A hidden star found where dust shouldn’t exist

Now, researchers at the University of Arizona believe they have found a crucial missing piece. They have identified a companion star that repeatedly passes through the same region where this unusually hot dust remains.

A Record-Setting Discovery

The findings were published in The Astronomical Journal and led by Thomas Stuber, a postdoctoral research associate at the University of Arizona's Steward Observatory. Using the European Southern Observatory's MATISSE instrument, the team achieved the highest-contrast detection of a stellar companion ever recorded with this technology.

This discovery gives scientists a rare natural "laboratory" for studying hot exozodiacal dust. This type of dust has become a major obstacle in the search for Earth-like planets around other stars.

Why Hot Exozodiacal Dust Is So Confusing

Hot exozodiacal dust challenges basic ideas about how planetary systems behave. The particles are incredibly small, comparable to smoke from a fire, and they orbit extremely close to their stars. The intense heat and radiation in these regions should destroy the dust almost immediately.

"If we see dust in such large amounts, it needs to be replaced rapidly, or there needs to be some sort of mechanism that extends the lifetime of the dust," Stuber said.

A Problem for Finding Other Earths

The mystery becomes even more important because hot dust often appears around stars that scientists hope might host Earth-like planets. NASA's planned Habitable Worlds Observatory (HWO), expected to launch in the 2040s, is designed to block out starlight using advanced coronagraphs so faint planets can be seen.

Hot dust interferes with this process by creating what researchers call "coronagraphic leakage" -- scattered light that can hide the signals of potentially habitable worlds. Learning where this dust comes from and how it behaves will be essential for future planet-hunting missions.

A Surprise Revealed by Interferometry

To investigate the system more closely, Stuber's team used interferometry, a method that combines light from multiple telescopes to simulate a much larger one. The researchers observed Kappa Tucanae A repeatedly between 2022 and 2024.

The international team initially planned to track changes in the dust over time. Instead, they uncovered something unexpected: a companion star moving on a highly elongated orbit. At its closest approach, it comes within 0.3 astronomical units of the main star -- closer than any planet in our solar system gets to the sun.

A Stellar Laboratory Takes Shape

According to Stuber, this finding changes how scientists view the entire system. Rather than being a simple mystery, Kappa Tucanae A now serves as a complex environment for studying extreme stellar interactions. The companion star travels far from the system before plunging back through the dust-filled inner region.

"There's basically no way that this companion is not somehow connected to that dust production," said Steward Observatory Associate Astronomer Steve Ertel, a co-author of the study. "It has to be dynamically interacting with the dust."

Decades of Technical Expertise

The breakthrough reflects years of leadership in interferometry at Steward Observatory. Its Large Binocular Telescope Interferometer (LBTI), funded by NASA and located on Mount Graham, transformed the study of warm exozodiacal dust, which is less extreme than the hot dust seen around Kappa Tucanae A.

The instrument's stability and sensitivity helped establish Steward as a global center for exozodiacal dust research. This success attracted major support from NASA, the National Science Foundation, and private donors, and placed the observatory at the forefront of exoplanet science.

Building the Next Generation of Instruments

That experience is now shaping future technology. Steward researchers are contributing to a new European nulling interferometer that will be 50 times more sensitive than earlier instruments.

The connection is personal as well as technical. Denis Defrère, who leads development of the European instrument, previously trained at Steward Observatory as a postdoctoral researcher and helped build the LBTI.

"Steward has established itself as the global leader to this kind of research, which is really critical for exo-Earth imaging," said Ertel, who received a NASA grant to study exozodiacal dust using the new instrument.

New Paths for Understanding Cosmic Dust

The Kappa Tucanae A system opens many new research opportunities. By examining how the companion star interacts with the dust, scientists hope to learn more about where hot dust comes from, what it is made of, how large the particles are, and how they are distributed.

The work may clarify whether magnetic fields trap charged dust particles, as suggested by Steward researchers George Rieke and András Gáspár. It could also test whether frequent comet activity replenishes the dust, a process studied by Steward researcher Virginie Faramaz-Gorka, who is also a co-author on the paper. Other, entirely different physical processes may also be at play.

Looking Ahead to Future Discoveries

The findings suggest that other stars with hot dust may also host hidden companions. Researchers at Steward Observatory now plan to reexamine systems observed in the past, searching for stars that may have been overlooked.

As NASA's Habitable Worlds Observatory moves closer to launch, discoveries like this provide essential insight into the environments astronomers will face.

"Considering the Kappa Tucanae A system was observed many times before, we did not even expect to find this companion star," Stuber said. "This makes it even more exciting to now have this unique system that opens up new pathways to explore the enigmatic hot exozodiacal dust."