Gliese 176

Gliese 176 is a small star with an orbiting exoplanet in the constellation of Taurus. With an apparent visual magnitude of 9.95, it is too faint to be visible to the naked eye. It is located at a distance of 30.9 light years based on parallax measurements, and is drifting further away with a heliocentric radial velocity of 26.4 km/s.

This is an M-type main-sequence star, sometimes called a red dwarf, with a stellar classification of M2V. It has 49% of the Sun's mass and 47% of the radius of the Sun. The star is radiating just 3.5% of the luminosity of the Sun from its photosphere at an effective temperature of 3,632 K. It is estimated to be around nine billion years old, and is spinning slowly with a rotation period of 40 days. The star is orbited by a Super-Earth.

Planetary system
A planetary companion to Gliese 176 was announced in 2008. Radial velocity observations with the Hobby-Eberly Telescope (HET) showed a 10.24-day periodicity, which was interpreted as being caused by a planet. With a semi-amplitude of 11.6 m/s, its minimum mass equated to 24.5 Earth masses, or approximately 1.4 Neptune masses.

Observations with the HARPS spectrograph could not confirm the 10.24-day variation. Instead, two other periodicities were detected at 8.78 and 40.0 days, with amplitudes below the HET observational errors. The 40-day variation coincides with the rotational period of the star and is therefore caused by activity, but the shorter-period variation is not explained by activity and is therefore caused by a planet. Its semi-amplitude of 4.1 m/s corresponds to a minimum mass of 8.4 Earth masses, making the planet a Super-Earth.

In an independent study, observations with Keck-HIRES also failed to confirm the 10.24-day signal. An 8.77-day periodicity - corresponding to the planet announced by the HARPS team - was detected to intermediate significance, though it was not deemed significant enough to claim a planetary cause with their data alone.