Astronomers have announced the detection of two large, gaseous exoplanets orbiting distant M-dwarf stars, a discovery that challenges prevailing theories of planetary formation. The new worlds, designated TOI-5916 b and TOI-6158 b, are both comparable in size to Jupiter but orbit small, cool stars where such massive planets are thought to be exceedingly rare. The findings, made by a University of California, Irvine-led team using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), provide crucial new data points for understanding how planetary systems evolve around the most common type of stars in the galaxy.
The existence of these “gas giants” in close proximity to low-mass host stars pushes the boundaries of the leading formation model, known as core accretion. This model posits that giant planets grow by first accumulating a solid core that becomes massive enough to rapidly pull in huge amounts of gas from the surrounding protoplanetary disk. M-dwarf systems are believed to have less massive disks that dissipate more quickly than those around larger stars like our sun, creating a challenging environment for the birth of giants. These newly identified planets, therefore, serve as critical test cases for theories explaining how and where such worlds can form.
Discovery via the Transit Method
The planets were first identified using data from NASA’s TESS mission, which is designed to survey hundreds of thousands of the brightest nearby stars to find transiting exoplanets. The satellite’s sensitive cameras detect tiny, periodic dips in a star’s brightness, which can indicate that an orbiting planet is passing in front of it from our point of view. TESS observed the host stars for both new planets, allowing scientists to pinpoint the initial candidates and measure their radii and orbital periods based on the transit depth and frequency. For TOI-5916, the data revealed a transit depth of about 1%, corresponding to a planet just larger than Jupiter, while TOI-6158 b showed a transit corresponding to a planet slightly smaller than Jupiter.
Characterizing the New Worlds
Following their initial detection by TESS, the planetary candidates were confirmed through a rigorous process of ground-based observation. This follow-up work was essential to validate the discoveries and precisely measure the planets’ masses.
Profile of TOI-5916 b
TOI-5916 b is a gas giant with a radius 1.05 times that of Jupiter but a mass of only 0.69 times Jupiter’s. This gives it a surprisingly low, Saturn-like density of 0.73 grams per cubic centimeter. The planet orbits its host star in an extremely tight embrace, completing a full circle in just 2.37 days at a distance of only 0.028 astronomical units (AU). Due to this proximity, its estimated equilibrium temperature is 716 K (about 829°F), classifying it as a “warm Jupiter.”
Profile of TOI-6158 b
The second planet, TOI-6158 b, has a radius of 0.93 times Jupiter and is even less massive, at 0.42 times Jupiter’s mass. Its density is also low, calculated at 0.66 grams per cubic centimeter. It orbits its star every 3.04 days at a distance of 0.033 AU. Its greater distance from its slightly cooler star results in a lower equilibrium temperature of 636 K (about 685°F), also placing it in the warm Jupiter category.
Small Stars Hosting Giant Planets
Both TOI-5916 b and TOI-6158 b orbit mid-M-dwarf stars, which are significantly smaller and cooler than the sun. The star TOI-5916, located 639 light-years from Earth, has a surface temperature of 3541 K and is about half the mass and radius of the sun. Its companion, TOI-6158, is 592 light-years away and has a similar size, with a temperature of 3467 K and about 49% of the sun’s mass and 47% of its radius. Discovering Jupiter-sized planets around such small parent bodies is a key piece of the puzzle, as these systems represent an extreme outcome of planet formation.
Confirming Mass with Radial Velocity
To confirm the planetary nature of the TESS signals and determine their masses, the research team used the Habitable-zone Planet Finder (HPF) spectrograph. This high-precision instrument measures a star’s radial velocity—its slight movement, or “wobble,” toward and away from Earth. This wobble is caused by the gravitational tug of an orbiting planet. By measuring the amplitude of the wobble, astronomers can calculate the planet’s mass. The HPF measured a radial velocity semi-amplitude of 85 meters per second for the star TOI-5916, confirming the mass of its planet. For TOI-6158, a smaller amplitude of 45 meters per second was detected, corresponding to its lower-mass planet. Additional photometric observations from the Red Buttes Observatory and Swope Observatory helped rule out false positives and further refine the transit data.
Implications for Planetary Science
The discovery of these two systems adds to a small but growing collection of Giant Exoplanets around M-dwarf Stars, known as GEMS, which currently numbers just 32 worlds. Each new addition provides a crucial data point for testing and refining formation models. The very short orbital periods of TOI-5916 b and TOI-6158 b suggest that they likely formed farther out in their respective systems and then migrated inward. This process, known as disk-driven migration, would have had to occur within the first 10 million years of the system’s life before the protoplanetary disk dissipated. The existence of these planets suggests that the processes that build gas giants can operate more efficiently or under a wider range of conditions than previously thought. Future studies, potentially including atmospheric characterization with the James Webb Space Telescope, could provide further insights into their composition and formation history, shedding light on the incredible diversity of planetary systems throughout the cosmos.
