Astronomers have unveiled an exciting new planetary system orbiting a star not unlike our own sun—though far younger and nestled nearly 500 light years away. The star, TOI-6109, shines within the Alpha Persei cluster and has recently revealed two alien worlds hiding in its glow. Using NASA’s Transiting Exoplanet Survey Satellite (TESS), researchers detected subtle dips in the star’s light, tiny shadows cast by orbiting planets. Follow-up observations confirmed the existence of these worlds, marking a remarkable step in our quest to understand how planets form and evolve.
TOI-6109 is special because of its youth. At just 75 million years old, this star is a cosmic adolescent compared to our middle-aged 4.6-billion-year-old sun. Studying planets around such a young star is like peering into a planetary nursery, offering astronomers a rare glimpse of the earliest chapters in the lives of alien worlds.
The Discovery of TOI-6109 b and TOI-6109 c
The two newfound exoplanets, TOI-6109 b and TOI-6109 c, are slightly larger than Neptune and orbit extremely close to their star. TOI-6109 b circles its host in just 5.69 days, skimming along at a distance of 0.06 astronomical units (AU)—far closer than Mercury’s orbit around our sun. Its sibling, TOI-6109 c, takes 8.54 days to complete an orbit, residing only slightly farther out at 0.078 AU.
Both planets measure about 4.8 times the size of Earth, placing them in the intriguing category of “sub-Saturns.” These worlds are too large to be rocky like Earth but smaller than the gas giants Jupiter and Saturn, existing in a size range that has no true counterpart in our solar system. Their existence challenges our understanding of planetary formation, raising questions about how such intermediate planets arise and whether they evolve into gas giants or shrink into smaller worlds over time.
A Delicate Cosmic Dance
What makes TOI-6109 b and c especially fascinating is the way they orbit. The two worlds are locked in orbits just outside a 3:2 resonance, meaning their periods are nearly—but not exactly—aligned in a simple ratio. This near-resonant configuration creates tiny gravitational tugs between the planets, producing measurable changes in the timing of their transits across the star’s face.
Astronomers can use these transit timing variations (TTVs) to estimate the planets’ masses without relying solely on radial velocity measurements, which are difficult to make around such a young and active star. These mass estimates are critical, as they allow scientists to probe the composition and density of the planets—key clues in understanding whether these worlds are puffy, gas-rich envelopes or denser, ice-laden spheres.
A Star Much Like Our Sun
Despite its youth, TOI-6109 bears a striking resemblance to our own sun. Classified as a G3-type star, it is only slightly cooler than the sun, with a surface temperature of 5,660 Kelvin and a luminosity about 88 percent that of our solar system’s center. Its mass and size are nearly identical to the sun’s, making it an invaluable analog for studying planetary systems that might resemble our own in their earliest stages.
By comparing TOI-6109’s system to our solar system’s history, scientists hope to reconstruct how planets migrate inward or outward, how they settle into stable orbits, and why some stars produce rocky worlds while others nurture only gas giants.
Why This Discovery Matters
The detection of TOI-6109’s planets is more than a tally mark in the growing catalog of alien worlds. It represents a rare opportunity to study planetary systems at a time when planets are still undergoing significant changes. Young systems like this help answer questions that cannot be solved by looking only at mature stars like our sun.
Do planets form where we find them today, or do they migrate across their systems in great journeys? Why do some planets end up in resonant orbits while others scatter chaotically? How do the atmospheres of young planets evolve under the fierce radiation of their host stars? The TOI-6109 system holds clues to all these mysteries.
TESS and the Search for Other Worlds
The discovery also highlights the continuing success of NASA’s TESS mission. Designed to scan the sky for the telltale dimming of stars caused by planetary transits, TESS has already identified nearly 7,700 exoplanet candidates, of which 693 have been confirmed. Its wide survey of about 200,000 nearby bright stars provides fertile ground for discoveries that reshape our understanding of planetary diversity.
TOI-6109 is just one star among thousands in TESS’s gaze, but the richness of its planetary system demonstrates the mission’s power to uncover unexpected treasures. Each detection adds a piece to the cosmic puzzle, helping us see the patterns and exceptions that define planetary systems across the galaxy.
The Road Ahead
Astronomers are eager to continue monitoring TOI-6109. The delicate near-resonant dance of its planets makes the system an ideal candidate for long-term transit observations. As scientists refine measurements of the planets’ masses and orbits, they will also gain insights into their atmospheric properties and dynamical interactions.
In the coming years, observatories like the James Webb Space Telescope (JWST) may even probe the atmospheres of TOI-6109 b and c, searching for chemical signatures that reveal their composition. Such data will illuminate not just these two worlds but the processes that shape planetary systems everywhere.
A Cosmic Story Still Being Written
The discovery of TOI-6109’s planetary system is a reminder that the universe is brimming with stories waiting to be told. Around a young star glowing in a distant cluster, two alien worlds circle in a quiet gravitational dance, carrying secrets about how planets are born and how they evolve.
For astronomers, TOI-6109 is not merely another dot of light in the sky—it is a laboratory of cosmic history, a place where theories can be tested against the raw evidence of youth. For the rest of us, it is a symbol of discovery, a sign that even as we learn more about the cosmos, the universe continues to surprise us.
In TOI-6109, we glimpse the beginnings of planetary lives, perhaps not unlike the story our own solar system once told. It is a reminder that while Earth is our home, the universe is vast and filled with worlds beyond counting—each one a chapter in the grand, unfolding story of creation.
More information: Anne Dattilo et al, THYME XIII: Two young Neptunes orbiting a 75-Myr star in the Alpha Persei Cluster, arXiv (2025). DOI: 10.48550/arxiv.2509.15313
