For years, scientists thought they had a pretty good handle on how planets form: bigger stars make bigger planets, and smaller stars, well, not so much. That’s why a recent discovery has sent ripples through the astronomy world. Imagine finding a fully grown elephant casually sharing a doghouse with a Chihuahua. That’s essentially the cosmic equivalent of what researchers just spotted: a massive planet, larger than Saturn, comfortably circling a star that’s a mere fraction of our Sun’s size. This unexpected find, detailed in the journal Nature Astronomy, isn’t just a quirky anomaly; it’s a cosmic curveball that’s forcing scientists to rethink the very blueprints of planet formation. It directly challenges our long-held understanding, suggesting the universe might be far more capable of crafting diverse planetary systems than we ever imagined.
How Astronomers Found This “Impossible” Planet
Finding a world like TOI-6894b (that’s its official name) was a meticulous process. The hunt began with the Transiting Exoplanet Survey Satellite, or TESS, a space telescope designed to spot planets by looking for tiny dips in a star’s brightness. These dips, known as “transits,” happen when a planet passes directly in front of its star from our viewpoint, briefly dimming the starlight.
Astronomers, led by Dr. Edward Bryant, systematically combed through TESS data from over 91,000 small, cool “red-dwarf” stars, specifically searching for these tell-tale transit signals. The first hint of TOI-6894b appeared in TESS observations from early 2020. More frequent observations over the next few years solidified this initial detection, revealing a consistent pattern of dimming.
Spotting such a faint signal from so far away isn’t easy. A major challenge is making sure it’s actually a planet and not something else, like two stars orbiting each other where one briefly blocks the other. To rule out these “imposters,” the research team used a network of ground-based telescopes around the world. These observatories performed additional observations, confirming the signal was indeed coming from the star TOI-6894 itself.
To further confirm the planet’s existence and measure its mass, astronomers employed powerful instruments to detect a subtle “wobble” in the star caused by the planet’s gravity. This technique, called the “radial velocity method,” provides crucial evidence that a true planet is tugging on the star. All this observational data—from TESS, ground-based light measurements, and wobble detections—was then fed into sophisticated computer programs. This rigorous analysis confirmed that the TOI-6894 system hosts a single, genuine planet, ruling out all other explanations.
Meet the Unexpected Giant: TOI-6894b
The detailed analysis painted a clear picture of this surprising planetary system. The host star, TOI-6894, is a type of small, cool, red star, roughly one-fifth the mass of our Sun. Its companion, TOI-6894b, is truly remarkable: a low-density gas giant with a radius slightly larger than Saturn’s, yet only about half its mass. This means it’s a puffy, lightweight giant for its size. It whips around its tiny host star every 3.37 days, a remarkably tight orbit.
What makes this planet even more intriguing is its potential for atmospheric study. Despite the star’s low temperature, TOI-6894b is relatively warm, with an average temperature of about 293 degrees Fahrenheit. Scientists anticipate its atmosphere will be rich in methane, a composition rarely seen on exoplanets. Even more exciting is the potential to find ammonia, which, if confirmed, would be a groundbreaking first for an exoplanet’s atmosphere. This means TOI-6894b could become a prime “laboratory” for studying cosmic chemistry.
The planet’s transits are incredibly deep—a remarkable 17% dimming of its star’s light. This makes it an exceptionally promising target for future atmospheric studies using powerful observatories like the James Webb Space Telescope, with observations already planned.
Rewriting the Cosmic Rulebook
The very existence of TOI-6894b throws a wrench into our prevailing “core-accretion” theory of planet formation. This theory suggests that giant planets need a massive solid core to form first, which then rapidly sucks in large amounts of gas from the surrounding cosmic dust and gas disc. Since low-mass stars usually have less material in their discs, and planet formation there takes longer, it seemed highly unlikely for such enormous worlds to form.
However, the paper explores alternative explanations. One possibility is a modified version of core accretion, where a substantial core still forms, but the gas accretion is more gradual. The fact that TOI-6894b itself is rich in heavier elements (metals)—its metal content is about 12 times higher than its star’s—supports the idea of a significant build-up of heavy elements during its formation.
Another intriguing idea is “gravitational instability.” In this scenario, dense clumps within the protoplanetary disc directly collapse under their own gravity to form planets. While some computer models support this for planets of similar mass around low-mass stars, others produce different results depending on their starting assumptions. Future studies of the planet’s atmosphere, especially its atmospheric metallicity, could help scientists figure out which formation story is more accurate.
Our understanding of the discs around low-mass stars is still developing. Measurements of dust in these discs might underestimate the total solid material, as larger “pebbles” could be invisible to current instruments. Also, observations of very young discs show significantly more dust, hinting that giant planets might form much earlier in a star’s life. Given how rare planets like TOI-6894b appear to be and the limited number of low-mass star discs we’ve studied, it’s not surprising we haven’t yet observed a disc massive enough to easily explain its formation through standard models.
This “unexpected planet” serves as a powerful reminder that the universe continues to defy our expectations, constantly pushing the boundaries of scientific theories. It underscores the vital role of ongoing exploration and observation in unraveling the mysteries of how worlds are made. TOI-6894b is more than just a planet; it’s a cosmic puzzle demanding a new solution, and its study promises to unlock deeper insights into the astonishing diversity of worlds beyond our solar system.
Paper Summary
Methodology
The discovery began with TESS data, identifying TOI-6894b as a candidate transiting planet. Ground-based telescopes were then used for follow-up observations to confirm the transit signal and rule out false alarms like eclipsing binaries. High-resolution spectroscopic observations from telescopes also measured the star’s ‘wobble,’ confirming the planet’s mass. All collected data, including TESS, ground-based light measurements, and wobble data, were jointly analyzed using advanced statistical methods to determine the system’s precise properties.
Results
The study confirmed TOI-6894b as a low-density gas giant, slightly larger than Saturn but only about half its mass, orbiting a small star roughly 20% the Sun’s mass every 3.37 days. Its temperature is around 418 Kelvin. A key finding is its incredibly deep transits (17% dimming), making it an excellent target for studying its atmosphere, which is expected to be rich in methane and potentially ammonia. This discovery challenges current theories about how giant planets form, especially around such small stars, and the planet’s high metal content suggests unique formation processes.
Limitations
The study acknowledges limitations in understanding how such a planet formed. Current disc mass measurements might underestimate available material for planet formation. Also, the efficiency of planet formation around small stars is still uncertain, and the small number of observed low-mass star discs limits our understanding. Different computer simulations of planet formation also yield varying results, highlighting the complexity of this process.
Funding/Disclosures
All data used in this study, including TESS photometry and ground-based observations from various telescopes, are publicly available or accessible through specific archives and portals. Further details on the research design are provided in the Nature Portfolio Reporting Summary.
Publication Information
The research was published in Nature Astronomy on June 4, 2025. The paper is titled ‘A transiting giant planet in orbit around a 0.2-solar-mass host star’ and can be found via DOI: https://doi.org/10.1038/s41550-025-02552-4. The lead author is Dr. Edward Bryant (University of Warwick and UCL’s Mullard Space Science Laboratory), with Dr. Andrés Jordán (Millennium Institute of Astrophysics and Adolfo Ibáñez University) among the co-authors.
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