LHS 1678

LHS 1678 (TOI-696) is an astrometric binary star system, located about 65 light-years from the Earth in the constellation Caelum. It is made up of a red dwarf and a companion star whose nature is still uncertain, but is likely to be a brown dwarf. The red dwarf star is known to host three small, close-in exoplanets. The apparent magnitude of LHS 1678 is 12.5, which is too dim to be seen with the naked eye or a small telescope.

Characteristics
LHS 1678 is a binary star, more precisely an astrometric binary. It is made up of a red dwarf star and a likely brown dwarf star. They are sometimes called LHS 1678 A and B, respectively. The projected separation of the system, i.e. the observed separation between both stars, is a maximum of 5 AU. The orbit is expected to be highly inclined, and the orbital period is in the order of decades.

The system is located in the southern celestial hemisphere, within the constellation Caelum. The apparent magnitude is 12.5, which is too faint to be seen to the naked eye or even a small telescope. Gaia DR3 provides a parallax of $10.94$ to LHS 1678, which translates into a distance of 19.86 pc to the system. LHS 1678 is moving away from Earth at a velocity of 10.9 km/s. The system has a high proper motion, from 1983 to 2022, it has moved 0.5 arcminutes across the sky.

LHS 1678 A
The primary component of the system is a red dwarf star (spectral type M3V, sometimes called LHS 1678 A ) which has 0.345 times the mass and 0.329 times the Sun's radius. It has a surface effective temperature of 3490 K, which is significantly cooler than the Sun, and is emitting a luminosity equivalent to 1.45% of the solar luminosity. LHS 1678 A is metal-poor compared to the Sun, with an abundance of iron equivalent to 44% of the solar level. Its rotational period is between $94.6$ (the Sun's rotational period is 24.5 days for reference ), and its age is estimated to be 4.22 billion years. It hosts three exoplanets orbiting it which are smaller than Earth.

LHS 1678 A occupes an unusual position in the HR diagram. It is located in a narrow position in the diagram, characterized by a gap in the lower main sequence. This gap, which can be more accurately described as a deficit of stars, is associated with a transition from partially convective interiors to fully convective interiors in red dwarfs.

The star has little variability in its brightness, no signs of starspots or stellar flares have been found during two months of observations. It also shows no signs of magnetic activity.

LHS 1678 B
The secondary companion is a probable brown dwarf, sometimes called LHS 1678 B, that has been detected trough long-baseline astrometry from RECONS. Observations from the Very Large Telescope's NaCo adaptive optics rule out any companion with a mass larger than at a projected separation larger than 5 astronomical units. The companion has been not observed directly. It may be instead a Jovian planet, but the astrometric monitoring data indicates that it is likely a brown dwarf. The possibility of the companion to be a white dwarf is ruled out by the astrometry and radial velocity of the system. It is more likely in or below the hydrogen burning limit, but is nature remains uncertain.

The nearest star to the system is Alpha Caeli, at a distance of 3.3 light-years. Alpha Caeli is also the brightest star in Caelum.

Planetary system
The red dwarf LHS 1678 hosts three exoplanets, which are smaller than Earth and were discovered by the transit method. The first exoplanets discovered were LHS 1678 b and c, discovered by Silverstein et al. 2022, which also announced the existence of a third possible exoplanet. This planet was later confirmed in 2024, and received the designation LHS 1678 d.

All planets are close to their host star: LHS 1678 b, c and d have orbital periods of about 21 hours, 3.7 and 5 days respectively. LHS 1678 c and d are close to a 4:3 orbital resonance, meaning that for every four orbits completed by LHS 1678 c, LHS 1678 d completes three orbits. Orbital resonances in planetary systems are occasionally linked to transit-timing variations, but no transit-timing variations were detected during a 3-year span.

The masses of the planets have been not measured, but future high-quality radial velocity measurements might measure their masses. Estimates of $239.368$, $-967.597$ and $0.345$ for the planets b, c and d were placed using the forecaster procedure, with large margins of error.