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GJ 876

Title: Can GJ 876 host four planets in resonance?
Authors: Enrico Gerlach, Nader Haghighipour

Prior to the detection of its outermost Uranus-mass object, it had been suggested that GJ 876 could host an Earth-sized planet in a 15-day orbit. Observation, however, did not support this idea, but instead revealed evidence for the existence of a larger body in a ~125-day orbit, near a three-body resonance with the two giant planets of this system. In this paper, we present a detailed analysis of the dynamics of the four-planet system of GJ 876, and examine the possibility of the existence of other planetary objects interior to its outermost body. We have developed a numerical scheme that enables us to search the orbital parameter-space very effectively and, in a short time, identify regions where an object may be stable. We present details of this integration method and discuss its application to the GJ 876 four-planet system. The results of our initial analysis suggested possible stable orbits at regions exterior to the orbit of the outermost planet and also indicated that an island of stability may exist in and around the 15-day orbit. However, examining the long-term stability of an object in that region by direct integration revealed that the 15-day orbit becomes unstable and that the system of GJ 876 is most likely dynamically full. We present the results of our study and discuss their implications for the formation and final orbital architecture of this system.

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Posts: 131433
Smallest Exoplanet

The smallest planet circling a main sequence star has been found.

High precision, high cadence radial velocity monitoring over the past 8 years at the W. M. Keck Observatory reveals evidence for a third planet orbiting the nearby (4.69 pc) dM4 star GJ 876 (HIP 113020).

The residuals of three-body Newtonian fits, which include GJ 876 and Jupiter mass companions b and c, show significant power at a periodicity of 1.9379 days. Self-consistently fitting the radial velocity data with a model that includes an additional body with this period significantly improves the quality of the fit.
These four-body (three-planet) Newtonian fits find that the minimum mass of companion ''d'' is m sin i = 5.89 ± 0.54 Earth masses and that its orbital period is 1.93776 (± 0.00007) days.
Assuming coplanar orbits, an inclination of the GJ 876 planetary system to the plane of the sky of ~ 50 degrees gives the best fit. This inclination yields a mass for companion d of m = 7.53 ± 0.70 Earth masses, making it by far the lowest mass companion yet found around a main sequence star other than our Sun.
Precise photometric observations at Fairborn Observatory confirm low-level brightness variability in GJ 876 and provide the first explicit determination of the star's 96.7-day rotation period. Even higher precision short-term photometric measurements obtained at Las Campanas imply that planet d does not transit GJ 876.

Position(2000): RA 22 53 16.73 Dec -14 15 49.3
Image of GJ 876

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The corresponding semi-major axis is 0.021 AU, making it clearly the smallest of any planet found in Doppler surveys. Note that this is 10 stellar radii, roughly coincident with the number of stellar radii separating GJ 882 (51 Pegasi) b from its host star. Planet d is also probably closer to its star in an absolute sense than are any of the planets found by transit.
A significantly better fit to the data is obtained by assuming that the normal to the three planetsí orbits is inclined
to the line of sight by 50° than by assuming this inclination to be 90° .
For this 50° fit, the actual mass of the inner planet is 7.53 ± 0.70M.

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