# Neptune-Sized Exoplanet Orbits Backward, Challenging Formation Theories GJ 3090 b's retrograde orbit around its star puzzles astronomers and offers new planetary formation insights. - **Source:** [All Articles | Discover Magazine](https://www.discovermagazine.com/rss/all) - **Published:** 2026-09-22 - **Original article:** - **On Brifing:** https://brifing.si/s/2a7e66ed-e36a-4419-9ea8-e4aa95db952c ## Unusual planetary motion GJ 3090 b, a Neptune-sized exoplanet 73 light-years from Earth, orbits its M dwarf star in the opposite direction to the star's rotation. This retrograde orbit was confirmed through measurements showing a 136-degree orbital obliquity, starkly contrasting with our solar system's planets that align within 3-7 degrees of the sun's equatorial plane. The discovery challenges conventional understanding of planetary system formation where planets typically orbit in the same direction as their host star's rotation. ## Discovery and characteristics First detected in 2022 by NASA's TESS space telescope, GJ 3090 b has a radius 2.2 times Earth's and a mass 4.5 times Earth's. Recent observations using the NIRPS instrument on Chile's ESO La Silla 3.6m Telescope precisely measured its three-dimensional orbit. This marks the first known instance of a planet with a backward orbit around an M dwarf star, a type of small, cool star that's common in our galaxy but rarely found with such planetary anomalies. ## Formation hypotheses Astronomers initially theorized gravitational interactions with a massive nearby planet or companion star might have altered GJ 3090 b's orbit. However, no such objects were found. Instead, researchers propose the star may have acquired a second, misaligned disk of gas and dust during formation. Planets forming from this material would inherit its unusual orientation, explaining GJ 3090 b's retrograde path without requiring disruptive external forces. ## Measurement techniques The NIRPS instrument's near-infrared spectroscopy enabled precise measurement of the planet's orbital obliquity. By analyzing the angle between the star's equatorial plane and the planet's orbital plane, researchers confirmed the extreme 136-degree tilt. This technique provides crucial three-dimensional orbital data that simple transit observations from TESS couldn't reveal, demonstrating how complementary observation methods advance exoplanet research. ## Comparative planetary science GJ 3090 b's retrograde orbit contrasts sharply with planets in our solar system, where all eight planets orbit within 7 degrees of the sun's equatorial plane. This difference suggests M dwarf systems may form through more chaotic processes than sun-like stars. The discovery implies that the environment around young stars plays a greater role in planetary architecture than previously thought, potentially explaining other exoplanetary anomalies. ## Theoretical implications The misaligned disk hypothesis suggests planetary systems can incorporate material from multiple sources with different orientations. If confirmed, this would expand our understanding of how protoplanetary disks evolve and interact with their stellar environments. The findings challenge the assumption that planetary systems form from single, coherent disks, opening new questions about the diversity of planetary system architectures. ## Future research directions Astronomers emphasize the need for additional observations to verify the misaligned disk formation scenario. Future studies will search for similar orbital configurations around other M dwarfs, which could reveal how common such formation pathways are. The team also plans to investigate whether GJ 3090 b's atmosphere shows compositional clues about its formation history, which might provide further evidence for the dual-disk hypothesis. ## Scientific significance GJ 3090 b's discovery demonstrates that current planetary formation models may not account for all possible formation scenarios, especially around M dwarf stars. By showing how environmental conditions can dramatically alter planetary architectures, this finding will influence future exoplanet research and telescope design. The system serves as a natural laboratory for testing theories about disk dynamics and planetary migration in young stellar systems. ## Sources covering this story - https://www.discovermagazine.com/neptune-sized-exoplanet-mysteriously-orbits-its-star-backward-raising-questions-about-planetary-formation-49713 — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._