Groombridge 34
A nearby red dwarf binary with flare variability and exoplanets.
Groombridge 34 is a binary star system found in the northern constellation Andromeda. It appears as the 34th entry in *A Catalogue of Circumpolar Stars*, which was released after the death of its author, British astronomer Stephen Groombridge, in 1838. According to parallax data from the Gaia spacecraft, the system lies roughly 11.6 light-years from the Sun, making it one of the closest stellar neighbors to our Solar System. Both stars are faint red dwarfs, invisible without a telescope, and their brightness fluctuates unpredictably due to flares, which has led to them receiving variable star designations. The two stars orbit a common center of mass in a highly elongated path, separated by about 93 astronomical units and completing one orbit every 1,065 years. The system moves across the sky at a relatively high rate of 2.9 arcseconds per year and is receding from the Solar System at 11.6 kilometers per second. It reached its closest approach to the Sun roughly 15,000 years ago, coming within 11 light-years. The larger and brighter star, Groombridge 34 A, is known as GX Andromedae. It is a main-sequence red dwarf with a spectral type of M1.4, and its brightness varies due to stellar flares. Gaia observations indicate a rotation period of 44 days and a magnetic activity cycle lasting about 9 years. The smaller companion, GQ Andromedae, is also a flare-prone red dwarf on the main sequence. It has a spectral type of M4.1, meaning its surface temperature is lower than that of the primary. In August 2014, a planet was reported orbiting Groombridge 34 A, based on radial velocity data from the Eta-Earth Survey using the HIRES instrument at Keck Observatory. At the time, it was the sixth-closest known exoplanet. Later, using the CARMENES spectrograph along with HARPS and HIRES data, researchers could not confirm that planet, but they proposed another one, designated Groombridge 34 Ac (or GJ 15 Ac). Subsequent HIRES observations covering a longer time span resolved the discrepancy, recovering both planets. The inner planet, Groombridge 34 Ab, has a minimum mass of 3.03 Earth masses and an orbital period of 11.4 days. The outer planet, Groombridge 34 Ac, has a minimum mass of 36 Earth masses and an orbital period of about 7,600 days. As of 2018, this system hosts the longest-period known Neptune-mass exoplanet.
- Distance
- 11.6 light-years (3.6 parsecs)
- Proper motion
- 2.9 arc seconds per year
- Radial velocity
- 11.6 km/s (moving away)
- Perihelion
- ~15,000 years ago at 11 ly (3.5 pc)
- Orbital period of binary
- ~1,065 years
- Separation of binary
- ~93 AU
- Eccentricity
- fairly eccentric
Lore & Background
The system was cataloged by Stephen Groombridge in his posthumously published 1838 catalogue. Both components are red dwarf main-sequence stars: the brighter, GX Andromedae, has spectral type M1.4, while the fainter GQ Andromedae is type M4.1. They orbit a common barycenter in a fairly eccentric orbit with a separation of about 93 AU and a period of around 1,065 years. The system has a high proper motion of 2.9 arc seconds per year and is moving away from the Sun at 11.6 km/s, having reached its closest approach about 15,000 years ago at 11 light-years. Both stars exhibit flare-driven variability. GX Andromedae has a rotation period of 44 days and a magnetic activity cycle of roughly 9 years, as observed by Gaia. GQ Andromedae, being cooler and less massive, also undergoes flare events. In August 2014, a planet around Groombridge 34 A was reported from radial velocity data by the Eta-Earth Survey using HIRES at Keck Observatory, making it the sixth-nearest-known exoplanet at the time. Later, CARMENES, HARPS, and HIRES data failed to detect that planet but proposed another, Groombridge 34 Ac. New HIRES observations covering a longer span reconciled the discrepancy, recovering both planets: Groombridge 34 Ab has a minimum mass of 3.03 Earth masses and an orbital period of 11.4 days, while Groombridge 34 Ac has a minimum mass of 36 Earth masses and an orbital period of about 7,600 days. As of 2018, this system hosts the longest-period known Neptune-mass exoplanet.
Reader's Guide
Groombridge 34 is notable as one of the nearest star systems to the Sun, lying only 11.6 light-years away. Its binary nature, with both components being flare-active red dwarfs, makes it a valuable laboratory for studying stellar activity and magnetic cycles in low-mass stars. The system's variable star designations—GX And for the primary and GQ And for the secondary—underscore its role in the study of flare variability. The discovery and subsequent confirmation of two exoplanets around the primary star, including a long-period Neptune-mass world, highlight the system's importance in exoplanet science. The initial detection of Groombridge 34 Ab in 2014 as the sixth-nearest-known exoplanet, followed by a period of uncertainty and eventual reconciliation through extended observations, illustrates the iterative nature of radial-velocity planet searches. The system's high proper motion and past perihelion passage also provide context for its kinematics and history within the solar neighborhood.
Did You Know?
- Both stars in the system are flare stars, designated GX Andromedae and GQ Andromedae.
- The system hosts a Neptune-mass exoplanet with the longest known orbital period as of 2018.
- The binary stars orbit each other with a period of about 1,065 years and a separation of roughly 93 AU.
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