In 2024, astrophysicist Stephen Giacalone discovered a brown dwarf that appeared to have formed through core accretion, making it, in effect, the largest planet ever to have existed. Forming planets are now known to encounter turbulent conditions in their solar systems. A new observation in 2026 provides compelling evidence of how the first generation of stars in dwarf galaxies died, chemically enriching their successors.
Brown dwarfs may simply be expanded planets.
Many astronomical objects follow clear rules and fit into neat categories, but brown dwarfs (celestial objects too massive to be mere planets, but too small to be true stars) still refuse to obey these rules.
Astronomers recently studied a sample of 70 objects, ranging from Jupiter-mass planets to brown dwarfs on the verge of star formation. By studying the relationship between the mass of these objects and certain characteristics of their solar systems (for example, whether the stars contain elements heavier than helium or how circular the objects’ orbits are), the researchers hoped to draw a clear line between massive objects forming like stars and smaller ones forming like planets. But they were disappointed, because the real Universe is a complex and intricate system.
As it turns out, the line between stars and planets may be more of a gray, fuzzy continuum, as UCLA astrophysicist Gregory Gilbert and his colleagues argue in a paper published in The Astronomical Journal in March 2026.
Stars, by definition, have a mass at least 80 times that of Jupiter, and they form from the outside in. When a clump of gas in a molecular cloud collapses under its own gravity, the tightly packed atoms in its core begin to fuse, releasing heat and light; a star is born.
A comparison of the sizes of planets, brown dwarfs, and the smallest stars. NASA/JPL-Caltech
On the other hand, giant gas planets, up to about the mass of Jupiter, form from the inside out. Initially, several dust grains coalesce in a disk of material around a newborn star, and their combined gravity is sufficient to attract even more dust. The material continues to accumulate faster and faster, forming a rocky core surrounded by thick layers of gas.
However, in between are a number of objects that astronomers are unsure whether to classify as “failed stars” or “expanded planets.”
Brown dwarfs, which range in mass from 13 to 80 times that of Jupiter, aren’t massive enough to fuse hydrogen into helium like a true star, but they are large enough to fuse deuterium—an isotope of hydrogen that contains a neutron along with the standard proton and electrons. Fusing deuterium into helium requires less pressure than fusing pure hydrogen. Then there are “sub-brown dwarfs”—gas giants that are truly enormous by planetary standards, but not large enough to be considered true brown dwarfs.
Ideally, there should be a clear cutoff: objects with masses above a certain value should be failed stars formed by the collapse of gas clouds, while objects with masses below this value should be expanded planets that have condensed from planetary disks.
However, astronomers have not yet been able to detect a single such line.
In 2024, astrophysicist Stephen Giacalone, one of the co-authors of the current study, discovered a brown dwarf that appeared to have formed through core accretion, making it, in effect, the largest planet ever to have existed. And some sub-brown dwarfs—giant planets not large enough to be considered brown dwarfs—appear to have formed through gravitational collapse, meaning they were so unsuccessful in becoming stars that they couldn’t even become brown dwarfs.
“It remains to be determined precisely how large the object might be as a result of core accretion, or how small as a result of disk instability or cloud fragmentation,” Gilbert and his colleagues wrote in their paper.
Gilbert and his colleagues used statistical models to test how the mass of their objects related to the chemical composition of their host stars and the shape of the objects’ orbits.
An analysis of the orbital eccentricity of these objects (a measure of how close to a perfect circle their orbit is) yields virtually the same result. Less massive objects tend to have more circular orbits, while the most massive objects, similar to brown dwarfs, have more pronounced eccentricities. However, Gilbert and his colleagues noted that this trend is very gradual.
An artist’s illustration of a brown dwarf. Nazariy Neshcherensky/iStock/Getty Images
“It is reasonable to assume that as the mass of an object increases, the probability of its formation by core accretion decreases, and the probability of its formation by gravitational instability [gas cloud collapse] increases,” the researchers wrote in their recent paper, “but this is more of a spectrum than a clear division of objects into two groups.
And then there’s metallicity. A planet can accrete enough material quickly enough to grow into a gas giant only if it forms in a star system with a very high metal content—that is, it contains many elements heavier than helium (primarily carbon, oxygen, and iron). So if there were a clear dividing line between the more massive objects formed by molecular cloud collapse and the less massive objects formed by accretion, researchers like Gilbert and his colleagues would expect to see smaller subbrown dwarfs forming only in metal-rich star systems. But that’s not what Gilbert and his colleagues actually saw in their data.
Instead, there appears to be no relationship between the mass of a gas supergiant and the metallicity of its solar system. This suggests that some of these objects formed through core accretion, while others formed more like stars—with the same end result and, often, the same mass. This means we currently cannot determine by observing an object whether it is a “failed star” or an incredibly successful planet.
“It’s possible that a clear dividing line between the formation channels does exist, but we haven’t yet detected it, either because we don’t have enough objects or because we haven’t yet studied the right combination of parameters,” Gilbert and his colleagues wrote in their recent paper.
A paper from NASA’s Backyard Worlds: Planet 9 project reports that volunteers have effectively doubled the number of known brown dwarfs, making more than 3,000 new discoveries in the past 10 years since the project began.
Brown dwarfs are Jupiter-sized balls of gas, less massive than stars. There’s one brown dwarf for every three to four stars near the Sun.
Although brown dwarfs are common, they are difficult to detect because they shine so dimly compared to stars. Having twice as many brown dwarfs to study allows astronomers to gain a deeper understanding of these elusive objects. This important new list of brown dwarfs has already revealed a new variety of objects—extreme T-type subdwarfs—and a host of other rarities, such as ultracool objects and a brown dwarf that appears to exhibit auroras. It has also helped us map the mass distribution in our galaxy and chart our cosmic environment.
An artist’s rendering of a brown dwarf created by Backyard Worlds: Planet 9 volunteer William Pendrill. Backyard Worlds: Planet 9 has announced the discovery of more than 3,000 such objects in the past 10 years, doubling the total. William Pendrill
The research results were published in a paper in the Astronomical Journal in May 2026, led by astronomer Adam Schneider of the U.S. Naval Observatory. They represent work conducted over ten years with the participation of a team of approximately 200,000 volunteers. Of the 75 authors of the paper, 61 are volunteers. The other two authors began their work on the team as volunteers and later went on to careers in astronomy.
Volunteers discovered brown dwarfs in images taken by NASA’s decommissioned Wide-field Infrared Survey Explorer (WISE) and Near-Earth-Object WISE Reactivation mission (NEOWISE-R) spacecraft. They studied the data using the citizen science platform Zooniverse, searching for moving objects in flickering images collected over a 16-year period. Some volunteers even contributed by creating their own search tools and data analysis software.
The Backyard Worlds: Planet 9 project continues to analyze more than 2 billion sources observed by the WISE and NEOWISE-R telescopes.
Planets like those in our solar system form through a bottom-up process, where small pieces of rock and ice stick together and grow in size over time. But the heavier the planet, the more difficult it is to explain its formation in this way.
Astronomers used NASA’s James Webb Space Telescope to study 29 Cygni b, an object approximately 15 times the mass of Jupiter orbiting a nearby star. They found abundant evidence that 29 Cygni b indeed formed through this “bottom-up” process, providing new insights into how the most massive planets form, as reported in The Astrophysical Journal Letters.
Planet formation is generally understood to occur within giant disks of gas and dust around stars through a process called accretion. Dust coalesces into small particles that collide and grow in size, forming protoplanets and then planets. The largest of these then accumulate gas, evolving into giants like Jupiter. Because gas giants take longer to form, and the disk of planet-forming material eventually evaporates and disappears, planetary systems contain far more small planets than large ones.
In contrast, stars form when a huge cloud of gas fragments, each fragment collapsing under its own gravity, becoming smaller and denser. A similar fragmentation process could theoretically occur within protoplanetary disks. This could explain why some very massive objects are found billions of kilometers from their host stars, in regions where the protoplanetary disk would have been too thin to accrete.
The star 29 Cygni b lies at the intersection of two possible mechanisms. It weighs 15 times more than Jupiter and orbits its star at an average distance of 1.5 billion miles (2.4 billion kilometers), roughly the same as Uranus in our solar system. The research team chose it as a study target because it could potentially be the result of either of these processes.
The exoplanet 29 Cygni b, depicted in this artist’s rendering, is a gas giant with a mass approximately 15 times that of Jupiter. Astronomers studied 29 Cygni b using NASA’s James Webb Space Telescope. They concluded that it likely formed through disk accretion rather than disk fragmentation. NASA, ESA, CSA, Joseph Olmsted (STScI)
“In computer models, disk fragmentation could easily produce masses significantly larger than 29 Cygni b. This is the smallest mass that could plausibly be obtained. But at the same time, it is approximately the largest mass that could be obtained through accretion,” said lead author William Balmer of Johns Hopkins University and the Space Telescope Science Institute in Baltimore.
The Balmer observing program used the Webb telescope’s Near-Infrared Camera (NIRCam) in coronagraphic mode to directly image the planet 29 Cygni b. This planet was the first of four objects targeted by the program, each known to have a mass between 1 and 15 times that of Jupiter. Furthermore, the team required the objects to orbit at a distance of approximately 9 billion miles (15 billion kilometers) from their stars.
All the planets were young and still hot from their formation, with temperatures ranging from 1,000 to 1,900 degrees Fahrenheit (530 to 1,000 degrees Celsius). This ensured that the chemical composition of their atmospheres was similar to that of the planets in HR 8799, a system Balmer had previously studied. By selecting the appropriate filters, the team was able to detect signatures of light absorption by carbon dioxide (CO₂) and carbon monoxide (CO), allowing them to determine the abundance of these heavier chemical elements, which astronomers collectively call metals.
They found compelling evidence that planet 29 Cygni b is enriched in metals compared to its host star, which is similar in composition to our Sun. Given the planet’s mass, the amount of heavy elements it contains is equivalent to approximately 150 Earth masses. This suggests that it accreted large amounts of metal-rich solids from the protoplanetary disk.
Astronomers used NASA’s James Webb Space Telescope to directly image 29 Cygni b, which has a mass 15 times that of Jupiter. They found evidence of heavy chemical elements such as carbon and oxygen, strongly suggesting that it formed as a planet through accretion within a protoplanetary disk. NASA, ESA, CSA, William Bulmer (JHU, STScI), Laurent Pueyo (STScI); Image processing: Alyssa Pagan (STScI)
The team also used the ground-based optical telescope CHARA (Center for High Angular Resolution Astronomy) to determine whether the planet’s orbit was aligned with the star’s rotation. They confirmed this alignment, as expected for an object formed from a protoplanetary disk.
“We were able to update the planet’s orbital data and also conduct observations of the host star to determine its orientation relative to this orbit,” said Ash Messier, co-author and a graduate student at Johns Hopkins University. “We showed that the planet’s tilt is closely aligned with the star’s rotation axis, which is similar to what we observe for planets in our solar system.”
“Taken together, these data strongly suggest that 29 Cygni b formed in a protoplanetary disk through rapid accretion of metal-rich material, rather than through gas fragmentation,” Balmer said. “In other words, it formed as a planet, not a star.”
As they collect data on the three other objects in their program, the team plans to look for evidence of compositional differences between the lower- and higher-mass planets. This should provide further insight into their formation mechanisms.
The James Webb Space Telescope is the world’s leading observatory for space science. Webb unravels the mysteries of our solar system, peers beyond it to distant worlds around other stars, and explores the enigmatic structure and origins of our universe, as well as our place in it. Webb is an international program led by NASA and its partners, ESA (the European Space Agency) and CSA (the Canadian Space Agency).
Another study shows that a powerful eruption from a young Sun-like star creates harsh conditions for nascent planets in its system.
A team of scientists from the Center for Astrophysics (CfA) used the Submillimeter Array (SMA), a telescope array on Mauna Kea in Hawaii, to observe the young star HD 283572. The crew observed HD 283572’s brightness increase hundreds of times in just a few hours. This observed eruption is considered one of the most powerful stellar outbursts ever observed, as reported in The Astrophysical Journal Letters in early 2024.
Located approximately 400 light-years away, HD 283572 is approximately 1.4 times more massive than the Sun, but at nearly 3 million years old, it is more than a thousand times younger than our Sun, which is approximately 4.6 billion years old. This means that when astronomers observed HD 283572, they were witnessing it at the same stage of its development as the Sun, when planets like Earth began to form.
An artist’s impression showing the formation of planets from a disk of material surrounding a star. NASA
As a result, these data may indicate that forming planets, including those in our Solar System, must encounter turbulent conditions.
“We were surprised to see an unusually bright flare from an ordinary young star,” said team leader and CfA scientist Joshua Bennett Lovell in a statement. “Any potential planets developing in this system would have been destroyed by the powerful flare. I wouldn’t want to grow up there!”
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Stellar flares like those observed by Lovell and his colleagues are thought to arise from the rotation of stars and the entanglement of their magnetic fields.
Just as a spring that’s too tightly wound accumulates kinetic energy that must be released, the stored magnetic energy in these twisted magnetic fields must be released. For stars, this results in the ejection of accelerated particles through their surfaces into space.
Location of the erupting young star HD 283572 and its brightness change between January 2022 and March 2024. CfA/JB Lovell et al.
The flares accompanying this eruption of stellar material, or plasma, can increase the star’s brightness by tens or hundreds of times and across a wide range of wavelengths. However, detecting such flares remains challenging, as these events occur essentially randomly, so there’s no clear indication of when to point a telescope at the star to capture its next outburst.
This was certainly true for HD 283572, which appeared to be dormant before its massive eruption.
“Every time we pointed the SMA telescope at the star again after this outburst, we saw nothing,” Lovell explained. “Our results confirm that such outbursts are rare at millimeter wavelengths but can be extremely powerful for stars of such a young age.”
Over the course of nine hours, the energy of HD 283572’s flare reached levels millions of times greater than those emitted by similar flares measured in the solar system’s vicinity. Such flares are so powerful that they are capable of destroying the atmospheres of young, forming planets.
The discovery of such a powerful flare during this crucial period in the life of a planetary system provides scientists with a key to understanding the pressures experienced by Earth and its twin planets during their formation approximately 4.5 billion years ago. The results may also hint at what extrasolar planets, or “exoplanets,” may be experiencing today during the earlier stages of their existence.
An illustration of an erupting young star that could pose challenges for exoplanet development. CfA/Melissa Weiss
The team of researchers who conducted this study is continuing to monitor HD 283572 to determine how often the young star erupts and to find out whether this glow could influence the formation of the atmospheres of planets around it.
Additionally, the SMA campaign is studying other young stars similar to HD 283572 to determine their typical properties and outburst frequency. According to the researchers, combining the SMA data with longer-wavelength observations will also provide a better understanding of the physics of these outbursts and the processes that generate them.
The 2026 observation provides compelling evidence of how the first generation of stars in dwarf galaxies died, chemically enriching their successors.
Scientists have taken on the role of “cosmic archaeologists” to discover a rare, iron-depleted second-generation star—essentially a fossil of the chemical evolution of our Universe. Just as artifact finds on Earth allow us to learn about lost generations of humans, this observation provides irrefutable evidence of how the first generation of stars perished, chemically enriching their successors. The results of the study were published on March 16, 2026, in the journal Nature Astronomy.
A second-generation star, or POP II, was discovered in the Pictor II dwarf galaxy, located approximately 150,000 light-years from Earth in the constellation Pictor, using the Dark Energy Camera (DECam) on the 4-meter Victor M. Blanco telescope. The star, designated Pic II-503, contains only 1/40,000th the iron found in the Sun, which is a third-generation star, or (somewhat confusingly) POP I. The fact that Pic II-503 has the lowest iron concentration ever observed outside the Milky Way makes it one of the oldest stars ever discovered.
The camera for measuring dark energy is installed in the Victor Blanco Telescope, pictured here along with other telescopes at Fermilab’s Cerro Tololo Inter-American Observatory in Chile.
However, this deficiency isn’t the most surprising thing about Pic II-503. The team also discovered that this POP II star possesses a massive carbon excess: its carbon-to-iron ratio is more than 1,500 times higher than that of the Sun. This excess reflects the unique carbon-rich characteristic of iron-poor stars found in the Milky Way’s hazy outer halo.
“Discoveries like these are cosmic archaeology, the discovery of rare stellar fossils preserving traces of the universe’s first stars,” Chris Davis, program director of the National Science Foundation’s NOIRLab, said in a statement.
The first stars in the Universe, or POP III stars, were born when the chemical composition of the cosmos was limited to hydrogen, helium, and a small amount of heavier elements, which astronomers collectively call “metals.” This meant that these POP III stars were predominantly hydrogen, with a small amount of helium and very small amounts of metals. The first carbon and iron formed in the cores of these stars—material that would spread into the interstellar medium when these stars exploded as supernovas at the end of their lives.
Interstellar clouds of gas and dust enriched in these metals eventually cooled and collapsed, giving birth to a second generation of stars, which became even more metal-rich thanks to the donation of heavier elements from their predecessors. This makes POP II a kind of time capsule, capturing an important stage in the chemical enrichment of the Universe.
“Finding a star that uniquely preserves heavy metals from the first stars was at the limit of what we thought possible, given the extreme rarity of these objects,” said team leader Anirudh Chiti of Stanford University in a statement. “By detecting the lowest iron abundance ever recorded in any ultrafaint dwarf galaxy, Pic II-503 opens unprecedented opportunities to study the initial formation of elements in primordial systems.”
The first confirmed example of a POP II star discovered in a dim dwarf galaxy, Pic II-503, was identified as having an extremely low metal content in data collected by the Mapping the Ancient Galaxy in CaHK (MAGIC) project on the DECam telescope. This 54-day observing program was designed with the explicit goal of identifying the oldest and most chemically primitive stars in the Milky Way and its companion dwarf galaxies.
Stars in the dim dwarf galaxy Pictor II, home to Pic II-503, a second-generation, iron-deficient star. CTIO/NOIRLab/DOE/NSF/AURA. Image processing: Provost TA (University of Alaska Anchorage/NSF NOIRLab), M. Zamani, and D. de Martin (NSF NOIRLab ).
Chiti and his colleagues combined MAGIC data with observations from the Very Large Telescope (VLT) in the Atacama Desert region of northern Chile and the Magellan Baade telescope to detect low iron and calcium abundances in Pic II-503—the lowest ever observed outside our home galaxy. This, in turn, revealed that Pic II-503 is the first documented case of chemical enrichment detected in a dwarf galaxy.
One possible explanation for the shockingly low iron-to-carbon ratio in Pic II-503 is that when POP III stars exploded as supernovae, these explosions had relatively low energies. This would mean that while lighter elements like carbon were ejected into the interstellar medium, heavier elements like iron fell back into the supernova debris.
The fact that Pic II-503 is found in one of the smallest dwarf galaxies ever observed, with a correspondingly low gravitational influence, supports the idea that POP III stars die in low-energy supernovae.
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