EBLM J0555-57

EBLM J0555-57 is a triple star system approximately 670 light-years from Earth. The system's discovery was released on July 12, 2017. EBLM J0555-57Ab, the smallest star in the system, orbits its primary star with a period of 7.8 days, and currently is the smallest known star with a mass sufficient to enable the fusion of hydrogen in its core.

System
EBLM J0555-57, also known as CD&minus;57 1311, is a triple star system in the constellation Pictor, which contains a visual binary system consisting of two sun-like stars separated by 2.5": EBLM J0555-57Aa, a magnitude 9.98 spectral type F8 star, and EBLM J0555-57B, a magnitude 10.76 star. No orbital motion has been detected but they have almost identical radial velocities and are assumed to be gravitationally bound.

Component A of the system is itself an eclipsing binary (EBLM J0555-57Ab orbiting EBLM J0555-57Aa). Eclipses, also known as transits in the context of planetary searches, have been detected in the near infrared, with brightness drops of 0.05% during the eclipse. The shape and duration of the transits allow the radii of the two stars to be determined. A full solution of the orbit gives a period of 7 days and 18 hours, with a low eccentricity of 0.09, an almost edge-on inclination of 89.84°, and a semi-major axis of 0.08 AU.

EBLM J0555-57Ab
EBLM J0555-57Ab has a mass of about $18.66$ Jupiter masses, or 0.081 solar masses. Its radius is 0.08 solar radii (about 59,000 km), comparable to Saturn, which has an equatorial radius of 60,268 km. The star is about 250 times more massive than Saturn. Current stellar models put its mass at the lower limit for hydrogen-burning stars. EBLM J0555-57Ab was discovered by a group of scientists at the University of Cambridge associated with the EBLM project (Eclipsing Binary, Low Mass), using data collected by the WASP project. WASP (Wide Angle Search for Planets) is searching for exoplanets using the transit method. The Findings were released on July 12th, 2017, it is unknown the exact date of the discovery. Additional properties of the star were determined using Doppler spectroscopy, to measure the periodic radial velocity variation of the primary star due to the gravitational influence of its companion. EBLM J0555-57Ab is the smallest hydrogen burning star currently known.