Metal-Poor Star Hosting a Sub-Neptune Planet
HD 175607 is an old G6 main-sequence star with a metallicity that is less than 25 percent of the Sun’s. Basically, the metallicity of a star is the fraction of a star’s mass that is comprised of elements heavier than hydrogen and helium. It is known that stars with a higher metallicity are more likely to have Jupiter-like planets compared to stars with a lower metallicity. Such a correlation can be expected as stars with a higher metallicity have more heavy elements, making it more conducive for the formation of Jupiter-like planets. However, such a correlation is not observed for lower-mass planets (i.e. planets with less than ~0.1 times the mass of Jupiter) such as Neptune-mass planets and super-Earths. The occurrence rate of such planets does not depend on the metallicity of the host star.
As part of a search for Neptune-mass planets and super-Earths around a sample of 109 low-metallicity stars with the High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph, Mortier et al. (2015) present the discovery of a sub-Neptune-mass planet around HD 175607. As a result of this discovery, HD 175607 is currently the most metal-poor FGK main-sequence star with a low-mass planet. The planet around HD 175607 is identified as HD 175607b, with the suffix “b” denoting its planetary nature. This sub-Neptune-mass planet has at least 8.98 ± 1.10 times the mass of Earth and it goes around its host star in a slightly eccentric orbit with an orbital period of 29.01 ± 0.02 days.
Mortier et al. (2015), “The HARPS search for southern extra-solar planets. XXXIX. HD175607 b, the most metal-poor G dwarf with an orbiting sub-Neptune”, arXiv:1511.03941 [astro-ph.EP]