Gliese 676

Planetary system

The first planet discovered, b, is a super-jovian first characterised in October 2009. The planet was formally announced in 2011, along with the first recognition of a trend not attributable to the companion star. Even after fitting a planet and a trend, it was noted that the residual velocities were still around 3.4 m/s, significantly larger than the instrumental errors of around 1.7 m/s. This tentatively implied the existence of other bodies in orbit, though nothing more could be said at the time.

The star was a test case for the HARPS-TERRA software for better reduction of data from the HARPS spectrometer in early 2012. Even with significantly lower margins of error on the data, less data was accessible than what was used in 2011. Still, the team reached a very similar conclusion to the previous team with a model of a planet and a trend. The residual velocities were still somewhat excessive, giving more weight to the existence of other bodies in the system, though still no conclusions could be made.

Between the time of the previous analysis and June 2012, the rest of the radial-velocity measurements used in 2011 were made public, allowing them to be reduced using HARPS-TERRA. These were then analysed via a Bayesian probability analysis, which was previously used to discover HD 10180 i and j, which confirmed planet b and made a first characterisation of planet c, which was previously only described as a trend. After the first two signals were introduced, the next most powerful signal was at around 35.5 days, with an analytic false alarm probability of 0.156. Through 10⁴ trials, the false alarm probability was found to be 0.44%, low enough for it to be included as a periodic, planetary signal. With a minimum mass of around 11 Earths, the planet lies at the accepted border between Super-Earths and gaseous, Neptune-like bodies of 10 Earths. After accepting the third signal, a strong peak at 3.6 days became apparent. With a false alarm probability much lower than that of the previously accepted body, it was immediately accepted. With a minimum mass of around 4.5 Earths, it is a small Super-Earth.

As of 2012, this system holds the record for the widest range of masses in a single planetary system, and also shows a hierarchy reminiscent of the solar system, with the gas giants at large distances from the star while the smaller bodies are much closer-in.

In 2016, the true mass of Gliese 676 Ab was measured via astrometry. A 2022 study revised this mass estimate, along with measuring the true mass of Gliese 676 Ac. There are two Super-Jupiter planets: "b" with a period of 1051 days (2.9 years) and a mass of 5.79 M_J, and "c" with a period of 13900 days (38.1 years) and a mass of 13.49 M_J, which is at the borderline between planets and brown dwarfs.