HD 10180

HD 10180 is a Sun-like star in the southern constellation Hydrus that is notable for its large planetary system. Since its discovery, at least six exoplanets have been observed orbiting it, and some studies have proposed up to nine potential planets, which would make it potentially the largest of all known planetary systems, including the Solar System.

Characteristics Based upon parallax measurements, it is located at a distance of 127 light-years (39 parsecs) from Earth. The apparent visual magnitude of this star is 7.33, which is too faint to be viewed with the naked eye although it can be readily observed with a small telescope. At a declination of −60°, this star cannot be seen at latitudes north of the tropics. HD 10180 is a G-type main-sequence star, and thus generates energy at its core through the thermonuclear fusion of hydrogen. The mass of this star is estimated as 6% greater than the Sun's mass, it has a radius of 120% that of the Sun, and is radiating 149% of the Sun's luminosity. The effective temperature of the star's chromosphere is 5,911 K, giving it a yellow-hued glow like the Sun. HD 10180 has a 20% higher abundance of elements other than hydrogen/helium compared to the Sun. With an estimated age of 7.3 billion years, it is a stable star with no significant magnetic activity. The estimated period of rotation is about 24 days. A survey in 2015 ruled out the existence of any stellar companions at projected distances from 13 to 324 astronomical units.

Planetary system On August 24, 2010, a research team led by Christophe Lovis of the University of Geneva announced that the star has at least five planets, and possibly as many as seven. The planets were detected using the HARPS spectrograph, in conjunction with the ESO's 3.6 m telescope at La Silla Observatory in Chile, using Doppler spectroscopy.

On April 5, 2012, astronomer Mikko Tuomi of the University of Hertfordshire submitted a paper to Astronomy and Astrophysics approved for publishing on April 6, 2012 that proposed a nine-planet model for the system. Re-analysing the data using Bayesian probability analysis, previously known planets' parameters were revised and further evidence was found for the innermost planet (b) as well as evidence of two additional planets (i and j). Subsequent studies since 2014 have found that a six-planet model is the best fit to the data. The system is not known to be a transiting planetary system, and as such planets are unlikely to be detected or verified by the transit method.

In 2017, an orbital simulation showed that the formation of dynamically stable families of comets in the HD 10180 system is unlikely. The identified reason for the instability of cometary orbits was the location of the most massive planet HD 10180 h in the outermost orbit.

Orbital arrangement

The system contains six planets with minimum masses from 12 to 46 times Earth's (ranging in mass from roughly Uranus to sub-Saturn) at orbital radii of 0.06, 0.13, 0.27, 0.49, 1.43 and 3.38 AU. In the Solar System this set of orbits would fit within the main asteroid belt. There are no planets known to be in mean-motion resonances, although the system has a number of near resonances including 3c:2i:1d and 3e:2j:1f. The approximate ratios of periods of adjacent orbits are (proceeding outward): 1:5, 1:3, 1:3, 2:5, 1:5, 3:11. Since the inclination of the planets' orbits is unknown, only minimum planetary masses can presently be obtained. Dynamical simulations suggest that the system cannot be stable if the true masses of the planets exceed the minimum masses by a factor of greater than three (corresponding to an inclination of less than 20°, where 90° is edge-on). A 2020 study set upper limits on the masses of the confirmed planets based on non-detections in Gaia astrometry: planet c is <8.626 MJ, planet d is <10.37 MJ, planet e is <20.44 MJ, planet f is <14.03 MJ, planet g is <10.62 MJ, and planet h is <22.63 MJ. While some of these upper limits are in the mass range of brown dwarfs, it is likely that the true masses are significantly smaller.

Planets

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