The Kepler-399 system contains 3 exoplanets. It is located 2415.31 light years away from the solar system.
| Mass | 0.9 solar masses |
| Radius | 0.68 solar radiae |
| Temperature | 5502 kelvin |
| Stellar Metallicity | 0.14 decimal exponent |
| Age | 3.98 billion years |
At less than 1.5 Earth masses, Kepler-399 b is a terrestrial planet, much like the terrestrial planets we find in our solar system, namely Mercury, Venus, Earth and Mars.
| Mass | 0.840 Earth masses |
| Density | 5.220 grams per cubic centimeter |
| Radius | 0.960 Earth radiae |
| Semi-major Axis | 0.103 AU |
| Eccentricity | 0 |
| Orbital Period | 14.425 days |
| Discovery Method | Transit |
| Discovery Facility | Kepler |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2014-03 |
| Reference | Rowe et al. 2014 |
At 2.636 Earth masses, Kepler-399 c is a so called Super Earth. Super Earths could be terrestrial worlds like Earth, but they could also be ocean worlds or terrestrial worlds wrapped in a substantial atmosphere, in which case some refer to them as Mini Neptunes.
| Mass | 2.630 Earth masses |
| Density | 4.940 grams per cubic centimeter |
| Radius | 1.430 Earth radiae |
| Semi-major Axis | 0.155 AU |
| Eccentricity | 0 |
| Orbital Period | 26.676 days |
| Discovery Method | Transit |
| Discovery Facility | Kepler |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2014-03 |
| Reference | Rowe et al. 2014 |
With a mass below 5 Earth masses, a density of 3.44, and a semi-major axis of 0.261 astronomical units, Kepler-399 d could, potentially, be an ocean world - a planet with no dry land.
| Mass | 4.230 Earth masses |
| Density | 3.440 grams per cubic centimeter |
| Radius | 1.890 Earth radiae |
| Semi-major Axis | 0.261 AU |
| Eccentricity | 0 |
| Orbital Period | 58.035 days |
| Discovery Method | Transit |
| Discovery Facility | Kepler |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2014-03 |
| Reference | Rowe et al. 2014 |