Hot Jupiters around M Dwarfs (HJMD)

• HJMDs are short-period giant planets orbiting low-mass M-dwarf stars, defined by deep transit depths and an occurrence rate of ~0.27% in early-type M dwarfs.
• Research reveals that these planets exhibit distinct metallicity and mass distribution trends compared to giants around FGK stars, reflecting formation and disk-mass differences.
• Investigations into HJMDs inform models of formation, migration, and spin-orbit alignment, while their favorable transit and RV signals enable high-value atmospheric studies.

Hot Jupiters around M dwarfs (HJMDs) are short-period giant planets orbiting low-mass main-sequence stars. In current transit-occurrence work on early-type M dwarfs, the operational planet domain is commonly taken as $7\,R_\oplus \leq R_p \leq 2\,R_J$ and $0.8 \leq P \leq 10$ days, while some demographic studies instead separate hot and warm giants by scaled semi-major axis, defining hot systems by $a/R_\ast \leq 20$ (Gan et al., 2022, Gan et al., 2024). The class is intrinsically rare, with the best current transit-based occurrence estimate for early-type M dwarfs at $0.27\pm0.09\%$, yet it has become disproportionately important because small M-dwarf radii produce unusually deep transits, host metallicities are strongly informative, and the emerging sample now reaches the level at which formation, migration, and spin-orbit architecture can be compared across stellar mass (Gan et al., 2022, Gan et al., 2024).

1. Definition and parameter space

Operational definitions of HJMDs are not uniform across the literature. The TESS occurrence analysis for early-type M dwarfs used $7\,R_\oplus \leq R_p \leq 2\,R_J$, $0.8 \leq P_b \leq 10$ days, and a stellar sample restricted to $2900 \leq T_{\rm eff} \leq 4000$ K and $0.45 \leq M_\ast \leq 0.65\,M_\odot$ (Gan et al., 2022). The homogeneous SpeX metallicity study selected M-dwarf hosts with $T_{\rm eff}<4000$ K and $M_\ast<0.65\,M_\odot$, considered giant planets in the interval $0.8 \leq P \leq 10$0, and divided them into hot and warm systems at $0.8 \leq P \leq 10$1 (Gan et al., 2024). A separate comparative study of transiting giants used a size threshold of $0.8 \leq P \leq 10$2 and highlighted the specific role of super-Jupiters at $0.8 \leq P \leq 10$3 in shaping host-mass trends (Kanodia, 2024).

These differences are not merely terminological. A period-selected sample, a scaled-separation sample, and a radius-selected sample need not contain identical objects, especially around compact, cool hosts for which a planet can be dynamically “hot” by period while remaining only moderately irradiated. As a result, statements about HJMD occurrence, metallicity, and mass distributions are definition-dependent at a level that remains non-negligible for the current sample sizes.

2. Historical development and representative systems

The class was established observationally by KOI-254b, confirmed as the first hot Jupiter around an M-type dwarf. The system has $0.8 \leq P \leq 10$4 days, $0.8 \leq P \leq 10$5, $0.8 \leq P \leq 10$6, and $0.8 \leq P \leq 10$7 AU, orbiting a host with $0.8 \leq P \leq 10$8 and $0.8 \leq P \leq 10$9; the host was already noted to be metal-rich (Johnson et al., 2011). By 2018, HATS-71b pushed the sample to a cooler host regime: an M3V star with $a/R_\ast \leq 20$0 K, a transit depth of about $a/R_\ast \leq 20$1, and a planet with $a/R_\ast \leq 20$2 and $a/R_\ast \leq 20$3 on a 3.7955 day orbit (Bakos et al., 2018).

Later discoveries expanded the phenomenology in two directions. TOI-3235 b is a short-period giant around an M4 dwarf with $a/R_\ast \leq 20$4, close to the partially-to-fully-convective transition, with $a/R_\ast \leq 20$5, $a/R_\ast \leq 20$6, and $a/R_\ast \leq 20$7 days (Hobson et al., 2023). TOI-4201b instead occupies the massive end of the known population, with $a/R_\ast \leq 20$8, $a/R_\ast \leq 20$9, host mass $0.27\pm0.09\%$0, and planet-to-star mass ratio $0.27\pm0.09\%$1 (Gan et al., 2023). By 2025, the NASA Exoplanet Archive contained 19 known HJMDs, 9 of them in binaries (Weisserman et al., 18 Aug 2025).

The historical trajectory is therefore from singular proof-of-existence to a still small but astrophysically structured population. That transition is what now permits population-level work on occurrence, metallicity, obliquity, and binary-assisted migration.

3. Occurrence rates and survey selection effects

Early dedicated transit surveys mainly delivered upper limits. The WFCAM Transit Survey analyzed 4523 M0–4 dwarfs in its 19h field and, with zero confirmed hot-Jupiter detections, placed a 95% upper limit of 1.7–2.0% on the occurrence of short-period hot Jupiters around M dwarfs (Kovács et al., 2013). Pan-Planets increased the target count to 65,258 M dwarfs and derived either $0.27\pm0.09\%$2 if one candidate were confirmed or a 95% upper limit of 0.34% if none were real (Obermeier et al., 2015). A TESS Primary Mission search of 60,819 early-type M dwarfs then produced the first robust occurrence estimate from a homogeneous transit pipeline with explicit completeness correction, yielding $0.27\pm0.09\%$3 for $0.27\pm0.09\%$4 and $0.27\pm0.09\%$5 d (Gan et al., 2022).

Survey	M-dwarf sample	HJMD result
WTS	4523 M0–4 dwarfs	95% upper limit 1.7–2.0%
Pan-Planets	65,258 M dwarfs	$0.27\pm0.09\%$6 if one candidate is real; otherwise 0.34% upper limit
TESS early-M search	60,819 stars	$0.27\pm0.09\%$7

A later synthesis summarized the contrast with higher-mass hosts as $0.27\pm0.09\%$8 for early M dwarfs and $0.27\pm0.09\%$9 for FGK stars, implying that hot Jupiters are empirically 2–3 times less common around early M dwarfs than around AFGK stars (Weisserman et al., 18 Aug 2025).

The detectability of HJMDs is unusual because the same host-star properties that suppress occurrence enhance observability. Transit depth scales as $7\,R_\oplus \leq R_p \leq 2\,R_J$0, so a Jovian planet produces a deeper signal around an M dwarf than around an FGK star. RV sensitivity also benefits from low stellar mass through

$7\,R_\oplus \leq R_p \leq 2\,R_J$1

for $7\,R_\oplus \leq R_p \leq 2\,R_J$2, which raises the semi-amplitude at fixed $7\,R_\oplus \leq R_p \leq 2\,R_J$3 and $7\,R_\oplus \leq R_p \leq 2\,R_J$4 (Gan et al., 2022). The practical counterweights are equally clear in the survey literature: optical faintness, magnetic activity, starspots, long-cadence smearing, and strong window-function aliasing all complicate discovery and completeness calibration [(Kovács et al., 2013); (Obermeier et al., 2015)].

4. Metallicity, bulk demographics, and the super-Jupiter question

The strongest homogeneous metallicity result comes from a SpeX/IRTF analysis comparing 746 field M dwarfs without known giant planets against 22 M dwarfs hosting 27 giant planets. Using metallicities derived uniformly from the same near-infrared methodology, that study found that giant planets favor metal-rich M dwarfs at 4–5$7\,R_\oplus \leq R_p \leq 2\,R_J$5, with host-versus-field p-values of H-band A–D $7\,R_\oplus \leq R_p \leq 2\,R_J$6 and K-band A–D $7\,R_\oplus \leq R_p \leq 2\,R_J$7. Within the giant-planet host sample, the 10 hot systems and 17 warm systems had statistically indistinguishable host-metallicity distributions, no significant correlation was found between giant-planet mass and host metallicity, and no significant difference emerged between single-giant and multi-giant hosts (Gan et al., 2024).

An archive-based comparison reached a different conclusion for the hot-versus-warm split around M hosts. Using hot and warm categories divided at $7\,R_\oplus \leq R_p \leq 2\,R_J$8, it found median host metallicities of approximately $7\,R_\oplus \leq R_p \leq 2\,R_J$9 dex for hot Jupiters around M dwarfs and approximately $0.8 \leq P_b \leq 10$0 dex for warm Jupiters around M dwarfs, whereas hot and warm Jupiters around G stars both showed median [Fe/H] of approximately $0.8 \leq P_b \leq 10$1 dex (Gan et al., 2023). This suggests that metallicity trends within the M-dwarf giant-planet population remain sensitive to sample construction, heterogeneity of discovery channels, and the still small number of confirmed systems.

The mass distribution adds a second demographic asymmetry. A comparative analysis of transiting giant planets around FGKM hosts reported that the average mass of M-dwarf Jupiters is lower than that of solar-type counterparts primarily because super-Jupiters with $0.8 \leq P_b \leq 10$2 are scarce around M dwarfs; once super-Jupiters are excluded, the average masses of M-dwarf and FGK warm-Jupiters become strikingly similar (Kanodia, 2024). The same work proposed a minimum disk dust mass threshold for Jovian formation through core accretion, more often satisfied around higher-mass stars. This suggests that the M-dwarf difference is concentrated in the super-Jupiter tail rather than being a simple downward rescaling of the entire Jovian mass function.

5. Formation channels, migration, and spin-orbit architecture

Formation theory has treated HJMDs as difficult systems from the outset. Classical core-accretion models and later population syntheses anticipate few Jovian planets around $0.8 \leq P_b \leq 10$3, especially at short periods, because lower disk masses reduce the probability of forming sufficiently massive cores before gas dispersal (Hobson et al., 2023). The occurrence data are consistent with that broad expectation, and the TESS early-M analysis further argued that the decline in $0.8 \leq P_b \leq 10$4 from cold to hot Jupiters may be steeper for early-type M dwarfs than for FGK stars, with an inferred change from about $0.8 \leq P_b \leq 10$5 at 1–10 AU to about $0.8 \leq P_b \leq 10$6 at 0.01–0.1 AU for early M dwarfs, compared with roughly $0.8 \leq P_b \leq 10$7 to $0.8 \leq P_b \leq 10$8 for FGK stars (Gan et al., 2022).

Spin-orbit measurements have begun to test whether close-in M-dwarf giants arrived through dynamically quiet disk migration or through high-eccentricity pathways. TOI-4201b yielded the first Rossiter–McLaughlin detection for an HJMD, with sky-projected obliquity $0.8 \leq P_b \leq 10$9 and true obliquity $2900 \leq T_{\rm eff} \leq 4000$0, with a 95% upper limit of $2900 \leq T_{\rm eff} \leq 4000$1 (Gan et al., 2024). In that system, the aligned geometry and nearly circular orbit were interpreted as consistent with dynamically quiet formation or preserved primordial alignment, because the tidal estimates adopted there gave $2900 \leq T_{\rm eff} \leq 4000$2 Gyr or $2900 \leq T_{\rm eff} \leq 4000$3–200 Gyr, both long compared with the likely system age (Gan et al., 2024).

MAROON-X later extended the obliquity sample to TOI-3714 b and TOI-5293 A b. The measured values were $2900 \leq T_{\rm eff} \leq 4000$4 and $2900 \leq T_{\rm eff} \leq 4000$5 for TOI-3714, and $2900 \leq T_{\rm eff} \leq 4000$6 and $2900 \leq T_{\rm eff} \leq 4000$7 for TOI-5293 A (Weisserman et al., 18 Aug 2025). Both hosts are in wide binaries. In the binary-orbit analysis, about 30% of minimum mutual-inclination samples for TOI-5293 exceeded the Kozai–Lidov threshold of $2900 \leq T_{\rm eff} \leq 4000$8, compared with about 3% for TOI-3714, and the same work argued that early-M dwarfs can damp misaligned obliquities efficiently, with $2900 \leq T_{\rm eff} \leq 4000$9 yr for TOI-3714 b and $0.45 \leq M_\ast \leq 0.65\,M_\odot$0 yr for TOI-5293 A b (Weisserman et al., 18 Aug 2025).

The empirical conclusion is therefore clearer than the theoretical one. All three RM-measured HJMDs are aligned, but the interpretation remains model-dependent: one analysis favors dynamically quiet formation or preserved alignment, while another allows efficient Kozai–Lidov migration followed by rapid tidal realignment. This tension is plausibly a consequence of the small obliquity sample and differing tidal prescriptions rather than of contradictory data.

6. Thermal states, observables, and atmospheric characterization

A persistent misconception is that “hot Jupiter” and “high equilibrium temperature” are interchangeable around M dwarfs. They are not. TOI-3235 b is a hot Jupiter by period, with $0.45 \leq M_\ast \leq 0.65\,M_\odot$1 d, but has $0.45 \leq M_\ast \leq 0.65\,M_\odot$2 K; HATS-71b has $0.45 \leq M_\ast \leq 0.65\,M_\odot$3 d and $0.45 \leq M_\ast \leq 0.65\,M_\odot$4 K; TOI-4201b has $0.45 \leq M_\ast \leq 0.65\,M_\odot$5 d and $0.45 \leq M_\ast \leq 0.65\,M_\odot$6 K (Hobson et al., 2023, Bakos et al., 2018, Gan et al., 2023). Around cool, low-luminosity hosts, short-period orbits can still correspond to irradiation levels well below the canonical inflation regime.

At the same time, these systems are exceptionally favorable for transit and RV characterization. TOI-3235 b has $0.45 \leq M_\ast \leq 0.65\,M_\odot$7 and $0.45 \leq M_\ast \leq 0.65\,M_\odot$8; HATS-71b has a transit depth of about $0.45 \leq M_\ast \leq 0.65\,M_\odot$9; TOI-4201b has $T_{\rm eff}<4000$0 and $T_{\rm eff}<4000$1 (Hobson et al., 2023, Bakos et al., 2018, Gan et al., 2023). The RV amplitudes are likewise large: TOI-3235 b has $T_{\rm eff}<4000$2 m s$T_{\rm eff}<4000$3 and TOI-4201b has $T_{\rm eff}<4000$4 m s$T_{\rm eff}<4000$5 (Hobson et al., 2023, Gan et al., 2023). In atmospheric terms, TOI-3235 b is especially favorable, with Transmission Spectroscopy Metric $T_{\rm eff}<4000$6 and an expected transmission signal per scale height of about 300 ppm, whereas TOI-4201b is substantially less accessible with TSM $T_{\rm eff}<4000$7 (Hobson et al., 2023, Gan et al., 2023).

The wider significance of this regime is illustrated by JWST modeling of the short-period warm Jupiter TOI-519 b around an M3.5 star. In that case, 3D Global Climate Model calculations showed that phase-curve amplitude and offset in NIRSpec-PRISM and MIRI-LRS can diagnose clouds and hazes, that one transit can chemically characterize CH$T_{\rm eff}<4000$8 and H$T_{\rm eff}<4000$9O with very high signal-to-noise, and that NH$M_\ast<0.65\,M_\odot$0 could be detected for the first time in a giant exoplanet if disequilibrium mixing is strong (Teinturier et al., 2024). The recommended observing sequence was a MIRI-LRS phase curve followed by NIRSpec-PRISM (Teinturier et al., 2024). Because many HJMDs have similarly deep transits but somewhat higher irradiation, the same logic extends directly to the hotter subset: clouds, haze structure, and bulk chemistry can be probed with unusually high leverage relative to giant planets around larger stars.

HJMDs thus occupy a distinctive niche. They are rare enough to remain demographically revealing, yet observable enough to support precise occurrence studies, homogeneous metallicity work, RM obliquity measurements, and high-value atmospheric spectroscopy. Their scientific importance derives precisely from that combination of scarcity and leverage.