---
title: 'GJ 887: M Dwarf Planetary System'
url: https://www.emergentmind.com/topics/gj-877
type: topic
---

# GJ 887: M Dwarf Planetary System

GJ 877, as it appears in the abstract of the RedDots study "RedDots: Multiplanet system around M dwarf GJ 887 in the solar neighborhood," is a typographical error for **GJ 887**. All analysis in that study concerns **GJ 887 = Gl 887**, also catalogued as **HD 217987** and **Lacaille 9352**, a bright, nearby **M1V** dwarf at **3.2877 pc** that hosts a compact system of low-mass, nontransiting planets detected by radial velocities. The reanalysis reported a preferred **four-planet** architecture consisting of the previously known planets **b** and **c**, the newly confirmed **Earth-mass** planet **e**, and the newly confirmed **super-Earth** **d** in the habitable zone, together with an additional **2.21661 d** signal that remains a candidate rather than a confirmed planet [2602.08929].

## 1. Designation and object identity

The object studied is the nearby M dwarf commonly known as **Gliese 887**. Throughout the body of the RedDots paper the star is designated **GJ 887**, with SIMBAD aliases **HD 217987** and **Lacaille 9352**. The occurrence of **“GJ 877”** in the abstract is explicitly identified as a typographical error; the analysis refers throughout to **GJ 887 = Gl 887**, described there as the well-known bright, nearby M1 dwarf at **3.29 pc**.

This distinction is important because the planetary architecture, stellar properties, and observational inferences attributed to “GJ 877” in that abstract belong to **GJ 887**. A plausible implication is that any use of “GJ 877” in connection with the reported **9 d**, **21 d**, **50 d**, and **4.4 d** signals should be interpreted as shorthand for the GJ 887 system rather than as a separate stellar designation.

## 2. Stellar parameters and magnetic activity

GJ 887 is reported as an **M1V** star with fundamental parameters drawn from **Mann et al. (2015)** and **Gaia**. The adopted stellar parameters are: **$T_{\rm eff} = 3688 \pm 86~\mathrm{K}$**, **$M_* = 0.495 \pm 0.049~M_\odot$**, **$R_* = 0.468 \pm 0.022~R_\odot$**, **$L_* = 0.0368 \pm 0.0040~L_\odot$**, **$[\mathrm{Fe/H}] = -0.06 \pm 0.08$**, **$v \sin i < 2.5~\mathrm{km\,s^{-1}}$**, **$\log g = 4.156^{+0.040}_{-0.038}$**, **age $\approx 2.9^{+8.0}_{-2.2}$ Gyr**, **distance $d = 3.2877$ pc** with **parallax $304.135 \pm 0.020$ mas**, and apparent brightnesses **$V = 7.39$** and **TESS magnitude $T = 5.56$**.

The star is described as **relatively quiet compared to typical early-M dwarfs**, with **low spot coverage**, **low photometric variability**, and **low Ca II H&K and H$\alpha$ indices in earlier epochs**. In the RedDots analysis, the stellar rotation period is measured from Gaussian-process modeling of the RV time series as **$P_{\rm rot} = 38.7 \pm 0.5~\mathrm{d}$**, with consistent values reported as **38.6–38.7 $\pm$ 0.5–0.6 d** across models [2602.08929].

That rotation period is independently corroborated by activity signals near **39 d** in **ASAS photometry** and in several spectroscopic tracers: **HARPS FWHM**, **BIS**, **differential line width (dLW)**, **H$\alpha$**, and **Na D1/D2**. A harmonic near **19 d** is also present, as expected. The study further reports an **“active phase” in 2019**, during which the **39 d** periodicity appeared coherently in **RV**, **Na D1**, and **dLW**. This behavior supports the interpretation that the **38–39 d** signal is rotationally modulated stellar activity rather than a planetary orbit.

## 3. Observational basis

The RedDots collaboration extended the earlier data set with **101 new HARPS** and **12 new ESPRESSO** radial velocities, obtained with a cadence intended to test the origin of the putative **$50~\mathrm{d}$** signal. The full HARPS material comprises **850 spectra total**; after nightly binning, **277 RVs** remained. These were segmented into three instrumental states: **HARPSpre** with **88 RVs** and median internal uncertainty **0.44 m s$^{-1}$**, **HARPSpost** with **164 RVs** and median internal uncertainty **0.87 m s$^{-1}$**, and **HARPSpw** with **25 RVs** and median internal uncertainty **0.61 m s$^{-1}$**. **ESPRESSO** contributed **19 spectra total** and **12 nightly binned RVs** spanning three years, with median internal uncertainty **0.17 m s$^{-1}$**.

The analysis fit **relative offsets** and independent **jitter** parameters for each HARPS segment and for ESPRESSO. Additional public RVs from **Keck HIRES**, **UCLES**, and **PFS** were not used because their median uncertainties of **1–2 m s$^{-1}$** were lower precision than the HARPS and ESPRESSO data. Radial velocities were derived by **template matching using serval**, specifically because its **order-by-order fitting** performs better than cross-correlation for **M dwarfs**.

Activity diagnostics combined **ESO-DRS CCF indicators**—**BIS** and **FWHM**—with **serval indices**: **H$\alpha$**, **Na D1/D2**, **dLW**, and the **chromatic index (CRX)**. These observables were used to confirm **$P_{\rm rot} \approx 39~\mathrm{d}$** and to distinguish activity-driven variability from Keplerian signals.

Photometric context came from both ground and space observations. **ASAS** provided **523 points from 2001–2009** with **median photometric precision $\approx 0.03$**. **TESS** observed the star in **Sectors 2, 28, and 69**, each of approximately **27 d**, with **median PDCSAP flux uncertainties of $\sim 1.1 \times 10^{-4}$**. A **Transit Least Squares (TLS)** search found **no transit detections** at the RV periods, consistent with a nontransiting planetary architecture. The TESS sector window functions are explicitly described as **suboptimal for detecting $\sim 40$ d stellar rotation**.

## 4. Signal extraction and activity mitigation

The radial-velocity inference pipeline combined multiple complementary methods. Initial signal searches used **generalized Lomb–Scargle (GLS)**, **stacked Bayesian GLS (sBGLS)**, and **$\ell_1$-periodograms** based on compressed sensing. Keplerian modeling was performed with **juliet**, using **RadVel** for orbital fits and **dynesty** for dynamic nested sampling, yielding posterior distributions and Bayesian evidences **$\log Z$** for model comparison [2602.08929].

Stellar activity was modeled with Gaussian processes. The preferred covariance model was the **quasi-periodic (QP) kernel** implemented in **george**:
$$
k(\tau) = \sigma_i^2 \exp\!\left[-\alpha \tau^2 - \Gamma \sin^2\!\left(\pi \tau / P_{\rm rot}\right)\right],
$$
where **$\tau \equiv t - t'$**, **$\sigma_i$** is the GP amplitude for each instrument interval, **$\alpha$** sets the decay time of correlations with **spot lifetime $l \approx 1/\sqrt{\alpha}$**, **$\Gamma$** controls the strength of the periodic modulation, and **$P_{\rm rot}$** is the activity periodic timescale. The QP kernel was preferred because it provided a better balance of **parsimony and fidelity** than **SHO** and **double-SHO** kernels in **celerite**. The **dSHO** model, in particular, is reported to have a tendency to **overfit short-timescale variability** and to **partially absorb the $\sim 50$ d planetary signal**.

Coherence tests were central to separating planets from activity. **sBGLS** tracked the temporal growth of signal significance, while **apodized Gaussian-envelope fits** probed the stability of the semi-amplitude over the observing baseline. Under this analysis, the signals at approximately **4.4249 d**, **9.2619 d**, **21.784 d**, and **50.77 d** behaved as **coherent, stable planetary signals**, whereas the **38–39 d** rotation signature was **incoherent across the full baseline** and strongest during the **active phase**. This methodological distinction underpins the classification of the four principal periodicities as planetary and the **39 d** signal as stellar activity.

## 5. Planetary system architecture

Bayesian model comparison identifies a preferred **four-planet model**, denoted **D** in the study, together with one additional short-period candidate, **f**, that does not yet satisfy the adopted threshold for strong evidence. The confirmed components are all nontransiting and are characterized through **minimum masses** **$m_p \sin i$**.

| Object | Period | Minimum mass and status |
|---|---:|---|
| GJ 887 e | $4.42490 \pm 0.00014~\mathrm{d}$ | $1.46^{+0.19}_{-0.18}~M_\oplus$, confirmed |
| GJ 887 b | $9.2619 \pm 0.0005~\mathrm{d}$ | $3.9 \pm 0.5~M_\oplus$, confirmed |
| GJ 887 c | $21.784 \pm 0.004~\mathrm{d}$ | $6.5^{+1.0}_{-0.9}~M_\oplus$, confirmed |
| GJ 887 d | $50.77 \pm 0.05~\mathrm{d}$ | $6.1 \pm 1.4~M_\oplus$, confirmed |
| GJ 887 f | $2.21661 \pm 0.00010~\mathrm{d}$ | $0.47 \pm 0.11~M_\oplus$, candidate |

**GJ 887 e** is the newly confirmed **Earth-mass** planet. Its reported parameters are **$K = 0.91 \pm 0.10~\mathrm{m\,s^{-1}}$**, **$e$ fixed to 0**, **$t_0 = 2453488.05 \pm 0.13$ BJD**, **$a = 0.0417^{+0.0014}_{-0.0015}$ AU**, **$S/S_\oplus = 20.9^{+4.0}_{-2.9}$**, and **$T_{\rm eq} = 544^{+21}_{-20}$ K** for **$A = 0.3$** and no redistribution. Its detection metrics are unusually strong for a sub-meter-per-second signal: **GLS residual peak FAP $\sim 4.6 \times 10^{-11}$**, **$\ell_1$ FAP $\sim 2.6 \times 10^{-4}$**, **$\Delta \log Z = 27.15$** relative to the two-planet model, and an amplitude detection of approximately **$9.8\sigma$**.

**GJ 887 b**, previously known, is reported with **$K = 2.00 \pm 0.17~\mathrm{m\,s^{-1}}$**, **$e = 0.14 \pm 0.06$**, **$\omega = 3.1 \pm 2.2$ rad**, **$t_0 = 2453492.75 \pm 0.27$ BJD**, **$a = 0.0683^{+0.0022}_{-0.0024}$ AU**, **$S/S_\oplus = 7.8^{+1.3}_{-1.1}$**, and **$T_{\rm eq} = 426 \pm 16$ K**. Its signal is supported by **GLS FAP $\sim 5.8 \times 10^{-8}$**, **$\ell_1$ FAP $\sim 1.4 \times 10^{-10}$**, and strong Bayesian evidence.

**GJ 887 c**, also previously known, has **$K = 2.54 \pm 0.27~\mathrm{m\,s^{-1}}$**, **$e = 0.17 \pm 0.06$**, **$\omega = 3.5^{+2.0}_{-2.4}$ rad**, **$t_0 = 2453498.2 \pm 0.9$ BJD**, **$a = 0.121^{+0.004}_{-0.005}$ AU**, **$S/S_\oplus = 2.5 \pm 0.4$**, and **$T_{\rm eq} = 320 \pm 12$–$13$ K**. The study notes that its coherence dips during the star’s active phase but recovers afterward, while apodized fits show a stable amplitude overall.

**GJ 887 d** is the newly confirmed **super-Earth** in the habitable zone. The favored model adopts a circular orbit with **$K = 1.7 \pm 0.4~\mathrm{m\,s^{-1}}$**, **$t_0 = 2453496 \pm 4$ BJD**, **$a = 0.212^{+0.007}_{-0.008}$ AU**, **$S/S_\oplus = 0.81^{+0.13}_{-0.12}$**, and **$T_{\rm eq} = 241^{+10}_{-9}$ K**. The residual-periodogram and evidence-based metrics are more marginal than for the shorter-period planets but remain supportive: **GLS residual peak FAP $\sim 1.6 \times 10^{-6}$**, **$\ell_1$ FAP $\sim 5.2 \times 10^{-3}$**, an amplitude detection of approximately **$4.6\sigma$**, and **$\Delta \log Z = 6.48$** relative to the three-planet model before conservative prior correction. The paper explicitly notes that the **dSHO GP** can absorb part of the long-period power, yet the **four-planet model remains preferred across kernels** [2602.08929].

The additional signal, **GJ 887 f**, has **$P = 2.21661 \pm 0.00010~\mathrm{d}$**, **$K = 0.37 \pm 0.09~\mathrm{m\,s^{-1}}$**, **$e$ fixed to 0**, **$t_0 = 2453486.46 \pm 0.18$–$0.19$ BJD**, **$a = 0.0263 \pm 0.0009$ AU**, **$S/S_\oplus = 53^{+9}_{-8}$**, and **$T_{\rm eq} = 685^{+26}_{-25}$ K**. Its amplitude is detected at approximately **$4.5\sigma$**, and the residuals of the four-planet GP model give **GLS FAP $\sim 8.3 \times 10^{-3}$** with **$\Delta \log Z = 4.42$** relative to the four-planet model. Because this lies below the adopted **$\Delta \log Z > 5$** threshold for strong evidence, and because prior-width correction makes the improvement negative, it is classified as **promising but not yet confirmed**.

## 6. Dynamical structure and habitability context

Long-term dynamical considerations were addressed with **SPOCK** following **Tamayo et al. (2020)**. Posterior samples were weighted by predicted long-term stability; this down-weighted high-eccentricity samples for **b** and **c**, but the median parameters remained essentially unchanged. The adjacent pairs in the five-planet configuration **febcd** have mutual Hill separations of **$\Delta(fe) \approx 28.6^{+0.9}_{-1.0}$**, **$\Delta(eb) \approx 21.6 \pm 0.6$**, **$\Delta(bc) \approx 20.0 \pm 0.6$**, and **$\Delta(cd) \approx 18.6^{+0.9}_{-0.8}$**. These values are described as typical of **compact multiplanet systems** and consistent with stable orbital spacing [2602.08929].

The putative **f:e** pair is of particular dynamical interest. Their period ratio is **$1.99626 \pm 0.00010$**, close to a **2:1 mean-motion resonance**. The posterior distribution of the resonant angle **$\phi = 2\lambda_2 - \lambda_1$** is reported as strongly clustered, with **resultant length $R = 0.84$**, near **$\pi/2$**, and **dispersion $\sigma \approx 0.58$ rad**, which is suggestive of **libration**. The study nevertheless states that further **ultra-precise RVs** and dedicated **N-body integrations** are required to determine whether the angle truly librates rather than circulates.

Habitability considerations center on **GJ 887 d**. With **$S/S_\oplus = 0.81^{+0.13}_{-0.12}$** and **$T_{\rm eq} \approx 241 \pm 9$–$10$ K** for **Bond albedo $A = 0.3$** and zero heat redistribution, the planet is stated to lie comfortably within the **Kopparapu et al. (2014)** habitable zone for a low-mass planet orbiting an **M1V** star. The estimated habitable-zone boundaries correspond to orbital periods of approximately **43–122 d**, placing the **50.77 d** orbit squarely within the zone. By distance, **GJ 887 d** is described as the **second-closest known habitable-zone planet after Proxima Cen b**, given **Proxima Cen b at 1.30 pc** and **GJ 887 at 3.29 pc**.

The stellar environment is treated cautiously. The star’s **low average magnetic activity** is cited as favorable, and **Mesquita et al. (2022)** is noted as suggesting that the **space weather environment may be Earth-like**. At the same time, **occasional flares have been observed** according to **Loyd et al. (2020)**, and the paper emphasizes that actual habitability depends on the planet’s **magnetic field** and **atmosphere**, both of which are presently unknown.

## 7. Observational constraints, ambiguities, and future work

The system is currently constrained primarily by radial velocities rather than transits. **TLS** analyses of **TESS Sectors 2, 28, and 69** found no transits at any of the RV periods. Using probabilistic mass–radius relations from **Chen & Kipping (2017)**, the expected transit depths span from approximately **$2.2 \times 10^{-3}$** for **c** down to approximately **$2.5 \times 10^{-4}$** for **f**; these depths would in principle be detectable given the reported TESS precision. Their absence therefore supports a **non-transiting architecture**, and all reported masses remain **minimum masses $m_p \sin i$**. The paper notes that true masses will require **inclination constraints**, for example from **future astrometry** [2602.08929].

The analysis also includes explicit sensitivity tests. Synthetic-signal injection and recovery showed that short-period signals at **$K \approx 0.94~\mathrm{m\,s^{-1}}$** and **$0.37~\mathrm{m\,s^{-1}}$** were recovered with **$\Delta \log Z \approx 29$** and **$\approx 5$**, respectively, consistent with the empirical behavior of **e** and **f**. A long-period **66 d** signal at **$K \approx 1.8~\mathrm{m\,s^{-1}}$**, chosen to resemble **d**, was recovered strongly but weakened when the semi-amplitude was reduced to **$\approx 1.2~\mathrm{m\,s^{-1}}$**, demonstrating the possibility of **GP absorption near $P_{\rm rot}$**. A formal detection-limit analysis in the five-planet residuals found a mean threshold of **$\sim 25 \pm 3~\mathrm{cm\,s^{-1}}$** over **0.5–100 d**, approximately **23 cm s$^{-1}$** near **2 d**, and approximately **26 cm s$^{-1}$** beyond **20 d**.

Several limitations remain material to interpretation. The paper explicitly cautions that **GP kernels can absorb planetary power near the rotation period or its harmonics**, and that the evidential strength of the **50.77 d** signal depends somewhat on the activity model. It also stresses that the **2.2166 d** candidate may represent a genuine resonant planet, but that an **eccentric single-planet solution could mimic the pair at 2:1**, following the alternative explanation discussed by **Anglada-Escudé (2010)**. The rotational **38–39 d** signature, by contrast, is regarded as activity because it is **incoherent across the full baseline**.

Future observing strategies are correspondingly targeted. The RedDots program emphasizes **nightly observing** to reduce aliasing and to capture activity cycles, and the paper identifies **continued sub-m s$^{-1}$ RV monitoring** with **ESPRESSO**, **NIRPS**, and future facilities as the principal route to confirming **f**, refining eccentricities, and constraining resonant dynamics. It also highlights **coordinated photometry**, such as with **CHEOPS**, to search for shallower or longer-duration transits missed by TESS.

GJ 887 d is additionally discussed as a possible target for future direct imaging. Assuming **Earth-like albedo $A = 0.3$**, the expected reflected-light properties are **angular separation $\theta = 64.6^{+2.1}_{-2.2}$ mas** and **contrast $C = 38.1^{+2.8}_{-2.4} \times 10^{-9}$**. For the **Habitable Worlds Observatory (HWO)** concept, the relevant benchmark cited is a contrast floor of approximately **$(2.5$–$4)\times 10^{-11}$** with **inner working angle $\approx 65$ mas** in **Tier C**. On those numbers, the paper states that the brightness ratio is well above the contrast floor, while the principal difficulty is that the separation lies almost exactly at the **IWA**. **LIFE**, the proposed mid-IR interferometer, is also identified as a relevant future facility because of the star’s brightness and proximity.

Source: https://www.emergentmind.com/topics/gj-877