M51-ULS-1

M51-ULS-1

Location and artist depiction of M51-ULS-1
Observation data Epoch J2000.0       Equinox J2000.0
Constellation	Canes Venatici
Right ascension	13h 29m 43.31s
Declination	+47° 11′ 34.8″
Apparent magnitude  (V)	24.01
Characteristics
A
Evolutionary stage	Blue supergiant
Spectral type	B2-8la
B
Evolutionary stage	Black hole or neutron star
Astrometry
Distance	28 million  ly
Orbit
Primary	M51-ULS-1 A
Companion	M51-ULS-1 B
Semi-major axis (a)	50 R☉ (<3 AU)
Details
A
Mass	20  M☉
Radius	<25  R☉
Luminosity	260,010  L☉
Temperature	26,068  K
Age	4-16  Myr
B
Mass	1.4-10  M☉
Radius	10 [a] -30 [b]   km
Age	4-16  Myr
Other designations
RX J132943+47115, IXO 79, CXOU J132943.3+471135
Database references
SIMBAD	data

M51-ULS-1, also known as RX J132943+47115, is a high-mass X-ray binary (HMXB) system in the constellation of Canes Venatici. ^{[1] [2] [3]} The binary system is located in the Whirlpool Galaxy, a nearby spiral galaxy some 28 million light years (or 8.6 million parsecs) away. ^{[1] [2]} The binary system has an apparent visual magnitude of 24.01. ^[1] The binary system was discovered in 1995 in a ROSAT survey of X-ray sources in the Whirlpool Galaxy and NGC 5195. ^[4]

Characteristics

M51-ULS-1 is a high-mass X-ray binary system in the Whirlpool Galaxy consisting of two components, a blue supergiant, and a compact object. ^{[2] [3] [5]} The binary system has an predicted age between 4 and 16 million years old; it was also found that the system is probably no older than 100 million years old. ^[3] The compact object orbits the more massive blue supergiant star in a orbit with a semi-major axis of 50 R_☉, and it was found that the semi-major axis of the binary is no larger than 3 astronomical units. ^[3]

The primary star, M51-ULS-1 A, is a massive, luminous early to late blue supergiant star. ^{[2] [3] [6]} The primary star has a spectral type of B2-8la discovered using Hubble Space Telescope photometry. ^[3] The primary star has a mass of about 20 M_☉. ^[2] Because of the star's mass it is predicted that the primary star might undergo a hydrogen-poor supernovae in the future. ^[3] The primary star has an estimated size of <25 R_☉, ^{[3] [6]} based on a luminosity of ~260,000 L_☉ ^[c] and a derived effective temperature of approximately 26,000 K. ^{[3] [d]}

The secondary object, M51-ULS-1 B, is a stellar remnant and is either a black hole or neutron star. ^{[2] [3] [5]} If the secondary object is a black hole, it would have a mass of 10 M_☉; classifying the secondary object as a stellar-mass black hole. ^[3] If it is a black hole, using its mass it would have a Schwarzschild radius of roughly 30 kilometers. ^[3] If the secondary object is a neutron star, it would have mass of 1.4 M_☉. ^[3] It is known that the secondary object is accreting 10^-6M_☉ worth of material every year. ^[3]

Planetary system

In September 2020, a candidate exoplanet named M51-ULS-1 b was discovered in the system using the Chandra X-ray Observatory. ^{[2] [3] [5]} The putative exoplanet is mainly notable for being the first extragalactic planet candidate with strong evidence. ^{[2] [3] [5]} The potential exoplanet was detected by the eclipses of the X-ray source (XRS); ^[3] i. e. M51-ULS-1 B, the compact object orbiting the primary star. The candidate exoplanet is likely slightly smaller than the planet Saturn. ^{[3] [7]} The unconfirmed exoplanet orbits both of the objects in the system in about 70 years, and has a semi-major axis of 45 AU. ^{[2] [3] [5]} It was suggested that the planet could be a white dwarf, however this was ruled out because of the fact it would cause a lensing event and not a dip in flux. ^[3] It was also proposed that it may be a cloud of gas, however the discoverers found this as an unlikely explanation. ^[3] Due to M51-ULS-1 b orbiting a potential black hole it may be classified as a blanet.

The M51-ULS-1 planetary system ^{[3] [7]}

Companion (in order from star)	Mass	Semimajor axis (AU)	Orbital period (years)	Eccentricity	Inclination	Radius
b (unconfirmed)	—	45	70	—	—	0.7  RJ

See also

• PA-99-N2, another system containing a potential extragalactic planet.
• M33 X-7, another extragalactic high-mass X-ray binary.
• Extragalactic planet
• Circumbinary planet
• Whirlpool Galaxy

Notes

1. Assumed radius if the secondary object is a neutron star
2. Schwarzschild radius if the secondary object is a black hole
3. 10³⁹ erg/s = 260,010.4004 L_☉
4. Applying the Stefan–Boltzmann law with a nominal solar effective temperature of 5,772  K:

   ${\displaystyle {\sqrt {{\biggl (}{\frac {5,772}{26,068}}{\biggr )}^{4}\cdot {260,010}}}=25\ R_{\odot }}$.

References

1. "RX J132943+47115". SIMBAD . Centre de données astronomiques de Strasbourg . Retrieved 2026-01-25.
2. "First Evidence of a Planet Identified Beyond Our Galaxy". NASA. Retrieved 2026-01-25.
3. Di Stefano, R. (2020). "M51-ULS-1b: The First Candidate for a Planet in an External Galaxy". NASA Ads. arXiv: 2009.08987 . Retrieved 2026-01-25.
4. Ehle, M.; Pietsch, W.; Beck, R. (1995). "ROSAT high-resolution X-ray observations of the galaxies M51 and NGC5195". Astronomy and Astrophysics. 295: 289. Bibcode:1995A&A...295..289E.}
5. "Chandra Sees Evidence for Possible Planet in Another Galaxy". Chandra X-ray Observatory. Retrieved 2026-01-25.
6. Terashima, Yuichi; Inoue, Hirohiko; Wilson, Andrew S. (2006). "Hubble Space Telescope Identification of the Optical Counterparts of Ultraluminous X-Ray Sources in M51". The Astrophysical Journal. 645 (1): 264. arXiv: astro-ph/0603528 . Bibcode:2006ApJ...645..264T. doi:10.1086/504251.
7. "EPE Results for M51-ULS-1 b". Extrasolar Planets Encyclopaedia. Retrieved 2026-01-25.