanic Cloud, or LMC, the largest of the small galaxies that orbit our Milky Way galaxy. At just 160,000 light-years away, the LMC offers a close look at highly active star birth sites like the one in this image. This photogenic nebula, named LHA 120-N44, or N44, is in the constellation Dorado.

N44 is dominated by two features: a vast central void and a shell of dense, dusty gas. The central void is a ‘superbubble’ spanning roughly 210 by 140 light-years across. The glittering stars at the center of the void are responsible for its creation; through their powerful stellar winds and explosive supernovae, these stars expelled much of the gas from which they were born.

When the stars of N44’s central star cluster swept away this gas, the expelled gas compressed and formed a shell around the superbubble. New stars are forming in this compressed gas shell, making N44 an interesting target for astronomers who are using the nebula to study how stars form in this environment. Their goal is to understand how long it takes from the collapse of cold gas clouds into dense knots to the moment nuclear fusion ignites in the heart of a newborn star.

The data in this image is from an observing program (#14689; PI: Gouliermis) that used Hubble to survey N44 and take a census of its stars, cataloging nearly half a million stars within the cluster as well as interlopers drifting in front of it. Of the stars surveyed, nearly 30,000 are what astronomers call pre-main-sequence stars, which have yet to begin fusing hydrogen into helium in their cores. Astronomers discovered this treasure trove of baby stars thanks to the high sensitivity and fine spatial resolution of Hubble’s instruments that can pick out faint objects in crowded clusters.

The gas shell surrounding the superbubble is energized by ultraviolet radiation from massive stars, causing it to glow and highlighting several distinct features. Each feature within the broader N44 star-forming complex was cataloged by astronomer Karl Henize in the 1950s. One feature is a smaller bubble, cataloged as N44F, that is located near the upper-right corner of this image. N44F is an interstellar bubble blown by the intense stellar winds of a single hot and massive star. As this previously released Hubble closeup shows, the star’s furious winds and radiation have sculpted the surrounding bubble and created pillars of dusty gas.

Hubble’s sensitive observations of the lowest-mass stars in this region open a new window onto star formation in regions that, like the LMC or the galaxies in the early universe, are poor in elements heavier than helium.

Text Credit: ESA/Hubble

Media Contact:

Claire Andreoli
NASA’s Goddard Space Flight Center, Greenbelt, MD
claire.andreoli@nasa.gov

Recent observations with NASA’s Hubble Space Telescope have revealed a giant, evolving, 10-sided atmospheric wave encircling Saturn’s south pole. This discovery marks the first time a large, regular-sided jet pattern has been observed in the planet’s southern hemisphere. The feature appears remarkably similar to Saturn’s famous hexagon at its northern pole, but is also distinctly different, suggesting scientists may be witnessing a new atmospheric phenomenon develop on the iconic gas giant.

The results published Wednesday in the journal Science Advances. 

By piecing together several years of Hubble observations dating back to 2023, researchers found subtle hints of the structure beginning to emerge before it became a clearly defined pattern. Those observations were taken as part of Hubble’s Outer Planet Atmospheres Legacy (OPAL) program, which has photographed the outer planets annually for more than a decade. 

“We’ve never seen anything quite like this in Saturn’s southern hemisphere,” said Amy Simon, study co-author and OPAL principal investigator, NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The northern hexagon has been there every time we’ve looked for more than 40 years. This feature is different — it appears to be strengthening, giving us the rare opportunity to watch a giant atmospheric pattern develop.”

The discovery was possible because Saturn’s changing seasons gradually brought the planet’s south pole back into view from Earth, where astronomers who collectively analyze images of Saturn from ground-based observatories first identified it.

Agustín Sánchez-Lavega, lead author of the new study, is a researcher at the University of the Basque Country in Spain. The university manages a website, called Planetary Virtual Observatory Laboratory, that accepts ground-based images of solar system planets contributed by observers all over the world. It was in those images, first in 2024, that Sánchez-Lavega and amateur astronomers Trevor Barry and Jean-Paul Oger noticed a subtle undulating band along the southern pole. Additional 2025 imagery taken from the ground hinted even more strongly toward this decagon structure.

That’s when the Hubble observations come into the picture. Hubble’s view from space offers unmatched image sharpness and spatial resolution over full rotations of Saturn, without smearing by Earth’s atmosphere. 

“Given Saturn’s symmetry in its north-south jet stream system, we have been searching for a counterpart to Saturn’s northern hexagon on the south pole in Hubble images since 1990,” Sánchez-Lavega said. “Images from NASA’s Cassini spacecraft, which orbited Saturn between 2004 and 2017, showed no inkling of a long-lived formation, either. The Hubble data confirmed the feature’s presence back to 2023.”

The wave sits within one of Saturn’s powerful jet streams and extends through multiple layers of the atmosphere, indicating it is not just a cloud-level feature, but a vertically extended atmospheric structure. The decagon’s apparent position shifts slightly, because Hubble captures images from different wavelengths. Those different wavelengths probe different altitudes in Saturn’s atmosphere.

“The most intriguing part to me is that this seems to have just formed recently,” said Simon. “The question is, why did it suddenly form now when we haven’t seen one before?” 

The authors say further study is needed from Hubble and NASA’s James Webb Space Telescope, as well as analysis of computer models, to understand how the decagon formed, how long it may last, and how it compares to the long-lived hexagon in the north.

Hubble’s long duration in operation has allowed astronomers to track changes over time in solar system planets and other astronomical objects as well.

Rather than providing a single snapshot, the OPAL program allows scientists to follow seasonal changes, track short-lived storms, and identify other atmospheric features that evolve slowly over time.

“When we started the OPAL program, we expected compelling surprises, but we didn’t know what to expect specifically,” said Mike Wong, study co-author, University of California, Berkeley. “A lot of the discoveries we see coming from OPAL are not just based on one observation, but on years and years of data. Regular observations over time are enabling a lot of new findings.”

The team plans to continue observing Saturn to determine whether the decagon settles into a long-lived, stable configuration like the northern hexagon or continues to evolve. Future observations also could help scientists determine what drives the wave, what it reveals about the atmospheric dynamics of giant planets throughout the solar system, and how they may relate to those we see here on Earth.

The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Our home galaxy, the Milky Way, grew to its current size in part by consuming smaller galaxies. Now, new data from NASA’s Hubble Space Telescope shows definitive evidence of a dwarf galaxy merging with the young Milky Way galaxy in the earliest phases of its evolution. This finding extends our knowledge of our galaxy’s history 1.8 billion years farther back in time than before.

The results published Monday in the journal Nature Astronomy.

The Milky Way today is a massive spiral galaxy home to hundreds of billions of stars. However, our galaxy wasn’t always so large; it has grown by forming new stars from its gas clouds as well as collecting stars, gas, and dark matter from other galaxies through mergers.

The most recent massive merger in our galaxy’s history took place with the Sagittarius dwarf galaxy, beginning over 6 billion years ago and still ongoing today. Looking back into the even more distant past, researchers learned that the Milky Way galaxy consumed another dwarf galaxy called Gaia-Sausage-Enceladus 10 billion years ago. This ancient merger greatly affected the structure of our galaxy’s disk of stars. Other, smaller mergers occurred between these two.

But our galaxy’s history doesn’t stop there. Both observations and simulations have suggested that another large merger preceded these two, though the specifics of the event have been heavily debated. Now, Hubble has uncovered definitive evidence of an earlier merger that occurred about 11.8 billion years ago, or just 2 billion years after the big bang.

“Our home is the Milky Way galaxy, but we do not know how our house was built,” said Davide Massari, lead author, Astrophysics and Space Science Observatory of Bologna in Italy. “In this paper we discover where the first significant batch of bricks came from: a dwarf galaxy that we call LKH.”

Cosmic archaeological sites

Immense astronomical surveys and precision data from spacecraft like ESA’s (European Space Agency’s) Gaia mission have been instrumental in piecing together the history of our galaxy. The farther back into our galaxy’s history that scientists attempt to look, the more difficult it becomes to tell what happened. When our galaxy was young, it was smaller and much closer in size to the galaxies it clashed with. It was also more chaotic, and it’s possible that the signs of mergers have been erased over billions of years.

It’s into this murky past that Hubble peered. Researchers used Hubble to study some of the Milky Way galaxy’s globular clusters: immense, roughly spherical collections of tens of thousands to a few million stars. Globular clusters contain some of the oldest stars in our galaxy, and they can act as cosmic archaeological sites that preserve stars from other galaxies the Milky Way galaxy has collected.

“Thanks to the high resolution and depth of Hubble imaging, we could measure the age and the metal content of these clusters with unprecedented precision,” said Chiara Zerbinati, study co-author, University of Bologna in Italy. “Coupled with measurements from Gaia, this made it possible to distinguish a population of globular clusters that are different from the others. These are the clusters that were born in LKH, and they tell us when that galaxy was devoured by ours, and how massive it was.”

The team analyzed Hubble observations of 39 globular clusters in the inner 20,000 light-years of our galaxy, where evidence of the most ancient mergers should be preserved. They expected this sample to contain globular clusters that formed within the young Milky Way galaxy as well as those collected from the Gaia-Sausage-Enceladus dwarf galaxy about 10 billion years ago.

Using Hubble’s sensitive observations to determine each cluster’s precise age and associated metallicity — the abundance of elements heavier than helium — they determined there was a third population of globular clusters in the inner regions of our galaxy. The team found that these clusters are older than the group collected in the Gaia-Sausage-Enceladus merger, but younger than those born in the Milky Way, regardless of their metal content. These  clusters, therefore, came from a separate and even earlier merger — in which the Milky Way galaxy absorbed a dwarf galaxy containing roughly 500 million times the mass of the Sun in stars, a significant fraction of our galaxy’s mass at the time. They named this dwarf galaxy Low-energy-Kraken-Heracles, or LKH, in honor of three earlier research papers that championed the idea of a merger early in our galaxy’s history.

Such a large merger so early in the Milky Way galaxy’s formation has profound implications for the evolution of our galaxy.

“Some past studies have argued that the earliest phases of our galaxy’s evolution were defined by stars born only in our galaxy,” says Massari. “Here, we have shown that stars born in external galaxies also need to be considered.”

The team plans to continue their work to unravel the history of the Milky Way galaxy by studying its globular clusters, aiming to characterize all the massive mergers that our galaxy has experienced across cosmic history.

“Hubble is observing globular clusters that have never been studied before, and this will help us characterize the merger events that are far back in time in the Milky Way galaxy’s history,” said Fernando Aguado-Agelet, co-author, University of Vigo and the University of La Laguna in Spain.

The Hubble Space Telescope has been operating for more than three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.