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Proxima Centauri c

Proxima Centauri c is a super-Earth candidate exoplanet orbiting Proxima Centauri, the closest known star to the Sun at a distance of 4.24 light-years. A radial velocity signal suggestive of a planet with a minimum mass of approximately 5.8 Earth masses, an orbital period of 5.2 years, and a semi-major axis of 1.48 AU was reported in 2020 as part of the Red Dots campaign using the HARPS and UVES spectrographs.^[1]^[2] This would place it well outside the star's habitable zone, with an equilibrium temperature around 39 K.^[3] The signal was initially distinguished from stellar activity through photometric and spectroscopic analysis.^[1] However, subsequent studies have failed to confirm it: a 2022 reanalysis attributed the signal to systematic effects in HARPS data,^[4] and 2025 observations with the NIRPS spectrograph found no conclusive evidence, though hints of a weaker long-period signal persist.^[5] Early analyses proposed an orbital inclination of about 152 degrees, yielding a true mass of roughly 6 Earth masses and suggesting a Neptune-like composition, but these depend on the disputed RV signal.^[2] Unlike its inner sibling Proxima b, which lies in the habitable zone, the candidate Proxima c would receive minimal stellar radiation, making it a cold world unlikely to support liquid water without significant internal heating.^[1] The candidate's wide orbit and proximity to Earth make it a potential target for direct imaging with future telescopes, though attempts with the VLT's SPHERE instrument in 2020 did not conclusively detect it.^[1] Ongoing astrometric observations from Gaia are expected to help refine its parameters if confirmed, while studies of the stellar wind environment indicate Earth-like conditions that could influence any potential atmosphere.^[2]^[6] As part of the Alpha Centauri system, a confirmed Proxima c would contribute to understanding planet formation around low-mass stars, potentially challenging models due to its relatively high mass at large separation.^[1]

Discovery and Status

Initial Detection

The initial detection of Proxima Centauri c was announced in April 2019 by a team led by Mario Damasso at the Breakthrough Discuss conference, based on radial velocity data analysis from the HARPS instrument on ESO's 3.6-m telescope at La Silla Observatory and the UVES instrument on the Very Large Telescope. The candidate planet was identified as a super-Earth in a distant orbit, marking the second potential planet in the system after Proxima b.^[1] This search was motivated by the 2016 discovery of Proxima b, an Earth-mass planet in the habitable zone, which encouraged astronomers to investigate for additional companions using long-term radial velocity monitoring to probe outer regions of the system.^[1] The signal was detected through periodogram analysis of the radial velocity time series, revealing a significant peak at approximately 5.2 years corresponding to the orbital period. The analysis yielded an orbital period of 1923 +96/-82 days, a radial velocity semi-amplitude of 1.09 ± 0.25 m/s, and a minimum mass of 5.8 ± 1.9 Earth masses (m sin i). These values were derived from a dataset spanning over 17 years with hundreds of measurements, though the signal's low amplitude required careful modeling to distinguish it from stellar activity.^[1]

Confirmation Efforts and Controversies

Following the initial detection, subsequent radial velocity efforts using the ESPRESSO spectrograph focused on validating the signal through complementary methods. In 2020, an analysis combining radial velocity measurements with Gaia DR2 astrometric data constrained the prograde orbital inclination to approximately 152° ± 14°, implying a true mass of about 12^{+12}_{-5} Earth masses if the planetary nature is confirmed.^[2] This astrometric approach provided the first estimate beyond the minimum mass derived from radial velocities alone, though uncertainties in the proper motion anomaly limited definitive confirmation.^[2] Direct imaging attempts using the SPHERE instrument on the Very Large Telescope between 2016 and 2019 also sought to visualize the candidate planet at its expected separation of around 1 arcsecond. No robust counterpart was detected, with a candidate signal at low signal-to-noise ratio (S/N ≈ 6) and a low false alarm probability of ~0.9%, but deemed not robust due to inconsistencies with the expected orbital motion, along with correlated instrumental noise and background sources, preventing clear identification.^[7] Radial velocity reanalyses incorporating additional data from 2022 to 2025 have shown the original signal persisting in combined datasets but with challenges from potential contaminants. A 2022 study using 114 ESPRESSO observations (spanning 2019–2021) alongside archival HARPS data confirmed signals for Proxima b and a new inner candidate (Proxima d) but found no significant detection of the 1900-day signal for Proxima c, attributed to the limited baseline for such a long period. Gaussian process modeling of stellar activity, including rotation periods around 83 days, mitigated some noise but highlighted possible aliasing effects. Subsequent integration of HARPS (442 spectra from 2003–2022) and ESPRESSO data with new NIRPS infrared observations (149 binned measurements from 2021–2024) in a 2025 analysis yielded refined parameters for Proxima b and d while providing only inconclusive evidence for Proxima c, with a guided search detecting a marginal semi-amplitude of 58 ± 28 cm/s—lower than the original 1.2 m/s estimate. This reanalysis modeled stellar rotation and activity-induced signals using Gaussian processes, revealing potential contamination from long-term magnetic cycles (≈18 years) or gravitational interactions with Proxima b, though no definitive non-planetary origin was established.^[8]^[5] Debates persist regarding whether the radial velocity signal represents a true planet or astrophysical noise, fueled by periodogram analyses assessing false positives. The 2022 ESPRESSO study employed generalized Lomb-Scargle periodograms on cross-correlation functions and line-by-line velocities, showing the Proxima c peak lacking significance (false alarm probability >1%) in new data, suggesting the original detection might stem from unmodeled systematics or data gaps rather than a planetary orbit. A 2025 reexamination extended this scrutiny with full-information periodograms on the combined 722-measurement dataset (spanning 24.5 years), identifying multiple activity-related peaks (false alarm probability <0.01 at periods like 108 and 270 days) that could mimic or obscure the candidate signal, further questioning its planetary interpretation without ruling it out entirely.^[8]^[5] As of 2025, Proxima Centauri c remains an unconfirmed candidate, with ongoing uncertainties preventing full validation despite persistent signals in historical data. Latest analyses indicate inconclusive evidence, treating it as a super-Earth-like world pending further observations to distinguish planetary motion from stellar noise.^[5]

Host Star and System Context

Properties of Proxima Centauri

Proxima Centauri is a red dwarf star classified as spectral type M5.5Ve, indicating a late-type M dwarf with emission lines from chromospheric activity. It has a mass of 0.1221 ± 0.0022 solar masses and a radius of 0.154 ± 0.005 solar radii, making it significantly smaller and less massive than the Sun. The effective temperature of the star is 3042 ± 87 K, which contributes to its low bolometric luminosity of 0.00148 ± 0.00012 solar luminosities.^[9] The star is estimated to be approximately 4.8 Gyr old, aligning with the age of the broader Alpha Centauri triple system to which it belongs. Proxima Centauri has a metallicity of [Fe/H] = -0.12, a value that plays a role in theoretical models of planet formation by influencing the availability of heavy elements in the protoplanetary disk. At a distance of 4.2465 ± 0.0013 light-years from the Sun, it holds the distinction of being the closest known star to our Solar System.^[2] Proxima Centauri displays pronounced magnetic activity, including frequent flares and elevated X-ray emission generated by its internal dynamo process. This activity is facilitated by the star's rotation period of approximately 83 days, which sustains the convective motions necessary for the dynamo. The system includes additional planetary companions, such as Proxima b and Proxima d, providing context for multi-planet dynamics around this active host.^[10]

Position in the Alpha Centauri System

Proxima Centauri serves as the outermost and closest member of the Alpha Centauri triple star system, gravitationally bound to the inner binary pair Alpha Centauri A and B. The current separation between Proxima Centauri and the Alpha Centauri AB binary is approximately 0.21 light-years, or about 13,000 AU, with Proxima orbiting the barycenter of the system over a period of roughly 550,000 years.^[11] This wide, loosely bound configuration positions Proxima as the nearest known star to the Sun at 4.24 light-years, while the binary AB pair orbits each other at a much closer distance of around 23 AU.^[11] Within this system, Proxima Centauri hosts multiple confirmed and candidate planets that highlight its role in the broader architecture. Proxima b, an approximately Earth-mass world at 0.05 AU, resides in the inner habitable zone with an orbital period of 11.2 days.^[12] Proxima d, a sub-Earth-mass planet at 0.029 AU, was confirmed in 2022 via radial velocity observations and completes its orbit in just 5.1 days.^[13] Proxima c, a candidate super-Earth at around 1.5 AU, fits into this multi-planet setup, though its detection remains tentative based on radial velocity signals. As of 2025, further observations with NIRPS have not confirmed its existence, with evidence suggesting the signal may be due to stellar activity or a lower-mass companion.^[5] The gravitational influence of the Alpha Centauri AB binary on Proxima Centauri's protoplanetary disk is subtle due to the vast separation but potentially significant over long timescales, as periodic close approaches during Proxima's eccentric orbit could perturb disk material. N-body simulations of the triple system confirm that these effects do not destabilize inner planetary orbits, with Proxima c's path remaining secure over billions of years despite the external tidal forces from AB.^[14] The overall dynamics of Proxima's planetary system underscore opportunities for stable multi-planet configurations within the Alpha Centauri framework, where the wide stellar separation allows inner worlds like Proxima b and c to evolve independently while sharing potential mean-motion resonances that enhance orbital longevity.^[15] These interactions suggest that Proxima's planets could maintain coherence against external perturbations, informing models of planet formation in wide hierarchical triples.^[15]

Orbital Parameters

Key Orbital Elements

Proxima Centauri c was proposed to orbit its host star at a semi-major axis of 1.48 ± 0.08 AU, corresponding to an average separation well beyond the inner planet Proxima b.^[1] This distance would result in an orbital period of approximately 1900 days (5.2 years), as determined from radial velocity measurements spanning several years.^[1] The orbit was modeled with low eccentricity, consistent with a nearly circular path (e ≈ 0, with upper limit e < 0.58 at 68% confidence).^[1] Astrometric analysis suggested an orbital inclination of 152° ± 14° relative to the line of sight (prograde solution), though this was based on limited data.^[2] The longitude of the ascending node and the argument of pericenter remained poorly constrained, as radial velocity provides only the line-of-sight component, and astrometry lacked precision.^[2] Assuming the proposed low eccentricity, the periastron distance would be near 1.48 AU, with minimal variation. This places the candidate well outside the habitable zone of the red dwarf host, where stellar flux is low. The equilibrium temperature was estimated at ~39 K, assuming zero Bond albedo and no atmosphere, using the formula for blackbody equilibrium:

T_\text{eq} = T_\star \sqrt{\frac{R_\star}{2a}} (1 - A)^{1/4}

where T_\star is the effective temperature of Proxima Centauri, R_\star its radius, a the semi-major axis, and A the albedo.^[1] This frigid baseline underscores the candidate's likely icy nature absent significant heating.

Dynamical Interactions

As originally proposed, Proxima Centauri c would experience weak secular dynamical interactions with its inner sibling planet Proxima b over a separation of approximately 1.4 AU. Secular perturbations from Proxima b would induce small oscillations in eccentricity, with amplitudes up to Δe ≈ 0.05 on timescales of about 10^5 years, based on evolution models. These arise from mutual gravitational influence in coplanar configurations, potentially modulating insolation over long timescales.^[16] ^[17] N-body integrations incorporating relativistic and tidal effects indicated high long-term orbital stability for the proposed system. Simulations showed a high probability of stability over 1 Gyr for low eccentricities (<0.2) and coplanar orbits, with instability at higher inclinations or eccentricities >0.5. Potential mean-motion resonances, such as 1:9 with Proxima b, were considered for stabilization, though unconfirmed.^[16] ^[17] However, as of 2025, high-precision radial velocity observations with the NIRPS instrument have refuted the existence of Proxima Centauri c, attributing the original signal to stellar activity rather than a planetary orbit.^[18] The broader Alpha Centauri system would exert negligible influence on such a candidate at ~1.5 AU, with binary Alpha Centauri AB (~15,000 AU away) affecting primarily early disk evolution. Simulations suggest disk truncation at 1–2 AU during formation, limiting material delivery to outer orbits. External stellar flybys pose minimal disruption risk over cosmic timescales (>10 Gyr).^[19] ^[20]

Physical Characteristics

Mass and Size Estimates

Proxima Centauri c remains an unconfirmed candidate, with its existence challenged by 2025 NIRPS observations finding no significant radial velocity (RV) signal at the proposed ~1900-day period, though hints of a weaker, slightly shorter periodicity (~1800 days) were noted.^[5] If real, its mass would be derived from RV measurements of its host star using HARPS and UVES spectrographs, detecting the planet's gravitational influence as a periodic wobble. The minimum mass, m \sin i, is $5.8 \pm 1.9 \, M_\oplus based on 2020 data from the Red Dots campaign, after modeling and subtracting the signal from Proxima b.^[1] This detection received astrometric support from Hubble data in 2020.^[21] The true mass depends on the orbital inclination i, unknown without transit or precise astrometry. Analysis combining the RV data with Hipparcos and Gaia DR2 proper motion yields i = 152^\circ \pm 14^\circ (prograde) or equivalently i \approx 28^\circ (retrograde), with \sin i \approx 0.47, implying a true mass of $12^{+12}_{-5} \, M_\oplus.^[2] This would place it roughly twice as massive as the minimum estimate and comparable to Uranus or Neptune in mass. Surface gravity would then be approximately 2–3 times Earth's (g_\oplus), depending on radius assumptions. No direct radius measurement exists, as no transit has been observed. Theoretical mass–radius relations for planets of 6–12 M_\oplus predict a radius of 2–4 R_\oplus, transitioning from super-Earth to mini-Neptune compositions with increasing volatile content; lower values assume rocky/iron-rich interiors, higher if water or gas envelopes are present. A bulk density of ~2–4 g/cm³ would be consistent with an ice-rich or volatile-dominated structure. RV measurements are complicated by the star's high activity, including flares and spots introducing jitter comparable to the planetary signal (~50 cm/s). This was mitigated through Gaussian process regression with quasi-periodic kernels, calibrated against simultaneous photometric data.

Interior Structure and Composition

If Proxima Centauri c exists with a minimum mass of $5.8 \pm 1.9 \, M_\oplus (or true mass ~12 M_\oplus), interior models suggest a super-Earth to mini-Neptune, likely with a rocky/icy core comprising 60–80% of the mass in silicate, iron, and high-pressure ice phases, possibly overlaid by a hydrogen-helium envelope of <5% mass retained from formation.^[22] These assume differentiation into layers, with the envelope vulnerable to photoevaporation from the active host star. For masses >10 M_\oplus, phase diagrams indicate high-pressure ices or supercritical fluids at the core-mantle boundary, especially if volatiles like water were incorporated during formation beyond the snow line.^[23] The planet would likely have formed via core accretion in the outer protoplanetary disk, accumulating solids before inward migration to ~1.5 AU as the snow line evolved.^[24] This fits standard models for massive planets around M dwarfs.^[19] Seismic models suggest potential plate tectonics if mantle water content is 0.1–1% by mass, enabling asthenospheric mobility unlike Venus's stagnant lid.^[25] However, the star's activity could disrupt surface processes.^[26] Differentiation models predict a metallic core ~0.3–0.4 times the planet's radius, potentially generating a dynamo-driven magnetic field in the liquid outer core, though weaker than Earth's due to scaling with mass; this could offer limited protection from stellar winds.^[27]

Atmosphere and Surface Conditions

Potential Atmospheric Models

Theoretical models suggest that the atmospheric mass fraction of Proxima Centauri c depends on its bulk composition and formation history. For a rocky super-Earth interpretation, consistent with its minimum mass of approximately 6 Earth masses, the atmosphere would likely be thin, comprising less than 0.1% of the planet's total mass, similar to known terrestrial exoplanets. However, if Proxima Centauri c is a mini-Neptune with a mass exceeding 6 Earth masses, it could retain a substantial hydrogen-helium envelope, potentially accounting for more than 1% of its mass, as seen in volatile-rich sub-Neptunes. Atmospheric retention on Proxima Centauri c is favored by its orbital distance of about 1.5 AU from the host star, where stellar wind interactions resemble Earth's environment and pose minimal erosion risk to denser atmospheres. Numerical simulations of Proxima Centauri's stellar wind indicate negligible Joule heating and weak interplanetary magnetic fields at this distance, supporting long-term stability for Earth-like conditions. Specifically, escape timescales for heavier gases like CO2 or N2 exceed 1 billion years under nominal stellar activity, though extreme ultraviolet flares from the active M dwarf could preferentially erode lighter components such as hydrogen.^[6] Outgassing from a volcanically active interior represents a primary mechanism for building Proxima Centauri c's secondary atmosphere, with models predicting dominance by CO2 for most crustal compositions under reducing to oxidizing conditions. If the planet maintains tectonic activity, N2-O2 mixtures could also emerge from equilibrated rock types, particularly in nitrogen-rich mantles. Photochemical processes driven by stellar radiation would likely produce organic hazes in these atmospheres, altering spectral signatures through scattering and absorption, as simulated for super-Earths around M dwarfs.^[28] Prospects for characterizing the atmosphere via transmission spectroscopy are limited, as high-cadence observations with TESS confirm no transits for Proxima Centauri c due to its orbital inclination. If a transit were to occur, absorption features from H2O or CH4 could be detectable in the near-infrared, but current data indicate this geometry is unlikely. Modeling of CO2-dominated atmospheres on cold exoplanets suggests that thin envelopes with surface pressures below 10 mbar may condense and subside to the surface without radiative trapping, potentially rendering Proxima Centauri c barren unless replenished by ongoing volcanism.^[29]

Estimated Climate and Temperature

The equilibrium temperature of Proxima Centauri c, calculated assuming rapid heat redistribution and a Bond albedo of approximately 0.3, is estimated at 39 K, with uncertainties ranging from 21 K to 55 K depending on orbital eccentricity, atmospheric redistribution efficiency, and albedo variations.^[30] This value is derived from the incident stellar flux at the planet's semi-major axis of 1.48 AU, using the formula for flux F = \frac{L_\star}{4\pi a^2}, where L_\star = 0.00151 \, L_\odot is Proxima Centauri's bolometric luminosity, followed by the blackbody effective temperature T_\mathrm{eff} = \left[ \frac{(1 - A) F}{4 \sigma} \right]^{1/4}, with A the Bond albedo and \sigma = 5.67 \times 10^{-8} W m^{-2} K^{-4} the Stefan-Boltzmann constant.^[31] Varying the albedo to represent different surface conditions alters T_\mathrm{eff} significantly; for a rocky surface with A = 0.1, the effective temperature is approximately 40–43 K, while an icy surface with A = 0.3 yields 36–39 K, reflecting reduced absorbed flux for higher-albedo scenarios.^[31] An Earth-like atmosphere with moderate greenhouse warming (optical depth τ ≈ 0.7–1.0 from H₂O, CO₂, and N₂) would raise surface temperatures by roughly 30–40 K above T_\mathrm{eff}, but given the low incident flux of ~0.94 W m⁻², this results in frigid conditions far below freezing, precluding liquid water without additional mechanisms. Due to the planet's large orbital separation (1.48 AU) and long period (~5.2 years), tidal forces are negligible, rendering tidal locking unlikely over the system's age of ~4.85 Gyr; the locking timescale exceeds the stellar age by orders of magnitude, as tidal torque scales inversely with a^6 (or steeper for eccentricity damping). This allows for a non-synchronous rotation period, likely on the order of days based on formation models for super-Earths, and permits an axial obliquity up to ~45° that could drive seasonal atmospheric mixing and moderate day-night contrasts. Three-dimensional general circulation models (GCMs) adapted for cold, low-flux exoplanets predict a globally frosted surface for Proxima Centauri c under weak greenhouse conditions, with permanent ice or CO₂ frost dominating due to temperatures below the H₂O sublimation point (~200 K at low pressure). However, scenarios with a thick CO₂-dominated atmosphere (surface pressure >10 bar, τ > 100) could trap sufficient heat to form localized pockets of liquid water or brines near subsolar points or volcanic regions, though such models remain speculative pending confirmation of atmospheric retention. The unknown rotation period introduces uncertainty in day-night heat transport, but slow rotation (days-long) would minimize extreme contrasts compared to rapid spinners, favoring more uniform global cooling.

Habitability Assessment

Placement in Habitable Zone

Proxima Centauri c, assuming its existence as a candidate planet, orbits its host star at a semi-major axis of approximately 1.48 AU, positioning it far beyond the habitable zone (HZ) boundaries calculated for the faint M5.5V red dwarf Proxima Centauri. The conservative HZ for such M dwarfs, defined by the range supporting liquid water on an Earth-like planet's surface under standard atmospheric conditions, extends from about 0.042 to 0.082 AU, with the inner edge set by the recent Venus limit and the outer by the moist greenhouse threshold.^[15] At its distance, the planet receives an incident stellar flux of roughly 1 W/m², equivalent to about 0.07% of Earth's insolation or 10^{-3} times the solar constant, rendering surface conditions profoundly cold without exceptional atmospheric retention.^[32] Optimistic HZ models, which account for denser atmospheres enabling stronger greenhouse effects, extend the outer boundary to around 0.2 AU via maximum CO₂ greenhouse limits, still well interior to Proxima c's orbit. In comparison, the confirmed inner sibling Proxima Centauri b orbits at 0.048 AU, aligning with the inner HZ edge where high insolation may drive Venus-like runaway greenhouse scenarios. The long-term dynamical stability of low-mass stars like Proxima Centauri implies gradual luminosity evolution, shifting the HZ outward over billions of years as the star brightens by factors of 1.5–2 over its remaining lifetime.^[33] However, even accounting for this expansion over the next 5 Gyr, Proxima c's orbital separation keeps it well outside viable HZ limits, even with thick, heat-trapping atmospheres. Recent 2025 radial velocity observations with NIRPS have not confirmed the planet's signal, providing only upper limits amid stellar activity challenges.^[34]

Barriers to Life-Supporting Conditions

Proxima Centauri c's position at a semi-major axis of 1.48 AU places it far outside the habitable zone of its host star, resulting in an extremely low equilibrium temperature of approximately 39 K without an atmosphere, causing surface temperatures to remain below 200 K and freezing any potential volatiles such as water ice. This extreme cold prevents the formation of liquid water on the surface, a key requirement for life as currently understood, unless a dense greenhouse atmosphere is present to trap heat. Models for outer habitable zone boundaries around M-dwarfs indicate that achieving surface temperatures above the water freezing point at such distances would require a CO₂-dominated atmosphere with pressures exceeding 10 bar to provide sufficient greenhouse forcing, though the feasibility of retaining such a massive atmosphere during the planet's formation and evolution remains uncertain given the star's youth and activity. The intense flare activity of Proxima Centauri poses additional challenges, with superflares releasing energies up to 10³⁴ erg that can deliver UV and X-ray radiation doses approximately 100 times greater than those from major solar events on Earth, potentially eroding atmospheric layers equivalent to ozone protection over timescales of about 1 million years through enhanced photolysis and ion escape. Although the radiation flux at Proxima Centauri c's distance is reduced by a factor of roughly (1.48/0.05)² ≈ 900 compared to the inner habitable zone, episodic flares could still contribute to cumulative atmospheric loss if a thin envelope is present. Tidal locking is unlikely for Proxima Centauri c due to its wide orbit and long orbital period of 1900 days, but dynamical interactions or capture mechanisms could result in a slow rotation rate, potentially driving superrotating atmospheric winds exceeding 100 m/s that inhibit global ocean circulation and limit the transport of nutrients and heat essential for supporting life. The planet's low stellar insolation, equivalent to less than 0.1% of Earth's, severely constrains energy availability for biological processes like photosynthesis, with climate models for similar low-flux environments predicting net primary productivity less than 1% of Earth's terrestrial levels even in optimistic scenarios with a retained atmosphere. Furthermore, with a minimum mass of 5.8 M_⊕, Proxima Centauri c may exhibit limited geological activity, as super-Earths in this mass range often feature a stagnant lid tectonic regime rather than active plate tectonics, suppressing volcanism and hindering the long-term recycling of atmospheric CO₂ needed to maintain a stable climate against carbon drawdown into the crust.

Observational Prospects

Current Detection Methods

The primary method for detecting and characterizing the candidate Proxima Centauri c has been the radial velocity (RV) technique, which measures the star's wobble due to the gravitational pull of orbiting planets. Instruments such as HARPS and ESPRESSO on the Very Large Telescope have achieved precisions of approximately 20 cm/s in ideal conditions, enabling the initial detection of the planet's signal in 2019 with a minimum mass of about 5.8 Earth masses (m sin i). However, Proxima Centauri's high stellar activity, including flares and starspots, introduces noise that mimics planetary signals and limits reliable detections to m sin i greater than roughly 3 Earth masses, complicating confirmation of lower-mass companions. As of August 2025, analysis of 420 high-resolution spectra from the Near Infra Red Planet Searcher (NIRPS) over 159 nights shows no significant evidence for Proxima c, achieving ~0.55 m/s precision and reducing activity-induced jitter compared to visible-wavelength instruments, but placing an upper limit on the RV semi-amplitude of 1.4 m/s (99.7% confidence). This is inconsistent with the prior estimate and suggests either a lower-mass planet or that the signal may arise from stellar activity, leaving the candidate unconfirmed despite hints of a long-period signal near 1800 days.^[5] Astrometric observations provide complementary constraints by detecting the star's positional shift due to planetary orbits, but current capabilities fall short for Proxima Centauri c. Data from Gaia Data Release 3 (2022) refined the proper motion of Proxima Centauri to high accuracy, offering indirect limits on potential companions through proper motion anomalies. Nonetheless, the expected astrometric signature of Proxima c—corresponding to an angular displacement of less than 1 milliarcsecond given its 1.5 AU orbit at a distance of 4 light-years—remains below Gaia's detection threshold in the current release, with full sensitivity anticipated only in future epoch astrometry from the mission's final data products. Direct imaging attempts have yielded non-detections, highlighting the challenges of observing a low-mass planet close to a bright, active M-dwarf host. Multi-epoch observations with VLT/SPHERE in the near-infrared reached a 5σ contrast limit of about 10^{-5} at the planet's projected separation of roughly 1 arcsecond, effectively ruling out giant planets (Jupiter-mass or larger) but providing no constraint on super-Earths like Proxima c due to its expected faint thermal emission. The transit method has not yielded detections, owing to the planet's unfavorable geometry. With a wide orbital separation and likely low orbital inclination (inferred from combined analyses), the geometric transit probability is less than 1%, making observations improbable even with space-based surveys like TESS, which monitored Proxima Centauri without identifying transits of c (or its sibling Proxima b). Future missions such as PLATO are unlikely to observe transits given this low probability. Combining RV and astrometric data has provided varying estimates of Proxima c's properties due to uncertainties in orbital inclination, though the candidate remains unconfirmed without resolved photometry. No photometric observations, such as phase curves, have been obtained to probe the planet's atmosphere or albedo, as transits remain unobserved and direct imaging unresolved.

Future Missions and Telescopes

The James Webb Space Telescope (JWST) offers promising capabilities for observing Proxima Centauri c through its Near-Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI), with planned post-2025 observations focusing on mid-IR imaging to probe the planet's potential thermal emission or reflected light. MIRI's coronagraphic mode, operating at wavelengths of 10-20 μm, could enable direct detection by suppressing the host star's glare, potentially resolving the candidate planet's status given its wide orbital separation of approximately 1.5 AU, which provides a favorable angular distance for imaging.^[35] The Extremely Large Telescope (ELT), scheduled for first light in 2028, will feature the ANDES high-resolution spectrograph, capable of achieving radial velocity (RV) precision down to 10 cm/s for M-dwarf stars like Proxima Centauri. This sensitivity aims to measure the true mass of Proxima Centauri c by determining the orbital inclination, overcoming current limitations in RV data where the minimum mass is known but the actual mass remains uncertain due to unknown geometry. ANDES will operate across optical to near-IR wavelengths (0.5–1.8 μm), enabling detailed characterization of low-mass planets in nearby systems.^[36] NASA's Nancy Grace Roman Space Telescope, launching in 2027, will conduct wide-field astrometry and imaging surveys to refine the orbital parameters of nearby exoplanets, including Proxima Centauri c. Its high-precision astrometry (expected to reach microarcsecond levels) could constrain the system's inclination and orbital elements, aiding in the confirmation or refutation of the planet by cross-validating RV signals with positional data over multi-year baselines. Long-term interstellar probe concepts, such as those under the Breakthrough Starshot initiative, propose flyby missions to the Proxima Centauri system in the 2040s, potentially providing in-situ data on Proxima Centauri c if confirmed. These gram-scale nanocrafts, propelled by laser sails to 15-20% of light speed, could capture close-up images and spectra during a high-speed pass, offering unprecedented details on the planet's surface, atmosphere, and composition despite the brief encounter duration.^[37] Ground-based efforts will be enhanced by the Square Kilometre Array (SKA), a radio telescope array entering early operations in the late 2020s, which can monitor Proxima Centauri's stellar activity through frequent radio flare observations. By correlating radio emissions with RV jitter, SKA data will help mitigate activity-induced noise in spectroscopic measurements, improving the signal-to-noise ratio for detecting subtle planetary signals like those from Proxima Centauri c.^[38]

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Orbital Stability and Secular Dynamics of the Proxima Centauri ...
Jan 16, 2024 · The two innermost planets of the Proxima Centauri system are separated by just 0.02 au, inducing strong gravitational interactions between them.
[16]
Dynamical evolution and stability maps of the Proxima Centauri ...
Sep 21, 2018 · In the coplanar case, we find that the orbit of Proxima b is stable for the semi-major axis ranging from 0.02 au to 0.1 au and the eccentricity ...Missing: body 99% 1 Gyr
[17]
EFFECTS OF PROXIMA CENTAURI ON PLANET FORMATION IN ...
Nov 4, 2016 · We performed simple simulations of planet formation to estimate the types and locations of planets that could exist in the Alpha Centauri system ...
[18]
[1709.03560] Was Proxima captured by alpha Centauri A and B?
Sep 11, 2017 · From 100 clones, we find that 74% orbits of Proxima Centauri are bound to alpha Centauri A and B while 17% and 9% orbits become unbound in the ...Missing: AB disk
[19]
To cool is to keep: residual H/He atmospheres of super-Earths and ...
This remnant atmosphere then cools and contracts without loss on Gyr time-scales, and the planet becomes a super-Earth with an f ∼ 10−4 H/He envelope. We ...
[20]
[PDF] 2.07 Mineralogy of Super-Earth Planets - Yale University
CMB and ICB designate Earth core–mantle boundary and inner core boundary, respectively. ... Hermann A, Ashcroft NW, and Hoffmann R (2012) High pressure ices.
[21]
Exploring plausible formation scenarios for the planet candidate ...
The metallicity of Proxima is measured to be [Fe/H] = 0.21 (Anglada-Escudé et al. 2016). When considering the distribution of solids, our baseline assumption is ...
[22]
The fate of water within Earth and super-Earths and implications for ...
Apr 17, 2017 · Water is thought to be critical for the development of plate tectonics, because it lowers viscosities in the asthenosphere, enabling subduction.
[23]
Deep mantle melting, global water circulation and its implications for ...
Dec 10, 2020 · Consequently, cooler regions of melting act as a water valve to maintain the water content of the MTZ near its threshold level (~ 0.1–1.0 wt%). ...
[24]
Initial Thermal States of Super‐Earth Exoplanets and Implications for ...
Feb 23, 2025 · The accretion of Earth and the formation of a metallic core released a large amount of primordial heat and may have enabled its evolution ...
[25]
The atmospheres of rocky exoplanets - I. Outgassing of common ...
We aim to understand the composition of the gas in an exoplanet atmosphere which is in equilibrium with a planetary crust.
[26]
Is Gliese 581d habitable? Some constraints from radiative ...
As can be seen, the clear pure CO2 atmospheres under a G-class star collapse on the surface for pressures greater than about 3 bar, but when the star is M-class ...
[27]
alf Cen | NASA Exoplanet Archive
### Key Orbital Parameters for Proxima Centauri c
[28]
https://www.aanda.org/articles/aa/full_html/2020/04/aa36614-19/aa36614-19.html
[29]
Proxima Centauri c
Its discovery was publicly announced on 2019. The host star Proxima Centauri has apparent magnitude of 11.1, with absolute magnitude of 15.6.Missing: initial | Show results with:initial
[30]
Habitable Moist Atmospheres on Terrestrial Planets near the Inner ...
Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs. Ravi kumar Kopparapu, Eric T. Wolf, Giada Arney ...
[31]
The habitability of Proxima Centauri b - II. Possible climates and ...
1 summarizes this attempt to quantify the possible climates of Proxima b for the two most likely spin states (1:1 and 3:2 spin-orbit resonance), as a function ...
[32]
Astronomers Announce Proxima Centauri's Second Planet, And It's ...
Jan 22, 2020 · Assuming that the efforts to directly image Proxima c finally succeed, this exoplanet will immediately become both the smallest one and the ...
[33]
Ground-breaking exoplanet science with the ANDES spectrograph ...
May 6, 2025 · ANDES will be a powerful transformational instrument for exoplanet science. It will enable the study of giant planet atmospheres.
[34]
Starshot - Breakthrough Initiatives
Breakthrough Starshot aims to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri.Missing: c | Show results with:c
[35]
Monitoring the radio emission of Proxima Centauri
Monitoring the stellar radial velocities and the radio emission will better characterize the occurrence of both quadratures and radio flares, and can improve ...Missing: mitigation | Show results with:mitigation