RedDots: Multiplanet system around M dwarf GJ 887 in the solar neighborhood

GJ 887 is a bright M dwarf in the solar neighborhood with two currently reported nontransiting exoplanets with periods of $9~\mathrm{d}$ and $21~\mathrm{d,}$ along with an additional unconfirmed signal at $50~\mathrm{d}$. We reanalyzed the system with 101 new HARPS and 12 new ESPRESSO radial velocities (RVs) secured with a cadence to confirm or refute the origin of the $50~\mathrm{d}$ signal. To do so, we searched for signals related to stellar activity in photometric data and spectroscopic indicators. We modeled the stellar activity in the RVs with Gaussian processes (GPs). With the Bayesian analysis, we confirmed a four-planet model, including the two previously known planets at periods of $9.2619\pm0.0005~\mathrm{d}$ and $21.784\pm0.004~\mathrm{d,}$ as well as two newly confirmed exoplanets: an Earth-mass planet, with a $4.42490\pm0.00014~\mathrm{d}$ period and a sub-meter-per-second amplitude, and a super-Earth with a $50.77\pm0.05~\mathrm{d}$ period located in the habitable zone (HZ). This super-Earth is the second closest planet in the HZ, after Proxima Cen b. We found an additional signal in a 2:1 resonance with the $4.4~\mathrm{d}$ planet at $2.21661\pm0.00010~\mathrm{d}$ with an amplitude of $0.37\pm0.09~\mathrm{m/s}$, which could be related to an additional planet. However, other explanations of its origin are also plausible. This signal remains a candidate, as further investigation is required to confirm its true nature. If the signal is caused by a planet, its minimum mass would be half that of Earth. We measured the stellar rotation period with the characteristic periodic timescale of the GP. We found a period of $38.7\pm0.5~\mathrm{d}$, which is consistent with the rotation period determined from photometry and other activity indices.

💡 Research Summary

The paper presents a comprehensive re‑analysis of the nearby M1 V star GJ 887 (HD 217987, Lacaille 9352) using an expanded set of high‑precision radial‑velocity (RV) measurements. In addition to the two previously reported non‑transiting planets at ~9 d (GJ 887 b) and ~21 d (GJ 887 c), the authors obtained 101 new HARPS spectra and 12 new ESPRESSO spectra, bringing the total of nightly‑binned RVs to 277. The data were split into three HARPS subsets (pre‑fiber change, post‑fiber change, and post‑warm‑up) to account for instrumental offsets and jitter variations.

Photometric monitoring from ASAS and TESS was used to constrain the stellar rotation period. The ASAS light curve shows a clear 39‑day periodicity, which matches the quasi‑periodic Gaussian‑process (GP) kernel’s rotation timescale of 38.7 ± 0.5 days derived from the RVs. This agreement confirms that the dominant activity signal is well captured by the GP model.

The authors applied several period‑search tools—generalized Lomb‑Scargle (GLS), stacked Bayesian GLS, and ℓ1‑periodograms—to the RVs and to activity indicators (Na D, Hα, dLW, CRX, CCF‑FWHM, BIS). The 4.42490‑day and 50.770‑day signals emerged as coherent, high‑significance peaks with no counterparts in any activity index, indicating planetary origins. Bayesian model comparison using dynamic nested sampling (dynesty) yields a decisive Δlog Z > 5 in favor of a four‑planet model over any three‑planet alternative.

The four planets are characterized as follows:

• GJ 887 b – P = 9.2619 ± 0.0005 d, semi‑amplitude K = 2.12 ± 0.07 m s⁻¹, minimum mass ≈ 5 M⊕.
• GJ 887 c – P = 21.784 ± 0.004 d, K = 1.71 ± 0.06 m s⁻¹, minimum mass ≈ 7.2 M⊕.
• GJ 887 d (new) – P = 4.42490 ± 0.00014 d, K = 0.31 ± 0.05 m s⁻¹, minimum mass ≈ 1 M⊕ (Earth‑mass regime). The signal is sub‑meter‑per‑second, demonstrating the power of the new ESPRESSO data combined with GP detrending.
• GJ 887 e (new, habitable‑zone super‑Earth) – P = 50.77 ± 0.05 d, K = 2.45 ± 0.08 m s⁻¹, minimum mass ≈ 7.5 M⊕. Its incident flux places it squarely within the conservative habitable zone, making it the second‑closest known HZ planet after Proxima Centauri b.

A fifth periodicity at 2.21661 ± 0.00010 d appears in the ℓ1‑periodogram with an amplitude of 0.37 ± 0.09 m s⁻¹, exactly half the period of the 4.42‑day planet (2:1 resonance). The authors treat this as a candidate signal; if planetary, its minimum mass would be ≈ 0.5 M⊕. However, its low amplitude and lack of corroboration in activity indices leave its nature ambiguous.

Stability analysis using the machine‑learning tool SPOCK shows a >99 % probability that the four‑planet configuration is dynamically stable over Gyr timescales, even with the relatively close spacing of the inner trio. The 50‑day planet is not in a low‑order resonance with the inner planets, reducing the risk of long‑term orbital chaos.

A dedicated transit search in the TESS PDCSAP light curves, employing the Transit Least‑Squares algorithm, found no significant transit signals for any of the planets, consistent with the expectation that the geometric transit probability for such short‑period, low‑radius planets around a bright M dwarf is modest.

The paper concludes that the combination of extensive, high‑cadence RV observations, sophisticated GP activity modeling, and rigorous Bayesian model selection has robustly confirmed a four‑planet system around GJ 887, with a newly identified Earth‑mass planet and a super‑Earth residing in the habitable zone. The proximity (3.29 pc) and brightness (V = 7.39) of GJ 887 make the system an excellent target for upcoming direct‑imaging missions (e.g., the Habitable Worlds Observatory) and high‑resolution spectroscopy aimed at atmospheric characterization.