Exoplanet · mini-Neptune
K2-21 c
K2-21 c is a mini-Neptune orbiting the star K2-21. Its year lasts 15.5 Earth days. It was discovered in 2015 by the Transit method.
| Host star | K2-21 |
|---|---|
| Distance | 272.8 light-years (83.6 pc) |
| Orbital period | 15.5 days |
| Orbital radius | 0.107 AU |
| Eccentricity | 0.210 |
| Radius | 2.49 R⊕ · 0.222 R♃ |
| Mass | 6.76 M⊕ · 0.0213 M♃ (estimated from the radius) |
| Density | 5.4 g/cm³ |
| Equilibrium temperature | 420 K (147 °C) |
| Stellar flux received | 5.1 × Earth |
| Discovery method | Transit |
| Discovery year | 2015 |
| Discovered by | K2 |
Habitable zone
Inside the inner edge of the habitable zone: it receives 5.1 times the light the Earth does, too hot to keep liquid water on its surface.
Its star in the sky
In the planet's sky its star is a disc 3.2° across: 6 times the Sun as we see it from Earth.
In the sky
A telescope is needed to see it: its apparent magnitude is 12.82. Visible from the whole Southern Hemisphere and north to latitude 76° N. South of latitude 76° S it never sets. It stands highest at midnight in August, the best time to observe it.
Light's journey
The light you see tonight set out around the year 1753. At the speed of Voyager 1, the fastest craft leaving the Solar System (16.9 km per second), the trip would take 4.8 million years.
Planets in the same system
- K2-21 b 9.33-day orbit · 1.84 R⊕
Data: NASA Exoplanet Archive, PSCompPars. Its look in the simulation is a rendering based on the measured radius, mass and temperature.