Using the Transiting Exoplanet Survey Satellite (TESS), a spacecraft designed to search for exoplanets, astronomers discovered an unusual quadruple star system. During another dedicated observing campaign, XRISM revealed that hot plasma signals follow the orbital motion of an otherwise invisible companion star. And a new study by Japanese scientists has shown that our Sun and numerous similar “solar twins” may have migrated together from the core of the Milky Way galaxy.
The largest four-star system in the world
Using NASA’s Transiting Exoplanet Survey Satellite (TESS), a spacecraft designed to search for exoplanets, astronomers have discovered an unusual quadruple star system. This system is the closest triple-star system ever discovered, comprising three stars and one star (3+1 stars), a subset of quadruple star systems. What’s particularly interesting is that the discoverers of this system were also able to determine its fate.
The TIC 120362137 system consists of a stable and tightly bound inner system of three stars orbiting each other, around which orbits a more distant outer star, observing the system from afar. Although the outer star is located at approximately the same distance from the trio as Jupiter’s distance from the Sun, the inner stellar subsystem would fit within the orbit of Mercury, the closest planet to the Sun, around our star. The results of the study were published on March 3, 2026, in the journal Nature Communications.
TIC 120362137 is an important discovery for researchers because, in addition to the fact that 3+1 systems (so-called hierarchical star systems, where several stars orbit each other in a relatively small region) are extremely rare, TIC 120362137 may also help better understand star formation processes and long-term orbital stability.
“TIC 120362137 is currently the most compact 3+1-type quadruple star system known,” team leader Tamás Borkovics, a researcher at the University of Szeged in Hungary, told Space.com.
However, the unusual nature of this system was not obvious at first glance.
“From a simple analysis of early TESS data, we realized that TIC 120362137 is a compact, tightly packed, triple eclipsing star system,” Borkowitz said. The researcher added that when the team first observed TIC 120362137, this previously unknown system initially appeared to consist of a pair of stars eclipsing each other every 3.3 Earth days, resulting in a dimming effect for one to two hours.
Illustration of the 3+1 star system TIC 120362137 by Robert Lee
“We know of thousands of such systems, called eclipsing binaries. So at that stage, there was nothing interesting or unusual,” he continued. “Then we realized that every 25-26 days, additional fadings lasting one to two days occurred, which clearly indicated that the system must contain a third star with an orbital period of about 51 days. Thus, we concluded that TIC 120362137 must be a triple eclipsing system.”
“However, at that time we still knew nothing about the fourth star.”
The team then observed several more eclipses, indicating the presence of a fourth star, whose presence was confirmed using the Tillinghast Echelle Spectrograph (TRES) on the 1.5-meter Tillinghast telescope located on Mount Hopkins in Arizona.
“TIC 120362137 is a record-breaker in that we found that the orbital period of the outermost star is only about 1,046 days, the shortest of any currently known 3+1 quadruple star,” Borkowitz said. “However, discovering such systems is very, very difficult. Detecting the fourth, outermost component by scanning eclipses in the same way as the inner system requires much more time, perhaps even decades or longer. Other ways of detecting the fourth star could occur, but only by chance.”
The team was also able to determine other characteristics of the stars in this system. They discovered that the three innermost stars are more massive and hotter than the Sun, while the outermost component, the fourth star, is cooler, less massive, and therefore similar to the Sun. Furthermore, using computer simulations, the researchers were able to determine the future of this 3+1 star system, which will ultimately remain just two white dwarf remnants.
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“First, the most massive star, the primary component of the innermost binary, will reach the red giant stage. In this state, it will merge with its partner, the secondary star of the innermost binary. We call this daughter stellar body A’,” Borkowitz said. “Then, after about 276 million years, in the second stage, this nova, the merged star A’, will merge with the third stellar component, star B, when both stars reach the red giant stage. We call this massive nova AB.”
He added that after this, the AB star will lose a significant portion of its mass, eventually collapsing to form a white dwarf. As this occurs, the distant fourth star will undergo a similar process, creating a second white dwarf.
“Our evolutionary model therefore predicts the formation of a binary system from these two white dwarfs, with an orbital period of about 44 days,” Borkowitz said. “The more massive white dwarf, with a mass of about 89% that of the Sun, forms after two mergers involving the three inner stars, while the less massive white dwarf, with a mass of about 29% that of the Sun, simply forms from the fourth, outermost star.”
Interaction between two companion stars
An invisible companion, absorbing matter from the naked-eye star Gamma-Cas, has been identified as the culprit behind the unusual X-rays emanating from the star system. This resolves a mystery that has puzzled astronomers for over fifty years, the European Space Agency reports.
Unique high-resolution observations conducted by the X-Ray Imaging and Spectroscopy Mission (XRISM) revealed that the X-ray emission is linked to the orbital motion of a companion white dwarf star, allowing astronomers to finally solve this mystery. A detailed description of the observations is presented in a new paper, co-authored by Yael Naze of the University of Liège (Belgium), published in March 2026 in the journal Astronomy & Astrophysics.
“For decades, numerous research groups have been working hard to unravel the mystery of gamma-Cas. And now, thanks to the high-precision XRISM observations, we’ve finally succeeded,” says Yael.
The star Gamma Cas (γ-Cas) is visible to Europeans every cloudless night. It forms the central “point” of the distinctive W-shaped constellation Cassiopeia.
Despite its visibility in the night sky, it has been shrouded in mystery since 1866, when Italian astronomer Angelo Secchi noticed something odd about its light profile. Its hydrogen “fingerprint” was bright, whereas for stars like our Sun, it usually appears as a dark line.
This strange feature gave rise to a new class of stars called Be stars, in which the “B” characteristic of hot, blue-white, massive stars has merged with the “e” formed by the peculiar emission of hydrogen.
It took several decades before astronomers realized that these emissions were emanating from a rotating disk of material ejected by a rapidly rotating star. Such disks can form and dissipate over time, causing variations in the star’s brightness. This makes it a popular target for amateur astronomers to this day.
High-resolution observations with the XRISM telescope have revealed the origin of curious X-ray emission emanating from the naked-eye star Gamma-Cas: material falling onto its white dwarf companion. ESA
As telescopic observations improved, it became possible to track the motion of Gamma-Kasa, revealing the presence of a low-mass companion star. Since the companion remains invisible to direct telescopic observation, astronomers speculate that it may be a white dwarf—a compact object with the mass of the Sun but the size of Earth.
Then, in the mid-1970s, a new mystery arose: Gamma-Cas was discovered to be emitting unusually high-energy X-rays. Further studies revealed that the source of this X-ray glow was primarily extremely hot plasma, with a temperature of 150 million degrees, emitting at a luminosity approximately 40 times greater than typically expected for such massive stars.
With the advent of X-ray space telescopes, including the European Space Agency’s XMM-Newton, NASA’s Chandra, and the German-led eROSITA, astronomers have discovered about two dozen gamma-Cas stars with similar, unusual X-ray emission, making them a special group among Be stars in general.
Over the years, the explanation for the high-energy X-ray emission has been narrowed down to two competing theories. Could the star’s local magnetic field interact with the magnetic field of the surrounding disk, creating hot material? Or are X-rays generated by the Be star’s disk incident on a white dwarf companion?
Finally, an instrument with sufficient precision to solve this mystery has arrived: the Resolve high-resolution spectrometer on the X-ray diffraction Imaging Survey (XRISM) telescope. During a dedicated observing campaign, XRISM revealed that the hot plasma signals follow the orbital motion of an otherwise invisible companion star. In other words, the white dwarf companion is absorbing matter from the gamma-ray Cas star, emitting X-rays in the process.
“Previous studies using XMM-Newton have truly paved the way for XRISM, allowing us to discard numerous theories and prove which of the last two competing theories is correct,” says Yael. “It’s incredibly exciting to finally have direct evidence to solve this mystery!”
Understanding that gamma-ray Cas objects are Be-type stars paired with an accreting white dwarf solves the mystery of the X-ray emission. But it also raises another intriguing question about how the broader population of such binary systems forms and evolves.
Such pairs were long thought to be common, primarily among low-mass stars. However, new research suggests they are rarer than previously thought and are typically observed in massive Be stars.
“We believe the key to understanding this lies in how exactly the interaction between the two stars occurs,” says Yael. “Now that we know the true nature of gamma-Cas, we can create models specifically for this class of stellar systems and update our understanding of binary evolution accordingly.”
Where to find gamma-Ca in the night sky. ESA
“It’s incredible to see how this mystery has gradually unraveled over the years,” says Alice Borghese, an ESA researcher specializing in high-energy astrophysics. “XMM-Newton has done a tremendous job of ruling out various theories about gamma-ray Cas. And now, thanks to the next generation of cutting-edge instruments, XRISM, we’ve reached the finish line.”
“This remarkable result underscores the close collaboration between the Japanese, European, and American XRISM teams,” adds Matteo Guainazzi, XRISM Project Scientist at ESA. “This international team brings together the technical and scientific expertise needed to solve the greatest mysteries of the X-ray Universe and open new avenues of research.”
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The XRISM (pronounced “cris-uh”) spacecraft was launched on September 7, 2023. It is a mission led by the Japan Aerospace Exploration Agency (JAXA) in partnership with NASA and ESA. It carries two instruments: the Resolve X-ray Calorimeter, which can measure the energy of individual X-ray photons to produce a spectrum with an unprecedented level of “energy resolution” (the instrument’s ability to distinguish the “colors” of X-rays), and a large-field X-ray CCD camera, called Xtend, for imaging the surrounding field.
The Sun may be part of a larger population of stars that migrated from the galactic core.
A new study has found that our Sun and many similar “solar twins” may have migrated together from the core of the Milky Way galaxy, potentially making the Solar System more habitable for life as we know it. Scientists detailed their findings on March 12, 2026, in two studies published in the journal Astronomy & Astrophysics.
Surrounding the Milky Way are solar twins—stars that are physically very similar to the Sun. By analyzing solar twins, astronomers hope to learn more about the Sun’s history.
In two new studies, scientists examined data from the European Space Agency’s Gaia satellite, which collected information on two billion stars to create the most accurate 3D map of the Milky Way ever created. They focused on 6,594 solar twins located within approximately 1,000 light-years of Earth. This collection of solar twins is approximately 30 times larger than previous studies of these stars.
Researchers have discovered many more solar twins with ages close to the Sun’s. By analyzing the sizes, temperatures, and compositions of these nearby solar twins, Taniguchi, Takuji Tsujimoto of the National Astronomical Observatory of Japan, and their colleagues were able to estimate the ages of these stars. By examining the age range, they noticed a broad peak of 1,551 stars, ranging in age from four to six billion years old. This population includes our Sun, which is approximately 4.6 billion years old.
Stars like our Sun formed as a result of a mass migration from the center of the Milky Way that occurred approximately 4–6 billion years ago. NAOJ
The discovery that the Sun and many of these solar twins are similar in age and located at roughly the same distance from the galactic center suggests that the Sun’s current position is no coincidence. Previous studies have shown that, based on the Sun’s “metallicity”—its abundance of elements heavier than hydrogen and helium—it formed more than 10,000 light-years closer to the galaxy’s inner regions, where metal abundances are higher than in the part of the galaxy where the Sun now resides.
The new results suggest that the Sun may be part of a larger population of stars that migrated out of the galactic core at roughly the same time—four to six billion years ago.
This discovery sheds light not only on the nature of our solar system, but also on the evolution of the galaxy itself. At the center of the Milky Way lies a gigantic rotating bar-like structure, which currently hinders such massive star migration. However, these new data reveal details about when this “co-rotating bar-like structure” formed. Indeed, the researchers hypothesized that the formation of this enormous, sprawling bar-like structure may have initially facilitated the concentration of gas, which helped initiate star formation and subsequently pushed stars outward.
The researchers said the new data could also shed light on what conditions may have favored the evolution of life on Earth.
“The inner regions of the Milky Way are thought to represent a more inhospitable environment for life, where energetic events like supernova explosions occur more frequently,” Taniguchi said. If the Sun migrated outward relatively soon after its birth, “the solar system could have spent most of its history in the quieter outer disk. In other words, the Sun may have entered a life-friendly environment not simply by chance, but as a result of the formation of a galactic bar.”
The scientists plan to expand their work to include more extensive data obtained by the Gaia satellite, scheduled for release in December 2026. They also intend to take a closer look at the composition of these solar twins, which, according to Taniguchi, “could help identify stars that were born in the same place and at the same time as the Sun—that is, true twins.”
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