The latest cosmic puzzle for scientists comes from the constellation Leo, roughly 240 lightyears from Earth. Astronomers, studying data from NASA’s Transiting Exoplanet Survey Satellite (TESS) and the European Southern Observatory’s Chile-based Very Large Telescope (VLT), have found an anomaly: a gas giant the size of Saturn orbiting a star, TOI-6894, that is only about a fifth the mass of our sun.
The unusual finding, published recently in Nature Astronomy, challenges the known theories of planetary formation, which suggest such diminutive stars should host only smaller worlds akin to Earth or Mars. Astronomer Edward Bryant of the University of Warwick, lead author of the study, says: “The question of how such a small star can host such a large planet is one that this discovery raises – and we are yet to answer.”
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TOI-6894 is now the smallest-known star to host a large planet, roughly 40% smaller than previous record holders. While no known planet is larger than its host star, the disparity in size between TOI-6894 and its lone planet is remarkably small; the star’s diameter is merely 2.5 times greater than that of its companion, a stark contrast to our own solar system where the sun’s diameter is ten times larger than Jupiter’s. And, this red dwarf is also exceptionally dim, about 250 times less luminous than our sun.
“These findings suggest that even the smallest stars in the universe can in some cases form very large planets,” noted Van Eylen, “that forces us to rethink some of our planet formation models.” Adding to the startling anomaly, this big planet orbits incredibly close to its star, completing a full revolution in approximately three days, resulting in a hot surface, though not quite as scorching as the “Hot Jupiters” found orbiting larger stars. It is less dense than Jupiter and Saturn, with a mass that is 56% that of Saturn and 17% that of Jupiter.
Cosmic lab of extremes
Indeed, the universe is a cosmic laboratory of extremes, where such “mismatches” are increasingly emerging. When HAT-P-67 b, a true titan with twice Jupiter’s radius, was found, its immense girth is deceptive as it is incredibly “fluffy,” possessing only a third of Jupiter’s mass, challenging our understanding of how such low-density giants form. Similarly, HD 100546 b, located 320 light-years away, is a behemoth that, along with its surrounding disk, initially estimated at 6.9 times Jupiter’s radius, blurring the lines between a planet and a brown dwarf.
Another record-breaker was TrES-4b, discovered in 2007. For quite some time, it held the title for both largest and least dense confirmed exoplanet, with a density akin to Saturn’s moon Methone. Its unusually large size for its mass is attributed to its scorching proximity to a highly luminous star and significant internal heat. WASP-12b further exemplifies the “puffed-up” phenomenon, being one of the least dense exoplanets known and dramatically close to its star – so close, in fact, it’s slowly being consumed.
Then there are the borderline cases: DENIS-P J082303.1-491201 b, a massive object around 28.5 times Jupiter’s mass, and Kepler-39b, which weighs in at 18 times Jupiter’s mass and is 1.22 times its size. These objects occupy the ambiguous territory between very large planets and brown dwarfs, highlighting the dynamic nature of planetary classification. Even ROXs 42Bb, a gas giant 2.5 times Jupiter’s radius and nine times its mass, prompts scientific debate about its formation, with some suggesting it might have formed more like a star than a typical planet.
Extreme climates and bizarre findings
Beyond sheer size, exoplanets offer extreme environments to study. KELT-9b, the hottest known exoplanet with searing temperatures reaching up to 4,300 degrees C (approximately 7,800 degrees F) has an extreme atmosphere that it contains vaporized iron and titanium, which astonishingly could condense into metallic rain. Another “Hot Jupiter,” HAT-P-7b, is theorized to have clouds of rubies and sapphires that rain down across its surface. Similarly, 55 Cancri e, often dubbed the “hell planet,” is theorized to have carbon-rich composition, that combined with extreme pressure, even suggests the possibility of diamond rain.
Evoking science fiction, Kepler-16b and Kepler-64b exoplanets are circumbinary, orbiting two stars at once, offering double sunsets to any hypothetical observer. In stark contrast, TrES-2b is the darkest known exoplanet, absorbing nearly all light that falls on it, making it less reflective than coal. Scientists speculate its perpetually dark environment might emanate a deep red glow from its burning atmosphere. The Neptune-sized GJ 436 b puzzles astronomers with a mysterious absence of methane in its hot atmosphere, while TOI-849 b is believed to be the core of a gas giant whose atmosphere was completely blasted away by its star.
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Beyond these atmospheric aberrations, the universe also hosts Rogue Planets like OTS 44 and Proplyd 133-353, which drift through space without a host star, some still retaining circumstellar disks. And the concept of “Eyeball Planets,” exemplified by LHS 1140 b, describes tidally locked worlds where one side permanently faces its star, potentially melting enough ice to form a liquid water ocean on that side, giving the planet a distinctive bull’s-eye appearance.
The discovery of these remarkable planets, from the “cosmic mismatch” of TOI-6894 b to the diamond-raining 55 Cancri e, underscores the incredible nature of the Universe, pushing the boundaries of our understanding of planetary formation, and stretching our imagination about the potential for life in the most unexpected corners of the cosmos, where each exoplanet has its unique story to tell.