As the Age of Exoplanet Discovery progresses, the search for planets around other stars is becoming more refined. NASA's Kepler and TESS missions were about bulk discovery of exoplanets. Building a large sample of exoplanets allowed astronomers to reach some understandings about the exoplanet population, and also pose questions that leads them deeper into that population and its characteristics.
But now, with more than 6,000 confirmed exoplanets, astronomers are beginning to tackle a very pointed question: Is there another Earth 2.0 out there? The term "Earth 2.0" might be a little tech-bro-ish for some, so the term "Earth-analogue" is also used. Regardless of how we refer to it, the idea that another planet similar to Earth might be out there is tangential to one of humanity's biggest questions: Are we alone?
Finding an Earth analogue is the goal behind missions like the proposed Habitable Worlds Observatory (HWO). The HWO is designed to find and image at least 25 Earth-like planets and search for biosignatures in their atmospheres. The HWO will use a coronagraph or starshade to block out the blinding light of the stars that these planets orbit.
*The design of the Habitable Worlds Observatory is nowhere near finalized. This illustration shows it with a starshade, but it may be built with a coronagraph. Both accomplish the same thing: blocking out starlight. Image Credit: NASA*
But there's a problem in the exoplanet search. Some stars are surrounded by hot exozodiacal dust whose light leaks into coronagraphs. It creates scattered light that astronomers call "coronagraphic leakage." This effectively pollutes the light signals and makes it very difficult to detect exoplanets, or Earth-like exoplanets in the HWO's case.
Finding an Earth analogue and confirming it will depend on multiple factors, and one of them is understanding coronagraphic leakage from exozodiacal light. New research examines a quintuple star system about 68 light-years away to try to understand how exozodiacal dust in that system is confusing observations. It's titled "Interferometric Detection and Orbit Modeling of the Subcomponent in the Hot-dust System κ Tuc A: A Low-mass Star on an Eccentric Orbit in a Hierarchical-quintuple System." The lead author is Thomas Stuber, a postdoctoral research associate at the University of Arizona's Steward Observatory.
Exozodiacal dust is very fine carbon and silicate dust found in the ecliptic plane in solar systems, ours included. It produces a faint, diffuse glow that contributes to the sky's natural light. Normally, there's not much of it. It's so fine that radiation pressure and heat from stars causes it to dissipate.
But Kappa Tucanae Aa has a lot of exozodiacal dust. "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. For that reason, the system is seen as a natural laboratory for studying the dust.
"The system κ Tuc A is part of a hierarchical-quintuple system and is a prime target for studies of hot-exozodiacal dust," Stuber and his co-researchers write. It's a prime target because the system displays excess near-infrared radiation that varies over time.
The excess infrared variability detected in the system is attributed to hot exozodiacal dust in the star's close vicinity. Stellar companions that are close can also cause the variability. Stars with known close companions are typically excluded from studies of the dust. Infrared variability was detected at Kappa Tucanae Aa in 2012, did not show up in observations in 2013, and was then observed again in 2014. In 2019 it was detected again, and those same observations found no companion star.
In this work, Stuber and his co-researchers observed the system between 2022 and 2024.
It turns out that the variability of the primary star, Kappa Tucanae Aa, could because of a newly-discovered companion, Kappa Tuc Ab, which was previously inferred by Gaia's astrometry measurements, and confirmed in this research. Kappa Tuc Ab has about 0.33 solar masses and is a cool, dim red dwarf. It's orbital period is about 8.14 years, and it follows an extremely elliptical orbit.
*This picture shows the faint white zodiacal light between the Milky Way and the sunset. It also shows three of the four telescopes that make up Very Large Telescope Interferometer (VLTI). Instruments named the Multi Aperture mid-Infrared Spectroscopic Experiment (MATISSE) and GRAVITY at the Very Large Telescope Interferometer played the leading role in this research by detecting the companion star. Image Credit: ESO/B. Tafreshi*
"There's basically no way that this companion is not somehow connected to that dust production," said co-author Stever Ertel in a press release. "It has to be dynamically interacting with the dust."
What's the nature of that interaction? The newly-detected star could in some way "stir up" the dust during its periastron, its closest approach to Kappa Tucanae Aa. Or it could be because the star alters the orbits of planetesimals or comets that are well below the detection threshold, which then replenish the dust. "This coexistence of hot dust and the stellar companion motivates dynamical studies of this intriguing planetary system, governing, for instance, how κ Tuc Ab interacts with the hot-dust distribution during its periastron passage or how it might excite unseen planetesimals onto cometary orbits that can replenish the dust in situ," the authors conclude.
Understanding extrazodiacal dust in this and other systems is a necessary step forward in the hunt for an Earth analogue. As the Habitable Worlds Observatory approaches, understanding how the dust affects coronagraphic leakage will be critical to understanding the signals the observatory gathers.
Other systems that have this dust could also be harbouring unseen companions, and the researchers intend to revisit other systems to scrutinize them.
"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."
