star
A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...
in the constellation 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 ...
. It is a
K-type main-sequence star
A K-type main-sequence star, also referred to as a K-type dwarf or an orange dwarf, is a main-sequence (hydrogen-burning) star of spectral type K and luminosity class V. These stars are intermediate in size between red M-type main-sequence stars ...
with an
apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of 10.250. It is 97 light-years (30 parsecs) away, based on
parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby objects ...
.
The rotation period of the star cannot be determined as in 2020, and can be either around 15 or 30 days, depending on interpretation of available data.
Planetary system
It has 3 transiting planets seen by the ''
Kepler
Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...
'' space observatory in their K2 survey. As of October 2017, it is the closest star discovered to have transiting
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 found by either the Kepler or K2 missions. The planets (b, c, d) have radii of 1.62, 1.27, and 2.09 times that of the Earth, and periods of 1.209, 3.648, and 6.201 days (ratios 1:3:5). Because of its close distance the system is considered an excellent target for studying atmospherics of exoplanets.
In late 2017, the masses of all three planets were determined using the Planet Finder Spectrograph on the Magellan II Telescope. Planet b was found to be very iron-rich, planet c appears to be mainly rocky, and planet d is a typical volatile-rich planet. GJ 9827b is noted as being one of the densest planets yet found, with its mass containing about ≥50% iron.
More precise radial velocity measurements released in late February 2018 revealed that all three planets have a lower density than Earth and have some amount of volatiles in their compositions. GJ 9827b and c are mainly rocky (containing less than 1% mass fraction of water, and negligible helium and hydrogen) with very thin volatile envelopes, while GJ 9827d is more akin to a Mini-Neptune. The loss of primordial atmosphere was indirectly confirmed in 2020 as no helium was detected at Gliese 9827 d though. With a mass of about 1.5 , GJ 9827c is one of the least massive planets detected with radial velocity. The atmospheres for GJ 9827b and GJ 9827d were not detected at all in 2021 observations.