HD 181720 is

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

with an orbiting

substellar companion A substellar object, sometimes called a substar, is an astronomical object, the mass of which is smaller than the smallest mass at which hydrogen fusion can be sustained (approximately 0.08 solar masses). This definition includes brown dwarfs and f ...

in the southern

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Sagittarius Sagittarius ( ) may refer to: *Sagittarius (constellation) *Sagittarius (astrology), a sign of the Zodiac * Sagittarius of Gap, a 6th century bishop *Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy Ships *'' S ...

. It is located at a distance of 196

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from the Sun based on

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements, but is drifting closer with a

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

of −45.4 km/s. The star has an absolute magnitude of 4.10, but at that distance it has an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

of 7.84, which is too faint to be seen with the naked eye. It shows a high

proper motion Proper motion is the astrometric measure of changes in the apparent places of stars or other celestial objects as they move relative to the center of mass of the Solar System. It is measured relative to the distant stars or a stable referenc ...

, traversing the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

at an

angular rate In physics, angular frequency (symbol ''ω''), also called angular speed and angular rate, is a scalar measure of the angle rate (the angle per unit time) or the temporal rate of change of the phase argument of a sinusoidal waveform or sine f ...

of . The

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

of HD 181720 presents as an ordinary

G-type main-sequence star A G-type main-sequence star (spectral type: G-V), also often, and imprecisely, called a yellow dwarf, or G star, is a main sequence, main-sequence star (luminosity class V) of stellar classification, spectral type G. Such a star has about 0.9 to ...

with a

stellar classification In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

of G1V. It is an older star with an estimated age of roughly 12 billion years and a minimal level of

magnetic activity A stellar magnetic field is a magnetic field generated by the motion of conductive Plasma (physics), plasma inside a star. This motion is created through convection, which is a form of energy transport involving the physical movement of material ...

in its

chromosphere A chromosphere ("sphere of color", from the Ancient Greek words χρῶμα (''khrôma'') 'color' and σφαῖρα (''sphaîra'') 'sphere') is the second layer of a Stellar atmosphere, star's atmosphere, located above the photosphere and below t ...

. The star has 87% of the mass of the Sun but the radius is now 42% larger than the Sun's. Its

metallicity In astronomy, metallicity is the Abundance of the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

content, as measured by the abundance of iron, is three-tenths as much as in the Sun. HD 181720 is radiating more than double the luminosity of the Sun from its

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

at an

effective temperature The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation. Effective temperature is often used as an estimate of a body's surface temperature ...

of 5,781 K. The star HD 181720 is named Sika. The name was selected in the

NameExoWorlds NameExoWorlds (also known as IAU NameExoWorlds) were various projects managed by the International Astronomical Union (I.A.U.) to encourage names to be submitted for astronomical objects, notably exoplanets. The accepted names would later be cons ...

campaign by

Ghana Ghana, officially the Republic of Ghana, is a country in West Africa. It is situated along the Gulf of Guinea and the Atlantic Ocean to the south, and shares borders with Côte d’Ivoire to the west, Burkina Faso to the north, and Togo to t ...

, during the 100th anniversary of the

IAU The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

. Sika means

gold Gold is a chemical element; it has chemical symbol Au (from Latin ) and atomic number 79. In its pure form, it is a brightness, bright, slightly orange-yellow, dense, soft, malleable, and ductile metal. Chemically, gold is a transition metal ...

in the

Ewe language Ewe (''Eʋe'' or ''Eʋegbe'' ) is a language spoken by approximately 5 million people in West Africa, mainly in Ghana and Togo. Ewe is part of a group of related languages commonly called the Gbe languages. The other major Gbe language is F ...

Planetary system

In 2009, a

gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

planet was found in orbit around the star. It was named "Toge" in 2019. The planets around such metal-poor stars are rare (only two known similar cases are HD 111232 and HD 22781, as of 2019). The upper bound on the mass of this component is 32 times the mass of Jupiter, which would place it in the

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

range.

References

{{DEFAULTSORT:HD 181720 G-type main-sequence stars Planetary systems with one confirmed planet Sagittarius (constellation) 181720 095262 CD-33 14164 J19225298-3255079 Sika

Planetary system

See also

References