K2-21, also known as EPIC 206011691, is a

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

star located away in the constellation Aquarius. It hosts two known

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

s, discovered in 2015 by the

transit method Methods of detecting exoplanets usually rely on indirect strategies – that is, they do not directly image the planet but deduce its existence from another signal. Any planet is an extremely faint light source compared to its parent star. For e ...

as part of Kepler's K2 mission. Both planets have significantly lower densities than Earth, indicating that they are not

rocky planets A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to th ...

and are better described as

mini-Neptune A Mini-Neptune (sometimes known as a gas dwarf or transitional planet) is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen-helium atmosphere, probably with deep layers of ice, rock or liquid oceans (made o ...

s. The inner planet, K2-21b, is less dense than the outer planet, K2-21c.

References

Aquarius (constellation) M-type main-sequence stars Planetary systems with two confirmed planets J22411288-1429202 240766850 {{reddwarf-star-stub