HD 179070, also known as Kepler-21, is a star with a closely orbiting exoplanet in the northern constellation of Lyra. At an apparent visual magnitude of 8.25 this was the brightest star observed by the Kepler spacecraft to host a validated planet until the discovery of an exoplanet orbiting HD 212657 in 2018. This system is located at a distance of 354lk=onNaNlk=on from the Sun based on parallax measurements, but is drifting closer with a radial velocity of −18.2 km/s.
The spectrum of HD 179070 presents as an evolving F-type subgiant star with a stellar classification of F6 IV. This suggests the star has exhausted the supply of hydrogen at its core and is evolving into a giant star. It is an estimated 2.6 billion years old and is spinning with a rotation period of 12.6 days. With 1.4 times the mass of the Sun it currently has 1.9 times the Sun's radius. The star is radiating five times the luminosity of the Sun from its photosphere at an effective temperature of 6,305 K.
A faint nearby source was detected in 2011 and determined to be a co-moving stellar companion in 2016. Designated HD 179070 B, it lies at an angular separation of along a position angle of 129° relative to the primary. At the distance of this star, this corresponds to a projected separation of . It is possible that this companion star had a significant influence on the exoplanet formation and subsequent orbital evolution.
A candidate transiting exoplanet was discovered based from the first four months of photometry data from the Kepler spacecraft. Confirmation was obtained in 2012 after extensive follow-up observations and analysis of the Kepler light curves.
The calculated density of the planet is approximately, similar to Earth's, which suggests a rocky composition. With an equilibrium temperature of 2,025 Kelvin, the top few-hundred kilometers of the planet is probably molten.
Calculations of the rate of orbital decay from tidal effects results in a decrease in the orbital period of 3.88 milliseconds per year, since this would be a change of only 4 seconds every thousand years it would be undetectable in any reasonable length of time.
Kepler-21 might have another exoplanet, a gas giant with at least 3.7 times the mass of Jupiter, named Kepler-21c.
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