Planet-Mass Candidate Companions

Updated 10 December 2025

Planet-mass candidate companions are substellar objects with masses below the deuterium-burning threshold (≲13–14 M_Jup) that test formation theories and mass function structures.
Detection methods such as radial velocity, direct imaging, and astrometry, combined with rigorous selection criteria, enable robust identification and classification.
Their diverse demographics across host types and orbital separations challenge traditional formation models and support alternative pathways like disk instability and migration.

Planet-mass candidate companions are substellar objects with estimated masses below the deuterium-burning limit (≲13–14 M_Jup) detected at a variety of separations from host stars or brown dwarfs. They are identified via radial velocity, direct imaging, astrometry, or a combination of these methods, and represent a key population for constraining formation pathways, population statistics, and the physical processes at the interface of planet and brown-dwarf regimes. The landscape of planet-mass candidates has been shaped by systematic surveys, innovative data analysis pipelines, and an expanding inventory of host environments, ranging from primordial disks to evolved giant stars.

1. Detection Techniques and Selection Criteria

Planet-mass companions are recognized through multiple detection modalities, each with specific mass and radius sensitivity, error modeling, and bias considerations:

Radial Velocity (RV) Surveys: Statistical orbit-fitting pipelines such as that developed for APOGEE utilize Keplerian parameterizations, mass-function inversion, and quality metrics like phase/velocity coverage indices (U_N, V_N) for robust planet-mass candidate selection. Stringent thresholds on derived minimum mass (m sin i ≤ 13 M_Jup), SNR (K/σ_tot ≥ 3), and orbit completeness (U_N V_N > 0.5) are applied to yield high-confidence samples (Troup et al., 2016). An analogous framework is employed in other RV projects, often incorporating additional spectroscopic and astrometric vetting (Sahlmann et al., 2010, Díaz et al., 2011).
Direct Imaging (DI): High-contrast imaging surveys (e.g., with VLT/NACO, SPHERE, Keck/NIRC2, HST/WFPC2) directly detect faint sources at large angular separations. Color-magnitude and color-color diagrams, together with sequence-specific model isochrones (e.g., DUSTY, BT-Settl, ATMO2020), are utilized to estimate mass and eliminate contaminant hypotheses (e.g., M dwarfs, galaxies, or field brown dwarfs) (Schmidt et al., 2016, Todorov et al., 2010, Liu et al., 19 May 2025, Kraus et al., 2013). Multi-epoch astrometry is essential for co-motion confirmation and resolving background-star scenarios.
Astrometry: Absolute astrometry, as implemented in GaiaPMEX, identifies non-single stars via excess noise statistics (RUWE) and proper motion anomaly (PMa). Bayesian mass–semimajor axis posteriors are built by comparing simulated companion-induced signatures to the observed astrometric residuals and are used to flag candidate planet-mass companions (Kiefer et al., 25 Sep 2024, Lagrange et al., 17 Jan 2025).
Combined Diagnostics: Cross-vetting with direct imaging, spectroscopy, and contemporaneous radial velocity can rule out false positives, yield dynamical masses, and break inclination degeneracies, enabling robust assignment of the companion to the planetary-mass regime (Lagrange et al., 17 Jan 2025).

2. Demographics and Physical Properties

The census of planet-mass candidate companions spans a broad range of host types and system architectures:

RV-detected Companions: In APOGEE, 57 planet-mass candidates have m sin i = 0.5–13 M_Jup, with periods 1–300 days and a median semimajor axis ≈0.15 AU. Host stars include main-sequence dwarfs, subgiants, and red giants (M_\star = 0.6–1.4 M_⊙), with metallicities –0.6 ≤ [Fe/H] ≤ +0.4 and dominant localization in the Galactic thin disk (Troup et al., 2016).
Directly Imaged Candidates: Numerous companions of ≈3–25 M_Jup are found at wide projective separations (100–3500 AU), including systems around young T-Tauri, M dwarfs, and even B-type stars (Schmidt et al., 2016, Chinchilla et al., 2019, Artigau et al., 2015, Janson et al., 2019, Faherty et al., 2021, Liu et al., 19 May 2025). Characterized spectral types range from M8–L8 γ, typically betraying low gravity/youth signatures and extremely red infrared colors.
Astrometric and Multi-method Candidates: GaiaPMEX catalogs thousands of solar-type main-sequence stars brighter than G=16 with astrometric signatures indicative of sub-13.5 M_Jup companions at 1–3 AU semi-major axes (Kiefer et al., 25 Sep 2024). Cross-validation isolates both known RV planets and new candidate hosts.
Notable System Architectures: Candidate companions can be found in hierarchical multiples (quadruples), circumbinary configurations, and in systems with both short-period and highly separated planet-mass bodies (e.g., CVSO 30, G80–21 b, AF Lep b, FW Tau, 2M J044144) (Schmidt et al., 2016, Lagrange et al., 17 Jan 2025, Todorov et al., 2010, Kraus et al., 2013).

3. The Brown Dwarf Desert and Mass Function Structure

A central outcome of massive RV and astrometric surveys is the empirical mapping of the substellar companion mass function:

Brown Dwarf Desert: There exists a well-established paucity of companions with 13 ≲ M ≲ 45 M_Jup on short period orbits (a ≲ 0.2 AU), termed the "brown-dwarf desert." True brown-dwarf orbits concentrate above ≈45 M_Jup, while planetary-mass companions cluster below ≈25 M_Jup, with a pronounced desert in between (Sahlmann et al., 2010, Díaz et al., 2011, Troup et al., 2016).
Planet Mass Function: The frequency of planet-mass companions is sharply rising at the lowest detectable masses, with significant occurrences below 5 M_Jup at short separations in sensitive surveys (Troup et al., 2016, Kiefer et al., 25 Sep 2024). However, direct-imaging surveys reveal that, at wide separations (≳200 AU), the occurrence rate of 6–20 M_Jup companions is ∼4% for solar-type stars in Upper Sco—an order of magnitude above the expectation from extrapolating the binary-star mass-ratio distribution (Ireland et al., 2010).
Host-Mass and Mass-Ratio Regimes: Wide planet-mass companions to massive stars (e.g., B9, MAB ≈ 2.5–5 M_⊙) with mass ratios q ≲ 0.01 are observed (e.g., HIP 79098 (AB)b), filling a previously scarcely populated parameter space (Janson et al., 2019). Mass-ratio continuity is observed from binary-star to planetary-mass regimes but is highly host-type and separation dependent (Kraus et al., 2013, Artigau et al., 2015).

4. Host System Diversity and Environmental Context

Planet-mass candidate companions span a vast range of system ages, host evolutionary phases, Galactic environments, and spatial configurations:

Host Type	Representative Examples	Masses (M_Jup)	Separations (AU)
Solar-type MS	APOGEE (MS hosts), Upper Sco (AO)	0.5–13	0.02–500
Red Giants/Subgiants	HD 155233b, HD 145457b, 75 Cet b	1.4–4.1	0.76–3.9
Early-M Dwarfs	USco1621 B, USco1556 B	14–15	2880–3500
B-type Stars	HIP 79098 (AB)b	16–25	345
Brown Dwarfs	2M J044144 B	5–10	15
T-Tauri	CVSO 30 b, CVSO 30 c	4–5	0.0084, 662
Hierarchical/Multiple	2M J044144 A+B + 2M J044145 A+B	5–10	15, 32, 1700

Wide companions challenge migration and stability theory; their survival and low binding energies (<10³³ J) raise questions about dynamical disruption and formation environments (Chinchilla et al., 2019, Todorov et al., 2010, Faherty et al., 2021). Shallow metallicity dependence in some samples (e.g., planet-mass companions to [Fe/H]<−0.5 stars) suggests alternative or additional formation pathways beyond core accretion (Troup et al., 2016, Díaz et al., 2011).

5. Implications for Formation Mechanisms

The observed demographics, orbital architectures, and environmental dependencies of planet-mass candidates inform models of substellar companion formation:

Core-Accretion: Efficient at forming gas giants with m ≃ 1–10 M_Jup at ≲ 3 AU, particularly around metal-rich, ≳1 M_⊙ hosts. However, in wide (≳ 100–200 AU) systems, core-accretion timescales generally exceed disk lifetimes, particularly in low-mass disks or at low metallicity (Troup et al., 2016, Ireland et al., 2010, Artigau et al., 2015).
Disk Instability and Gravitational Fragmentation: Disk- or cloud-core fragmentation is required to explain companions formed within ≲1 Myr, those with high mass ratios, or at wide separations (>100 AU). Disk instabilities can yield massive fragments, with subsequent dynamical evolution (e.g., planet-planet scattering in CVSO 30, hierarchical fragmentation in Taurus multiples) distributing companions from short-period to extreme orbits (Schmidt et al., 2016, Todorov et al., 2010, Kraus et al., 2013).
Migration and Scattering: Mechanisms such as planet–planet scattering or outward dynamical ejection are invoked to explain co-existence of close-in and wide companions of similar mass in the same system, extreme eccentricities, or wide “orphaned” planets (Schmidt et al., 2016).
Alternative Pathways: Formation via capture of a free-floating planet, formation in a dissolving cluster, or hybrid processes combining disk instability with dynamical scattering or planetary migration may operate in rare cases (e.g., BD+60 1417 W1243 (Faherty et al., 2021), ultra-wide companions at >1000 AU).

6. Outstanding Challenges and Future Prospects

Despite rapid advances, substantial uncertainties remain on the true population statistics, dynamical histories, and physical structures of planet-mass candidates:

Degeneracy in Mass–Semimajor Axis Inference: For astrometry, the mass–a degeneracy means that many high-RUWE Gaia candidates could be more massive objects at larger or smaller orbits; extensive follow-up (RV, DI, DR4 orbital fits) is required for confirmation (Kiefer et al., 25 Sep 2024, Lagrange et al., 17 Jan 2025).
Classification Challenges: The empirical “brown dwarf desert” motivates a mass-based planet–brown dwarf dividing line, but ambiguous cases (m sin i near 13–25 M_Jup) and inclination uncertainties hamper robust classification (Sahlmann et al., 2010, Díaz et al., 2011).
Atmospheric and Evolutionary Model Systematics: Model-dependent mass estimates (DUSTY, BT-Settl, COND, ATMO) incorporate uncertainties from age, distance, and surface gravity degeneracies. Benchmark systems for atmospheric retrieval and direct dynamical mass measurement are essential for calibration (Kraus et al., 2013, Faherty et al., 2021).
Dynamical Evolution and Survival: Ultra-wide systems (a > 1000 AU) have low binding energies and may be susceptible to dissolution within ≲0.1–0.3 Gyr in rich clusters; their current existence probes both recent star formation and future cluster dynamics (Chinchilla et al., 2019).
Legacy Surveys and Citizen Science: Projects such as APOGEE, GaiaPMEX, BEAST, Backyard Worlds, and YSES continue to expand the catalog of planet-mass candidates, opening parameter space in stellar host type, age, and orbital radius (Troup et al., 2016, Kiefer et al., 25 Sep 2024, Janson et al., 2019, Faherty et al., 2021, Liu et al., 19 May 2025).

Ongoing and forthcoming data releases (Gaia DR4+), high-contrast imaging surveys (ELT, JWST), and integrated multi-technique confirmation workflows will allow systematic closing of both occurrence rate and formation theory gaps, achieving a comprehensive empirical mapping of planet–mass candidate companions across the Galaxy.