TESS discovers two Jupiter-sized exoplanets—TOI-5916 b and TOI-6158 b—orbiting M-dwarf stars with Saturn-like densities and ultra-short periods.
A UCI-led team using NASA’s TESS announced the discovery of two new gas giants around mid-M-dwarf stars: TOI-5916 b (1.05 RJ, 0.69 MJ) orbiting in 2.37 days at 0.028 AU, and TOI-6158 b (0.93 RJ, 0.42 MJ) with a 3.04 day period at 0.033 AU. Both exhibit low densities (0.73 and 0.66 g/cm³) and equilibrium temperatures near 700 K, joining just 32 known giant exoplanets around M-dwarfs (GEMS).
The Curious Giant Planets Around Tiny Stars
Gas giants are rare around M-dwarfs due to limited protoplanetary disk mass. The GEMS survey targets TESS Objects of Interest (TOIs) around ~200,000 bright nearby stars, confirming planets via ground-based photometry (Red Buttes Observatory for TOI-5916, Swope for TOI-6158) and Habitable-zone Planet Finder radial velocities. TOI-5916’s host (Teff = 3541 K, 0.51 M⊙, 0.49 R⊙, 639 ly) and TOI-6158’s host (Teff = 3467 K, 0.49 M⊙, 0.47 R⊙, 592 ly) both enable high signal-to-noise transit detections despite faint stellar magnitudes.
What Happens During Transit and RV Confirmation
Phase-folded light curves (Figure: TOI-5916) show ~1% transit depths corresponding to 1.05 RJ and 0.93 RJ radii with 2.37 and 3.04 day periods. Habitable-zone Planet Finder measured RV semi-amplitudes of 85 m/s and 45 m/s, yielding masses of 0.69 MJ and 0.42 MJ and densities of 0.73 and 0.66 g/cm³. Equilibrium temperatures calculated from stellar luminosities and orbital distances are 716 K and 636 K, classifying both as warm Jupiters rather than hot Jupiters.
Why It Matters for Planet Formation Around M-Dwarfs
Core-accretion models predict long formation timescales for gas giants around low-mass stars due to depleted solids, making GEMS rare test cases for disk mass and migration theories. TOI-5916 b and TOI-6158 b’s short periods suggest disk-driven inward migration within 10 Myr. Their low densities indicate substantial hydrogen–helium envelopes, constraining envelope accretion efficiency against photoevaporative mass loss under M-dwarf XUV fluxes ~10× solar levels at 0.03 AU.
Observational Challenges in Characterizing GEMS
M-dwarf variability and flaring complicate transit depth stability and RV precision, requiring simultaneous photometric monitoring to correct spot-crossing and activity-induced RV jitter. High stellar rotational velocities (>2 km/s) broaden spectral lines, diminishing RV sensitivity. TESS 2-minute cadence and dedicated follow-up spectrographs with fiber scrambling and laser frequency comb calibration achieve <5 m/s precision needed for mass determination.
Link to Comparative Exoplanetology
Among 32 known GEMS, the newly discovered planets join a growing sample showing trend toward low densities (~0.5–1.0 g/cm³) at 0.02–0.05 AU. Comparison with TRAPPIST-1’s rockier planets highlights divergent formation pathways within M-dwarf systems. Upcoming JWST and ELT spectroscopy of transmitted atmospheres may detect molecular species (H₂O, CO, TiO), probing envelope metallicity and distinguishing migration history between in situ formation and high-eccentricity tidal circularization.
What the Future Holds for GEMS Surveys
Continued TESS monitoring through its extended mission phases will identify additional GEMS candidates for demographic studies. Ground-based radial velocity campaigns (EXPRES, NEID) expanding to fainter M-dwarfs aim to confirm >50 GEMS by 2027. Gaia DR4 astrometry will provide independent mass estimates for the brightest systems, improving bulk density accuracy. Combined data will refine mass–radius–insolation relations for gas giants across host mass spectrum.
Why This Discovery Is So Exciting for Exoplanet Science
Finding Jupiter-sized planets around M-dwarfs at 0.03 AU challenges prevailing disk-mass constraints, expanding understanding of planet formation diversity. TOI-5916 b and TOI-6158 b enable comparative studies of atmospheric retention under intense M-dwarf irradiation, critical for evaluating habitability potential of moons or Trojan companions. These results underscore TESS’s continuing yield of compelling targets for next-generation telescopes exploring planetary system architecture around the most common stars in our galaxy.
Conclusion
TOI-5916 b and TOI-6158 b add to the exclusive GEMS sample, offering unique laboratories for testing planet formation and atmospheric evolution around low-mass stars. As follow-up observations proceed with JWST and ELTs, these warm Jupiters will illuminate the processes shaping diverse planetary systems. Explore more about astronomy and space discoveries on our YouTube channel, So Join NSN Today.
