HD 10180
HD 10180, also designated 2MASS J01375356-6030414, is a Sun-like star in the southern constellation Hydrus that is notable for its large planetary system. Since its discovery, at least six planets, and possibly as many as nine, have been observed orbiting it, making 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 about 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 G1V-type 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 the 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). 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 65 times Earth's (ranging in mass from roughly Uranus to Saturn) at orbital radii of 0.06, 0.13, 0.27, 0.49, 1.42 and 3.4 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 it 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). Planets. HD 10180 b is a possible Earth-sized planet (minimum mass 1.4 times Earth's) located at 0.02 AU. Its orbital radius was originally estimated to have a near-circular orbit at a distance of 0.02225 ± 0.00035 AU (closer than Mercury, about one-seventh the distance and correspondingly hotter), taking 1.1 days to complete a full orbit. Planet b was confirmed in 2012 with a slightly smaller orbital radius and a more eccentric orbit. The false detection probability was initially 1.4%; its probability  was improved by Mikko Tuomi in 2012, but it was not confirmed by Kane in 2014. HD 10180 c, with a minimum mass comparable to that of Uranus, is a hot Neptune. Dynamical simulations suggest that if the mass gradient was any more than a factor of two, the system would not be stable. Its orbital period and eccentricity were originally estimated at 5.75979 ± 0.00062 and 0.045 ± 0.026 respectively; however, these were revised in 2012 in favour of a more eccentric orbit. The false detection probability is less than 0.1%. HD 10180 i is a possible but unconfirmed  hot super-Earth claimed by Mikko Tuomi in 2012. HD 10180 d is a hot Neptune. Its mass was initially estimated at >11.75 ± 0.65 (smaller than Uranus) and on a slightly eccentric orbit; however, this was re-estimated with a larger mass and less eccentric orbit in 2012. HD 10180 e was initially thought to be a hot Neptune with about twice the mass of Neptune. Its estimated orbital distance and eccentricity were downscaled in 2012. The false detection probability is less than 0.1%. However, a later analysis indicated the planet's true mass, which depends greatly on  its orbital inclination and only the lower-limit of that has been calculated to any degree of certainty, could in theory be much higher at 20 times Jupiter’s mass and so there is a small chance it could be a brown dwarf instead of a planet. HD 10180 j is a possible but unconfirmed hot super-Earth or gas dwarf claimed by Mikko Tuomi in 2012. HD 10180 f is a hot Neptune and of similar in mass to HD 10180 e. At orbital distance of 0.49AU and eccentricity of 0.13, its tight and wild orbit is analogous to  that of Mercury with a similar black-body-temperature range though with its immense mass, any greenhouse effect caused by an atmosphere would give it searing Venus-like or greater temperatures. Estimated orbital distance and eccentricity were downscaled slightly in 2012. The false detection probability is less than 0.1%. A later analysis indicated the planet's true mass, depending on its orbital inclination of which only the lower-limit has been calculated with any certainty, could in theory be much higher at 14 times Jupiter’s mass and so there is a small chance it could actually be a brown  dwarf. HD 10180 g is a giant planet with a mass larger than Neptune's. It has a significantly eccentric orbit at 1.4 AU and either crosses the system's predicted habitable zone or lies within it though it does not fit the current models for planetary habitability due to its large mass (24 times Earth). If it is a gas giant, it is likely of . There is a possibility that a natural satellite with sufficient atmospheric pressure could have liquid water on its surface. Its estimated orbital distance and eccentricity were downscaled in 2012 but remains in the habitable  zone. The false detection probability is less than 0.1%. HD 10180 h is the largest and outermost known planet in the system. Originally, it was thought to be a Saturn-sized giant planet with a minimum mass 65 times that of Earth. Orbiting at 3.4 AU, a distance comparable to the distance of the outer part of the asteroid belt from the Sun and as such it is likely a planet. The spurious detection probability is 0.6%. A later analysis indicated the planet's true mass, depending on its orbital inclination of which only the lower-limit has been estimated  to any degree of certainty, could in theory be much higher at 22 times Jupiter’s mass and so there is a possibility that it is massive enough to be a brown dwarf.