G876 update review


After 20 years I completed a literature review on Gliese 876 to obtain my Astronomy Master Degree, I have done an update.

14th Sept 2025


Gliese (GJ) 876: An Extended Thesis on Observations, Dynamics, Formation, and Habitability

Author: prepared by Charles Duarte






Abstract

Gliese 876 (GJ 876) is a nearby M4V red dwarf hosting a compact, multi-planet system notable for its resonant architecture, strong planet–planet interactions, and historical importance in exoplanet detection. This thesis synthesizes observational history, detailed planetary parameters, dynamical analyses (including the 1:2:4 Laplace-like resonance among planets c, b, and e), formation scenarios via disk-driven migration, long-term stability, tidal evolution, and habitability prospects for bodies in the system. It also reviews techniques used to detect and model the system (precision radial velocities, N-body fitting, astrometry, and photometry), summarizes numerical simulation results from the literature, and proposes a prioritized observational program for further characterization.

(Key load-bearing sources used in this draft include: the NASA Exoplanet Archive overview for GJ 876; Rivera et al. 2005 and Rivera et al. 2010 for discovery and characterization of planets d and e; Nelson et al. 2015 for 3D resonant architecture constraints; and several dynamical analyses and formation studies examining the Laplace resonance.


Table of contents

• Introduction

• Stellar properties and environment

• Observational history and data sources

• Planet-by-planet characterization 

• 4.1 GJ 876 b

• 4.2 GJ 876 c

• 4.3 GJ 876 d

• 4.4 GJ 876 e

• Resonant dynamics and architecture

• Numerical methods and modeling approaches

• Formation theories and migration

• Long-term stability, chaos, and tidal evolution

• Habitability and prospects for satellites

• Future observations and recommended programs

• Conclusions

• Appendix: collated parameter table from public catalogs

• References


1. Introduction


The study of exoplanetary systems orbiting M dwarfs has been transformational for planet detection and formation theory. Gliese 876 (hereafter GJ 876) stands out among nearby planetary systems because it hosts multiple giant planets and a low-mass inner planet in a configuration where gravitational interactions are directly observable in radial-velocity datasets. The system was among the earliest detections of planets around red dwarfs and remains an illustrative laboratory for resonant dynamics, disk migration, and the limits of detectability with Doppler techniques. This thesis aims to collect and synthesize observations, theoretical studies, and numerical experiments into a single extended document suitable as the basis for an academic thesis or an advanced review paper.

Key themes carried through this work are:

• the importance of self-consistent N-body modelling for parameter estimation in strongly interacting systems;

• the role of convergent migration and dissipative processes in establishing multi-body resonances;

• how observational constraints (radial velocity, astrometry, photometry) interplay to yield three-dimensional orbital architectures; and

• the limits of habitability in systems dominated by giant planets but located around cool, low-luminosity stars.

This document pulls parameter values and synthesis from major public resources and peer-reviewed studies, including catalog compilations (NASA Exoplanet Archive) and key discovery/analysis papers (Rivera et al. 2005, Rivera et al. 2010, Nelson et al. 2015, and subsequent re-analyses). 



2. Stellar properties and environment

GJ 876 is an M4V red dwarf residing at a distance of approximately 4.67 parsecs (≈15.2 light years) from the Sun, in the constellation Aquarius. Catalog values (e.g., Gaia parallaxes collated by public archives) put the parallax near 214.0 mas which corresponds to that distance. Stellar fundamental parameters commonly cited in the literature are a mass of roughly 0.30–0.36 M_☉, a radius ≈0.36 R_☉, and luminosity ≈0.013 L_☉ (the precise values depend on the adopted source and stellar models). The effective temperature is ~3300–3400 K; the star displays low-level photometric variability and X-ray activity typical of an M dwarf with starspots and a slow rotation.


2.1 Age and metallicity

Estimating ages for M dwarfs is challenging because their long evolutionary timescales produce small changes in observable properties over Gyr. Estimates for GJ 876’s age vary in the literature; activity proxies and kinematic population assignment typically yield ages in the Gyr range (values with significant uncertainty; some analyses place the star at a few Gyr). Metallicity estimates place GJ 876 at approximately solar to slightly sub-solar metallicity (estimates on the order of [Fe/H] ≈ -0.1 to 0.0 in different analyses). These parameters matter for planet formation modeling because disk masses and solid content scale with stellar metallicity and influence the potential for giant-planet formation around low-mass stars.


2.2 Habitable zone (CHZ)

Given the star's low luminosity, the conservative circumstellar habitable zone (CHZ) is compact, with conservative estimates for GJ 876 placing the inner and outer bounds roughly between 0.12 and 0.23 AU depending on climate model choices and the adopted effective flux boundaries. Those orbital distances are small compared to Solar System scales but relatively large compared to the semi-major axes of the system's innermost planets.


3. Observational history and data sources


GJ 876’s planets were discovered through high-precision radial-velocity (RV) campaigns that began in the 1990s. The outer gas giant, GJ 876 b, was announced in 1998, making it one of the first planets known to orbit an M dwarf. Subsequent RV monitoring revealed additional periodicities that were, at first, misinterpreted as orbital eccentricity features; careful dynamical analysis and additional data led to the recognition of the inner giant GJ 876 c (2001) and, later, a low-mass inner planet GJ 876 d (2005). The discovery of a fourth planet, GJ 876 e, with period ~124 days was reported in 2010, completing the presently accepted four-planet configuration in most models. Key RV datasets that have been combined by researchers include measurements from Keck/HIRES, HARPS, ELODIE/CORALIE, Lick, and others; the long time-baseline (decades) and multi-instrument coverage make GJ 876 a well-sampled RV system. 


3.1 The necessity of N-body fitting

An early, and now canonical, lesson from GJ 876 is that when mutual planet–planet gravitational interactions are strong enough to produce measurable perturbations on observational timescales, independent Keplerian fits to each planet are insufficient. Instead, researchers must use self-consistent Newtonian (N-body) models that integrate the equations of motion and fit the RV data by adjusting masses and orbital elements within the dynamical model. These dynamical fits can reveal parameters (e.g., true masses, mutual inclinations) that are inaccessible to simple Keplerian models because the interactions break the usual m sin i degeneracy. Rivera et al. (2005) and many follow-ups used N-body fitting techniques to derive improved parameter estimates and demonstrate resonant libration behavior in the system. 


3.2 Key discovery and follow-up papers

• Gliese 876 b (1998): Early RV detections from multiple teams reporting the outer giant and constraining its orbital period to ≈60–61 days. 

• Gliese 876 c (2001): Detection of a second giant interior to b (P≈30 days); the presence of two massive planets in near 2:1 resonance quickly became a focal point for dynamical analysis.

• Gliese 876 d (2005, Rivera et al. 2005): Announcement of a low-mass inner planet (m sin i ≈ 5.9 M_⊕, later refined to ≈6.8 M_⊕ depending on inclination assumptions) with P≈1.9379 days. This was one of the earliest low-mass planets discovered by RV.

• Gliese 876 e (2010, Rivera et al. 2010): Evidence for a fourth planet (Uranus-mass scale) with P≈124 days that participates in a three-body Laplace-like resonance with b and c.

3.3 Astrometry and photometry

Astrometric detections using Hubble Space Telescope Fine Guidance Sensors (Benedict et al., early 2000s) provided independent evidence constraining inclinations for some planets, notably the outer giant; later re-analyses combining RV and astrometry helped settle the system's nearly coplanar architecture. Photometric monitoring has been used to both search for transits and monitor stellar activity; to date, transits of the main massive planets have not been confirmed, and photometry primarily serves to quantify activity-related RV noise (jitter) and rotation periods. 

4. Planet-by-planet characterization

This section synthesizes representative parameter values (period, semi-major axis, mass, eccentricity, inclination constraints) from major catalogs and the peer-reviewed literature. Parameters differ across fits; where possible I note ranges and point to the primary sources.

4.1 GJ 876 b (outer giant)

• Discovery: 1998 (Doppler spectroscopy).

• Representative orbital period: ≈61.1 days.

• Semi-major axis: ≈0.208–0.22 AU.

• Minimum mass (m sin i): ≈1.9–2.3 M_J; true mass estimates from dynamical fits and astrometry place it around ~2 M_J depending on the assumed inclination. 

• Eccentricity: modest and time-variable due to resonant interactions (often reported as low-to-moderate, with values depending on epoch and fitting methodology). The planet participates in a strong 2:1 resonant relationship with GJ 876 c and the broader Laplace configuration including planet e.


Notes on composition and atmosphere

As a gas giant with mass comparable to or larger than Jupiter but orbiting a cool red dwarf, GJ 876 b likely has an atmosphere dominated by hydrogen and helium with traces of heavier volatiles and possible cloud decks, much like warm-to-cold gas giants in other systems. The planet's equilibrium temperature (depending on albedo assumptions) is modest (often estimated in the ~150–250 K range), but because the star emits primarily in the infrared, atmospheric energy balance and photochemistry differ from solar-type-hosted giants. Direct atmospheric characterization has not been achieved for GJ 876 b due to lack of transits and observational sensitivity limits, though future instruments might probe thermal emission or reflected-light signatures for nearby giants.

4.2 GJ 876 c (inner giant)

• Discovery: 2001. 

• Representative orbital period: ≈30.1 days.

• Semi-major axis: ≈0.13 AU.

• Minimum mass (m sin i): ≈0.6–0.75 M_J in many fits; true mass estimates depend on inclination and N-body modeling.

• Eccentricity: non-zero, and dynamically coupled to planet b — the eccentricities of the two giants exchange angular momentum on secular timescales because of resonance.

Dynamical role

GJ 876 c and b form a near 2:1 mean motion resonance; the pair displays significant librations of resonant angles, and their mutual interactions are the primary reason N-body modelling is required for accurate parameter estimation. The inner giant c typically has a smaller mass than b but exerts significant influence on b's orbital evolution because of proximity and resonance locking. 

4.3 GJ 876 d (inner low-mass planet)

• Discovery: 2005 (Rivera et al. 2005).

• Representative orbital period: 1.9378–1.9379 days.

• Semi-major axis: ≈0.0208 AU.

• Mass: m sin i ≈5.9 M_⊕ from initial fits; dynamical analyses and inclination estimates often give a mass closer to ≈6.7–7.5 M_⊕ (catalog values such as NASA list ≈6.8 M_⊕).

• Eccentricity: small-to-moderate; tidal dissipation at this close-in orbit likely damps eccentricity over time and may have shaped the current orbital parameters.

Notes

The inner planet d was one of the first RV-detected super-Earth/mini-Neptune class bodies found around a main-sequence star, predating many discoveries of small planets by transit surveys. Its very short period and proximity to the host complicate habitability — the intense stellar irradiation and tidal effects likely render the planet inhospitable to Earth-like life. Nevertheless, the planet's existence inside a multi-giant resonant system provides clues about the migration history and dynamical packing of the system.


4.4 GJ 876 e (outer Uranus-mass planet)

• Discovery: 2010 (Rivera et al. 2010).

• Representative orbital period: ≈124 days.

• Semi-major axis: ≈0.33–0.34 AU.

• Mass: often reported in the Uranus-mass regime (around 13–20 M_⊕ depending on fits) and sometimes expressed as a super-Neptune. Its discovery completed the observed 1:2:4 Laplace-like chain (c:b:e).


Resonant role

Planet e participates in a three-body Laplace resonance with b and c: a condition analogous to the Laplace resonance among Jupiter’s moons Io–Europa–Ganymede, where combinations of orbital longitudes produce a librating angle characteristic of a three-body resonance. The presence of a Laplace-like resonance provides strong evidence for convergent migration and dissipative capture during the system's formation epoch. 


5. Resonant dynamics and architecture

The GJ 876 system is one of the best-studied examples of multi-planet resonant architecture. The clearest resonant relationship is the near 2:1 mean-motion resonance between planets c and b (P_c ≈ 30.1 d; P_b ≈ 61.1 d). The addition of planet e at ~124 d completes an approximate 1:2:4 chain (c:b:e), and analyses show that a Laplace-like critical angle formed from the mean longitudes of the three planets librates — a signature of a three-body resonance. Several points about the resonant dynamics are essential:

5.1 Libration of resonant angles

In mean-motion resonances, specific combinations of orbital angles (resonant arguments) librate rather than circulate. For GJ 876, multiple resonant arguments associated with the 2:1 pair and the three-body Laplace angle show libration on timescales accessible to RV datasets. The libration amplitudes and centers contain information about the capture process, dissipation history, and current dynamical energy in the resonant degrees of freedom. Detailed studies (e.g., Rivera et al. 2010; Nelson et al. 2015) show that at least one resonant argument per planet pair and the Laplace argument librate, indicating a deep resonant connection among the outer three planets. 


5.2 Mutual inclination and three-dimensional structure

Radial-velocity data alone traditionally give m sin i for planetary masses, leaving the inclination unknown. However, in strongly interacting resonant systems like GJ 876, the dynamical signature of interactions provides constraints on the three-dimensional orbital architecture because the strength and pattern of perturbations depend on the true masses (not just m sin i) and mutual inclinations. Combining RV with astrometry (and applying N-body modeling) allows researchers to constrain the system's inclination and mutual inclinations; studies find the system is approximately coplanar with mutual inclinations small (few degrees) — compatible with formation via disk migration rather than violent scattering. Nelson et al. (2015) and other works perform Bayesian/N-body analyses to quantify these constraints.

5.3 Stability islands and chaotic zones

Despite the resonance providing stabilizing structure, numerical explorations reveal that the resonant domain in parameter space can be surrounded by chaotic regions. The Laplace resonance in GJ 876 appears to be a stabilization mechanism residing in a narrow island of regular motion; small changes to masses or orbital elements can lead the system into chaotic diffusion and instability on relatively short timescales. Studies mapping stability as a function of eccentricity and mutual inclination (e.g., Martí et al., Giuppone, others) show allowable parameter ranges for long-term stability but also emphasize the system's sensitivity to initial conditions. 

5.4 Implications for mass determination

The amplitude and phase of dynamical interactions are sensitive to planet masses and inclinations; dynamical fits that match the observed RV time-series can therefore break the m sin i degeneracy and yield estimates of the true planetary masses and sometimes mutual inclinations. This dynamical information is why GJ 876 has been used as a benchmark system for testing N-body MCMC fitting frameworks and for illustrating how mutual interactions help retrieve true system geometry from RVs alone in favorable cases. 


6. Numerical methods and modeling approaches

Accurate modeling of GJ 876 requires several methodological choices and numerical techniques. Below I summarize typical approaches used in the literature and comment on strengths and limitations.

6.1 Newtonian N-body integrations

N-body integrators (e.g., symplectic integrators like Wisdom-Holman variants or high-order adaptive integrators) are used to compute the planetary motions under mutual gravitational forces and the host star. Integrators must be sufficiently accurate to preserve the resonant dynamics over model-fitting timescales and must be coupled to parameter estimation frameworks (least-squares fitting, MCMC, nested sampling) to find posterior distributions of orbital parameters and masses. Rivera et al. and later teams used such integrations embedded in fitting routines to derive self-consistent solutions. 

6.2 Bayesian inference and posterior sampling

Given data (multi-instrument RVs with offsets and noise parameters), researchers commonly use Bayesian methods (MCMC, affine-invariant samplers, nested sampling) to explore parameter space while requiring dynamical stability (either as a post-processing filter or enforced during sampling). Including instrument-specific noise terms (jitter) and modeling correlated stellar activity signals can be important to avoid spurious planetary detections or biased parameter estimates. Nelson et al. (2015) exemplify a careful approach to three-dimensional N-body Bayesian fitting that yields credible intervals on mutual inclinations and resonant libration amplitudes. 

6.3 Stability constraints and long-term integrations

Because the RV dataset covers decades but not Gyr timescales, researchers often apply long-term stability tests on proposed orbital solutions by integrating them for 10^6–10^9 years (depending on computational resources) and rejecting unstable orbits. Stability criteria can include absence of close encounters, bounded eccentricity evolution, and the preservation of resonant locking. This two-step approach (fit to RVs, then stability screening) is standard for resonant systems such as GJ 876. Studies mapping stability islands use chaos indicators (Lyapunov exponents, MEGNO) and map parameter space to find stable regions compatible with observations. 


7. Formation theories and migration

The presence of multiple giant planets in tight orbits around a low-mass star presents formation challenges: core-accretion models predict that forming Jovian-mass planets around low-mass stars is less efficient due to smaller disk masses, yet GJ 876 hosts multiple massive planets. Two broad formation scenarios have been proposed and explored:

7.1 In situ formation

In situ formation of massive giants near their present locations is generally disfavored around low-mass stars because the required solid material to form massive cores within small orbital radii is unlikely. However, local accumulation through pebble accretion or enhanced surface density regions might allow for preferential formation of substantial cores closer in. Most studies, though, prefer migration-based explanations for GJ 876’s architecture.

7.2 Disk-driven convergent migration and resonant capture

The leading formation explanation is that the giant planets formed at larger separations where more solid material was available and migrated inward via interactions with the gaseous protoplanetary disk (Type II migration for gap-opening giants, Type I for lower-mass bodies). Convergent migration (outer planet migrating faster than inner, or differential migration) naturally leads to capture into mean-motion resonances. Hydrodynamical and N-body simulations with dissipative forces (torques and eccentricity damping) show that resonant capture into 2:1 and higher-order chains is robust for a wide range of parameter choices and that subsequent damping by the disk can reduce libration amplitudes and eccentricities to values consistent with observations. Studies such as Lee & Peale (2002) and more recent works (e.g., 2018 study on smooth disk migration) map the plausible migration histories that reproduce the GJ 876 resonant architecture. 

7.3 Role of tidal dissipation and disk dispersal

After resonant capture, tidal dissipation (especially for inner planets) and the eventual dispersal of the gaseous disk can alter the system’s orbital configuration. Tidal damping can circularize tight orbits (e.g., GJ 876 d) and change semi-major axes slightly; the final resonant amplitudes also depend on how abruptly the disk dissipates. Some models require moderate eccentricity damping during the migration epoch to match the current eccentricities and libration amplitudes, which sets constraints on disk viscosity and density. 


8. Long-term stability, chaos, and tidal evolution

Long-term integrations reveal a nuanced picture: the resonant chain is both a stabilizing structure and a feature that confines the system within a narrow island of regular motion inside a surrounding chaotic sea. The dynamics of the outermost resonant planet (e) are particularly important because small changes to its mass or eccentricity can broaden chaotic diffusion and potentially destabilize the system on Myr–Gyr timescales. Recent work mapping chaos indicators and diffusion supports the idea that the current configuration is long-lived but delicately balanced; the resonant lock likely formed early and persisted, with migration and damping processes shepherding the planets into this stable configuration. 


8.1 Tidal effects

Tides on the innermost planet (d) from the star are expected to be significant. For short-period planets like GJ 876 d (P≈1.94 d), tidal dissipation causes orbital circularization and can alter the semi-major axis slightly over timescales that depend on the planetary tidal quality factor Q_p and stellar tidal properties. The present eccentricity and spin state of d (likely tidally locked rotation) can give insights into its interior rheology and tidal dissipation efficiency.

8.2 Chaos and diffusion timescales

Chaotic diffusion in phase space — where small perturbations lead to exponential divergence of trajectories — is present in certain regions of parameter space; however, the observed best-fit solutions tend to reside in stable islands where diffusion timescales exceed the system age. Studies calculating Lyapunov times and MEGNO indices find that while the system can be chaotic in a technical sense, the chaotic timescales can be long enough to preserve the architecture for Gyr under present conditions. Nonetheless, the narrowness of the stable region explains why some parameter changes lead to rapid destabilization in simulations. 


9. Habitability and prospects for satellites

Assessing habitability in GJ 876 is subtle. The star’s CHZ lies at ~0.12–0.23 AU; none of the known planets are clearly terrestrial and located within stable, Earth-like insolation ranges with low tidal heating. The outer giants b and c lie near or inside what might be considered the habitable zone depending on assumptions, but being gas giants, they are unlikely to host surface conditions suitable for life. The possibility of habitable moons orbiting giant planets has been discussed in the literature, but forming and retaining large moons around close-in giants in a dynamically active resonant system is non-trivial. Additionally, M dwarfs present strong high-energy radiation environments (flares, coronal mass ejections) that can erode atmospheres of terrestrial worlds or satellites unless protected by magnetic fields or high-mass atmospheres. Overall, the prospects for conventional surface habitability among currently known bodies in GJ 876 are slim, although the system remains an interesting case study for satellite habitability modeling under resonant conditions. 

9.1 Tidal heating for moons

Hypothetical moons of the gas giants could experience significant tidal heating driven both by their host planet and by forced eccentricities generated through the system’s resonant architecture. This heating could, in principle, create subsurface habitable environments (analogous to Europa) but requires significant tidal energy input and long-term orbital stability of the moon — conditions that are not guaranteed in GJ 876’s complex dynamical environment.

10. Future observations and recommended programs

To further refine the GJ 876 picture and test formation theories, a prioritized observational program includes:

10.1 Continued high-precision radial-velocity monitoring

Expanding the RV time baseline and adding precision (sub-m s^-1 when possible) will help refine libration amplitudes, eccentricities, and secular oscillations and could reveal any additional low-mass companions or long-term trends. Combining data from multiple stabilized spectrographs (HIRES, HARPS, HARPS-N, ESPRESSO) with uniform calibration will improve parameter constraints. 

10.2 High-precision astrometry

Space-based astrometry (e.g., Gaia data releases) or targeted interferometric programs can constrain inclinations and absolute masses, complementing N-body RV fits and potentially tightening mutual inclination bounds. Existing HST-FGS astrometry provided early constraints; modern instruments can do far better and may remove lingering degeneracies. 

10.3 Transit searches and photometry

Although geometric transit probabilities for the known planets are modest, dedicated photometric monitoring during predicted transit windows (derived from dynamical models) could detect transits for some planets or place stringent upper limits on planetary radii. Even non-detections are informative because they constrain inclinations and the possibility of large moons. Long-term photometry also characterizes stellar activity and rotation, aiding RV noise modeling.

10.4 Direct imaging and reflected light

Direct imaging of close-in planets around M-dwarfs is challenging due to small angular separations and low planet-star contrasts, but advanced extreme-AO systems or future space telescopes with coronagraphs may, in principle, access reflected light or thermal emission from nearby, massive planets like GJ 876 b in favorable conditions. Such observations would revolutionize atmospheric knowledge but are technically ambitious.

10.5 Theoretical and numerical modeling

Further work on formation modeling (hydrodynamical simulations of convergent migration including realistic disk thermodynamics) and long-term tidal-dynamical coupling will help map plausible evolutionary tracks for the system. Additionally, detailed stability mapping that incorporates realistic post-formation perturbations (stellar encounters, long-term tidal evolution) would clarify how unique or common GJ 876-type resonant chains are.

11. Conclusions

Gliese 876 is a benchmark multi-planet system around an M dwarf notable for its resonant chain (1:2:4 Laplace-like resonance among planets c, b, and e), strong planet–planet interactions observable in radial velocities, and the presence of a close-in low-mass super-Earth/mini-Neptune (planet d). The system shows evidence of formation via convergent migration and resonant capture, and dynamical fits that incorporate N-body physics and stability constraints have refined planetary masses and geometries. Although habitability prospects for known planets appear low, GJ 876 remains a key testbed for migration theory, resonant dynamics, and the interpretation of interacting RV signals.

If you want this draft expanded further (detailed literature review with subsection-by-subsection citations, full collated parameter table and error bars from multiple catalogs, figures of resonant angle libration from published studies, or appendices with example N-body fitting scripts), tell me which items you want next and I will add them to this document.

12. Appendix: collated parameter table (representative values)

Note: users seeking precise numerical values for simulation should consult the NASA Exoplanet Archive or original papers for the latest parameter sets and uncertainties.

PlanetPeriod (days)a (AU)m sin i (M⊕/M_J)Representative massEccentricityCommentd1.93790.0208~5.9 M⊕~6.8 M⊕ (catalog)~0.21 (varies)Inner super-Earth/mini-Neptune. 30.100.13~0.6–0.75 M_J~0.7 M_Jnon-zero, coupledInner giant in 2:1 with b. 61.100.208~1.9–2.3 M_J~2 M_Jlow-to-moderate, variableOuter giant; resonant with c; participates in Laplace chain. citeturn0search6e1240.33~13–20 M⊕ (Uranus-mass)Uranus-mass rangelow-to-moderateOuter resonant planet completing 1:2:4 chain. 

13. References (select)

• Rivera, E. J., et al., "A 7.5 M⊕ Planet Orbiting the Nearby Star GJ 876." ApJ, 2005. 

• Rivera, E. J., et al., "A Uranus-Mass Fourth Planet for GJ 876 in an Extrasolar Laplace Configuration." ApJ, 2010. 

• Nelson, B. E., et al., "An Empirically Derived Three-Dimensional Laplace Resonance in the Gliese 876 Planetary System," 2015. 

• GJ 876 overview, NASA Exoplanet Archive. 

• Dynamical analysis and Laplace-resonance mapping (Martí et al., Giuppone & Martí, 2013). 

• "Forming Gliese 876 through Smooth Disk Migration," 2018 (Dawson et al.). 

• Chaos and diffusion analysis: "Chaotic diffusion in the Gliese-876 planetary system" (MNRAS). 


End of literature review 

Installing a remote Observatory

 

Installing a remote Observatory

Its been quite a while seen my last posting, although I have not been active here, I been very active installing and setting up a new remote system inside my observatory.

I will explain below the reasons why building this remote setup and how I achieve it.

But first, why building a remote /automation observatory, my current observatory is semi-automatic, not fully and not remote, meaning I need to get up in the middle of the night or early in the morning when the sequence is complete to switch off all the equipment.

Reason

Simply a automate telescope is a telescope system that can operate autonomously. It is principally a system used for imaging whether this is for capture of data for images of objects or scientific use. This could be as simple as something at the backyard or a system on the other side of the world. Robotic systems are useful because they allow you to capture data automatically. You can be doing something else during unsociable hours like sleeping! Systems such as this also allow you to capture data in more favorable locations for seeing, darkness and objects.

 

Necessity

The basic components for an automate system are just a telescope system that can be controlled by computer. automated does not necessarily mean ‘Remote System’. It is just a system that can operate manually initially, programed to run on its own on-side. For more advanced systems that are left on its own, then you also need secondary equipment that can control power, monitor the weather and react when conditions deemed as unsuitable or unsafe are met. Typically, this also includes an observatory whose roof is motorized, or a dome that will closed and open motorized, For remotely accessed systems a good internet connection is also needed.

Choosing the components

Choosing components for a automate system is not that simple, and you would need to inform youself. What will work depends on what you want to achieve. If you are going to remain close to your system and will turn it on / off etc manually less thought is needed. There are products that suggest a simple route to full unattended operation using the wifi to communicate at a short distance like the Primaluce Eagle. However, these do not offer the full connectivity or power management needed for true remote operation but do allocate attended operation. They simply replace your PC.

The Automated Observatory

For true remote operation you need an observatory with a motorised roof (a simple Roll Off Roof type or Dome) but this needs to be controllable. Motorising a roof is simple and no different to any other system that uses a motor to open and close something like a garage doors a common examples. Ideally the roof system should be computer controlled as this will allow integration with other Astronomy software. If using a dome this is totally different, the dome rotates, so for example while a roof top moves horizontal from wall ‘A’ to wall ‘B’ (open and close the roof), the Dome is a Shutter window open and close from ‘A’ to ‘B’, but the dome top rotates,  second level of control is needed as not only must you open & close the shutter of the dome, this must track with the telescope. This also requires your telescope to be installed correctly, and the slave data must be calculated.

 


The Automated Observatory – Dome Slave

Domes Telescopes on a fork mount with an equatorial wedge or a normal EQ Mount like I have, will need to be offset because the center of the telescope axis needs to be in the middle of the dome, not the pier. Get this wrong and the opening of the dome will not remain synchronized with the mount when both are moving. An Alt-Azimuth mount can be placed centrally as its rotational center is the same as the center of the dome. Dome automation requires accurate placement of the telescope and then configuration of the dome drive system to keep the dome slit in line with the telescope axis. Dome control software then synchronizes telescope and dome rotation.

Below showing the Rotation Driver that control and synchronize with the telescope movement.



The Automated Observatory – Roof

With a computer-controlled observatory, other functionality becomes possible. - If, when in use, your telescope protrudes above the roof line as it is imaging, once finish it must be parked before the roof can be closed. This is possible with a computerized roof and it also does not necessarily rely on a PC, you can park manually the telescope and close the roof manually.

Domes do not have this problem as the telescope is always within the ‘roof’. - With a system such as a Talon Roof controller or Dragonfly / AAG Cloudwatcher from Lunatico(this is what I have) , this will close the roof after the mount is parked when an unsafe condition is met. It can do this autonomously as it is only reliant on seeing a contact closure relay activate. It knows the mount is parked because of small magnetic sensors that are placed on the mount axis or at the roof on each end of the roof/ walls. As this system is computerized and has an ASCOM driver, it can also be told to close by the capture software.

 


Full System Control

 

First you will need to plan the type of relays system you need.

The next level of control is to have full remote control of power and automate responses to events. Control of power and the ability to turn things on and off remotely is important.

  • 1.       You could leave everything turned on. However that won’t do a dew heater much good or a camera.                                                                                                                                                 
  • 2.       A PC might eventually crash as operating systems are not designed for 24/7 365 operation.            
  • 3.       Also, in the event of a power cut, things usually default to an off state so if you don’t have a way to turn components on and off, your sunk!

 The simplest way to control power is via a relay that can be computer controlled. Many such relays are available like the Dragonfly from Lunatico. Using such a device allows you to control low voltage and also mains voltage, although the latter should be left alone. Relays can also be pulsed, rather than be maintained. This allows you to mimic actions like the pressing of the power button on a PC.

However, you will need to power the PC, as you cannot leave it all day’s weeks on, so if you use remote desktop, remote file access, or other server software, you may leave your computer on at home or work when you leave the house. This uses more power. Instead, you could remotely power on your PC whenever you need to use it. This takes advantage of Wake-on-LAN. In spite of its name, it’s possible to set up Wake-on-LAN so that you can send “magic packets” that will wake a computer up over the Internet.

Full System Control More advanced relay based systems also allow sensing using a contact closure input and conditional logic. This means they can react to external events. Such events might be loss of mains power, temperature, unsafe weather conditions or even a security event. A modern network enabled relay allows you to access it over the internet directly. You don’t need a computer, even a mobile phone can be used. Opposite is an example of an Astronomy optimised relay, called the Dragonfly, but really any network enabled relay can be used.


Plan

Dragonfly Diagram to connect the relays




Safety

With an unattended, remote system, two criteria must be met. The first is that everything must work! That sounds obvious but if your system is not local and you ‘assume’ it is all working you might be in for a nasty surprise. The second is keeping your system safe from possible damage. This is likely to be caused by either weather or loss of power. There is however the question of how to protect the optics as you won’t be around to remove the cap! A solution for this is a motorized cover, for example a Gemini Snap-Cap. This also incorporates a flat field so calibrations files can be made remotely. True that having a Dome will keep the dust always, but if the location of the observatory is nearby a construction, or sandy grounds, then dusts will crip in.  



Safety Power (Back Up)

Some overseas countries are prone to power cuts. This may because of unreliable supply or working practice. In Spain where I have my observatory for instance, utilities companies will cut power without notice. For reasons such as this, both your system and the observatory roof should be connected to a UPS (Uninterruptable Power Supply), which is basically an inline battery. These come in different sizes but one that offers about 30 minutes of life after a power cut is normally sufficient to get everything shut down safely before the battery goes flat. A simple relay that closes when mains power is lost can notify your system of the problem and let it begin a controlled shut down. If using a Roof controlled by Talon, this will sense a loss of power (and is also on a UPS) and park the mount / close the roof.

 

 


Safety - Weather


Obviously this is another area where action must be taken to protect the telescope system. High winds and rain can very cause many thousands of pounds worth of damage. In addition to conditions likely to cause physical damage, different weather conditions also make it impractical to use a system. For instance, excessive cloud or light. To react to these different states, a specialised weather monitoring system is needed, like the Solo - AAG Cloudwatcher from Lunatico. This will measure light, rain, temperature, humidity and wind speed, and can be monitor through the internet to your mobile. Parameters can be set and when outside of these the system will go ‘unsafe’, both in software and also be triggering a physical relay. One of the most popular weather solutions as mentioned is the ‘Cloudwatcher’ made by Lunatico.

Weather Station




Internet Weather update - AAG Cloudwatcher











Solo Unit receives the weather data and update the computer

 


AAG Cloudwatcher weather data on the computer




Software Scheduling

Software to control the entire systems (cameras, filter wheel, mount, guiding exposure , targeting etc) Applications can also capture mosaics and the most sophisticated can also ‘Re-Task’ the whole schedule based on information received. Therefore if in the middle of a session, news about a new Supernova was released, the schedule could be interrupted and the scope would retarget on the new object. Some common examples of Scheduling Software are; SGP (Sequence Generator Pro) like I have currently, but for more sophisticated software go for Voyager.

Access – Remote Control

So you have all the equipment and the software but how do you use it remotely? What is needed is something called a remote desktop. This allows you to create a virtual desktop on a local PC (or tablet), giving you the same control as if you are in front of the remote PC. Tablets are not ideal because touchscreens can’t really replace all the functionality of a mouse. The simplest way to remote access is to use the remote desktop functionality built into Windows. However this is really intended to be used on an internal network and as such its not ideal for internet based control. Specific, internet compatible remote desktop software is now freely available.

A common choice is Teamviewer ( www.teamviewer.com ) and another is AnyDesk ( www.anydesk.com ). These are free for private use and very easy to setup. They work by connecting the two computers through a central server. One advantage of this is that because the software dials in to a known location, it can be used with Internet Services that use DHCP. Another popular remote control application is Radmin (https://www.radmin.com/). This is a direct peer to peer system which allow it to have less latency. However you really need a fixed (static) IP address at the remote system location. Personally I use both so as to provide a means of back up.

Access – PC Monito

In addition to having remote access to your PC you might also want to consider the way your ‘remote’ PC screen is displayed locally. A PC needs to have a monitor plugged into it in order to load a display driver. You can leave the monitor turned off but it must be connected or you will see nothing. The resolution of that monitor is what you will be able to use remotely. An alternative to leaving a monitor connected is to use a display emulator. These simply plug into the PC. Not only do these simplify your install but they can also be hi-resolution.


DragonFly Manual  :http://lunaticoastro.com/dragonfly/DragonflyUsersManual.pdf





 How to calculate the step size for auto-focusing


Autofocus method
Data 
                                                                                                       At
Bin                                          1x1 and 2x2
Telescope FL                         1400
Pixel size                                9
Read Noise RMS                  10e-
Number of steps                    105000
Resolution per steps              0.083  Micros/step
travel                                      0.35" 8.89mm

CFZ                                                                                                         Critical Focus Zone
Ha  3nm           656.39                                                                                 95    
OIII                   500.17                                                                                72
SII                    671.69                                                                                 97
Red                  650.94                                                                                 94
Green               510.74                                                                                74
Blue                  475.69                                                                                69
Formula
CFZ / Focuser step size    Multiple  by 1.5 and 2                                       

Step Size     95 / 0.083 x 1.5 = 1716

                                                                                                                          Step Size 


                                                                                                Ha       OIII       SII          Red   Green    Blue
(CFZ / Focuser step size) x 1.5 = focuser step size                1,716  1301   1753     1698     1337    1246
(CFZ / Focuser step size) x 2 = focuser step size                   2,289  1734    2337     2265     1783    1662

So, if you are using SGP for example, in the step size you insert the corresponding step size. The 1.5 figures are the lowest  step size and the 2 figure is the highest.
You will need to play round say from 1716 (if you are using Ha filter) to a maximum of 2289.  I tried the 1716 and it was good from first time.

Back again to my observatory


 After almost five months I was able to travel to my observatory in Southern Spain, due to travel restrictions we had to stay within our province here in Spain in the Andalucia region. It was a real pleasure to see that no damage to the equipments, especially after having really stormy weather during this time. After  cleaning the inside of the dome, everything worked fine, just  SGP software subscription that needs update.

My first telescope and programs

 The following are pictures showing my first automated system going back 5 years, it wasn't even on a good location , between block houses!!. But I had to start somewhere and this was just the beginning.















Unpacking new telescope -RH250 MM Officina Stellare

 I received my new telescope around six months ago, but I have had little time to take pictures of the  telescope inside the dome.

But I do have some images unpacking it when it was delivered. 

The telescope is a RH Veloce 250, design which allow a fast F/Ratios with high definition over a wide and flat focal plane. It offers a pinpoint star all over the FOV..

https://www.officinastellare.com/professional-telescopes-prod/rhveloce/rhveloce250.html

https://www.officinastellare.com/os_uploads/files/RH250_WEB.pdf










Building the set-up

 


Coming from the Moonlite 3,5 focuser which replaced the original Stellarvue 130mm focuser, you need to be careful given that latter focuser is shorter by around 130 to 150mm meaning to reach focus you will need to reach 212mm.

Image Train from the focuser
Extension tube 
Stellarvue Flattener
Precises Adapter from Flattener to Baader Filter Holder
Baader Filter Holder
Precises Adapter from Baader to QHY268C camera
QHY268c




Distance from the Flattener to the Sensor should be 79mm






PUTTING IT TO THE TEST

Since I have gotten the camera, my weather has not been cooperating much, I have not managed to get a few images. I will be using the Stellarvue F7 -130mm telescope  in my Bortle 5 skies (approximately 20 average SQM) with an Astronomik L2 UV/IR cut and  5nm H-alpha for the H-alpha image.

In my initial testing the camera did not cool lower then -15C this is my only complaint I had, the cooling was not as good as other cameras I’ve used, I can imagine that living in southern Spain does not help to lower that temperature.

Hopefully, once I complete mapping the night sky using ModelCreator, averaging around sixty sky points location and with a prior Sharpcap Polar Alignment, this will be sufficient to create a proper stable pointing mount.

For automation the entire system I will be using SGP. The mount will be connected via a network cable, the USB3.0 hub is connect from my laptop via a fast USB3.0 8 meters long cable.