GP Comae Berenices

GP Comae Berenices, abbreviated to GP Com and also known as G 61-29, is a star system composed of a white dwarf orbited by a planetary mass object, likely the highly eroded core of another white dwarf star.^[7] The white dwarf is slowly accreting material from its satellite at a rate of (3.5±0.5)×10⁻¹¹ M_☉/year and was proven^[8] to be a low-activity AM CVn star.^[7][5] The star system is showing signs of a high abundance of ionized nitrogen from the accretion disk around the primary.^[9]

GP Comae Berenices

An ultraviolet band light curve for GP Comae Berenices, adapted from Smak (1975).[1] The error bar shown on the leftmost point applies to all points.
Observation data Epoch J2000      Equinox J2000
Constellation	Coma Berenices
Right ascension	13h 05m 42.401s[2]
Declination	+18° 01′ 03.76″[2]
Apparent magnitude (V)	15.69[3]
Characteristics
Evolutionary stage	White dwarf
Spectral type	DBe[4]
Apparent magnitude (g)	15.929[2]
Variable type	AM CVn
Astrometry
Proper motion (μ)	RA: −344.92±0.06 mas/yr[2] Dec.: 34.85±0.06 mas/yr[2]
Parallax (π)	13.7306±0.0452 mas[2]
Distance	237.5 ± 0.8 ly (72.8 ± 0.2 pc)
Details[5]
Mass	0.59±0.09 M☉
Temperature	14,800±500 K
Other designations
Gaia DR2 3938156295111047680, G 61-29, LTT 18284, WD 1303+18, 2MASS J13054243+1801039[6]
Database references
SIMBAD	data

In 1971, Brian Warner discovered that the star, then known as G61-29, is a variable star.^[10][11] it was given its variable star designation, GP Comae Berenices, in 1975.^[12]

Planetary system

The material emitted from the planetary mass companion is mostly helium, with a molar ratio of nitrogen up to 1.7%, very low neon levels and other elements not detectable at all.^[13] Approximately half of the luminosity of the system comes from the accretion disk.^[5] The planetary object is suspected to contain a strange quark matter core due to its unusually high density, which must be above 187.5 g/cm³ to prevent tidal disruption; the theoretical bound for planets composed solely of ordinary matter is on the order of 30 g/cm³. The object's orbit is expected to decay within 100 million years due to gravitational wave emission.^[14]

The GP Com planetary system^[5]

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (seconds)	Eccentricity	Inclination	Radius
b	10.5±1.5 MJ	0.0014[14]	2794	0	59.5±14.5[9]°	≤ 0.420±0.020 RJ