HD 3167 is a single, orange-hued star in the zodiac
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 origins of the e ...
of
Pisces
Pisces may refer to:
* Pisces, an obsolete (because of land vertebrates) taxonomic superclass including all fish
*Pisces (astrology), an astrological sign
*Pisces (constellation), a constellation
**Pisces Overdensity, an overdensity of stars in t ...
that hosts a
system
A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment (systems), environment, is described by its boundaries, ...
with three
exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
s. The star is too faint to be seen with the naked eye, having an apparent visual magnitude of 8.97. The distance to HD 3167 can be determined from its annual parallax shift of as measured by the
Gaia
In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthenog ...
light year
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...
s. It has a relatively high
proper motion
Proper motion is the astrometric measure of the observed changes in the apparent places of stars or other celestial objects in the sky, as seen from the center of mass of the Solar System, compared to the abstract background of the more dista ...
, 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 the rate of per year. Since it was first photographed during the Palomar observatory sky survey in 1953, it had moved over by 2017. The star is moving away from the Earth with an average heliocentric
radial velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
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 grati ...
of K0 V and no significant variability. The star has 86% of the
mass of the Sun
The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
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 5,261 K. The spin of the star displays a relatively low projected rotational velocity of around 1.7 km/s. It has a near solar
metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...
– a term astronomers use for the proportion of elements other than hydrogen and helium in a stellar atmosphere.
In 2019, a group of astronomers first reported that the orbits of the detected exoplanets hosted by the star are oddly unusual: two planets (HD 3167 c; HD 3167 d) revolve around the star on polar orbits, i.e. orbits that pass over the ''poles'' of the star. Later, in October 2021, the third planet (HD 3167 b) was found to orbit around the ''equator'' of the star instead, while confirming the other planets' orbital inclinations from the 2019 study.
Planetary system
In 2016, data collected during the extended K2 mission of the Kepler space telescope was used to identify two
transiting exoplanet
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of ...
candidates orbiting this star, designated HD 3167 b and HD 3167 c. This made it one of the closest and brightest such multi-transiting stars known at the time. The lack of chromospheric activity makes it ideal for the precise
radial velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the temporal rate of change, rate of change of the distance or Slant range, range between the two points. It is e ...
(RV) measurements needed to estimate the masses of its planets. Follow-up RV observations showed additional perturbation signals beyond the two planets already identified. This led to the discovery in 2017 of a third, non-transiting planet, designated HD 3167 d.
The close-orbiting body HD 3167 b has a mass of and radius . It most likely has had its atmosphere stripped away by the host star, leaving a rocky planet with about 15% iron by mass. HD 3167 b is orbiting HD 3167 with a period of , an orbital inclination of 83.4°, and an assumed
orbital eccentricity
In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values betwee ...
of zero – a circular orbit. The semimajor axis of its orbit is , or just four times the star's radius.
The second planet, HD 3167 c, has an orbital period of 29.8454 days and an eccentricity of less than 0.267. The semimajor axis is . It has and , giving it a low bulk density of . This suggests either a mini-
Neptune
Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
with a gaseous envelope consisting mainly of hydrogen and helium, or a planet consisting of mostly water. In 2020,
transmission spectroscopy
Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating f ...
measurements of its atmosphere strongly favored a model with a high (>700 × Solar) metallicity atmosphere, due to discovered molecular absorption bands which cannot be attributed to hydrogen or helium. The incident
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
from the host star is around 16 times the amount the Earth receives from the Sun, and it is less susceptible to atmospheric stripping than HD 3167 b.
The orbital inclination of HD 3167 d is inclined at least 1.3° away from the orbital planes of the other two exoplanets. Its orbit is expected to remain stable for periods longer than 100 million years only if this inclination is less than 40°. It has an orbital period of , placing it in between the other two orbits, and shows a minimum mass of . The true mass is most likely less than Neptune.
A fourth planet, HD 3167 e, first suggested as a candidate in 2019, was discovered in 2022 by the radial-velocity method.