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An eccentric Neptune-mass planet near the inner edge of the BD-11 4672 habitable zone

A new eccentric Neptune-mass planet has been detected by an international team led by Italian researchers within the GAPS (Global Architecture of Planetary Systems) collaboration, using 68 new radial velocity measurements collected with the HARPS-N spectrograph at Telescopio Nazionale Galileo in conjunction with 43 literature HARPS measurements in the Southern hemisphere.

The newly detected planet BD-11 4672 c has a minimum mass of 15 Earth masses and orbits a K dwarf star with a period of 74 days; the host star was already known to host a planet with a minimum mass of 0.65 Jupiter masses and an orbital period of 4.5 years. The search for planetary systems characterized by such architecture, hosting both an outer giant and an inner lower-mass planet, is key to further our understanding of formation and migration models. Indeed, different theoretical models predict, depending on the formation mechanism involved, that the presence of giant planets on outer orbits can promote or impede the formation and survival of lower-mass planets in the inner regions of the system.

A notable feature of the new planet BD-11 4672 c is its high orbital eccentricity of 0.4, making it one of the most eccentric Neptune-mass exoplanets at intermediate orbital periods known so far. Additionally, the outer planet b receives a stellar flux similar to that of Jupiter, while new planet c receives a flux comparable to that of Venus, making this planetary system an interesting analogue of the Solar System around a K-dwarf star. Also, BD-11 4672 c orbits near the inner edge of the circumstellar habitable zone, and dynamical stability tests indicate that its mass and significant eccentricity negatively affect the presence of possible yet-undetected temperate exoplanets, allowing stable orbits only for low-eccentricity Neptune-mass bodies on the outer portion of the habitable zone.

To detect BD-11 4672 c the authors applied a careful correction of the spurious 1-y periodicity. Such a subtle effect is due to the motion of Earth around the Sun that makes the spectral lines cross block stitchings of the CCD. GAPS used this correction to detect HD 164922 d, too.

The architecture of the BD-11 4672 planetary system compared with that of the Solar System, with the conservative and optimistic span of the circumstellar habitable zone shown as dark and light green areas.

Figure : The architecture of the BD-11 4672 planetary system compared with that of the Solar System, with the conservative and optimistic span of the circumstellar habitable zone shown as dark and light green areas.

Link to the papers:
BD-11 4672: https://ui.adsabs.harvard.edu/abs/2020arXiv200614393B/abstract
HD 164922 https://ui.adsabs.harvard.edu/abs/2020arXiv200503368B/abstract