The K2-138 system is located almost 600 light years from earth,the main star is a large early type-K main-sequence star which is orbited by 6 planets in which the first two were discovered in early april 2017 by citizen scientists and it's one of the few we know of that contains more then one planet around the star itself
all the planets in the system are bigger then earth in terms of mass and size with the lowest contain 3.1 time the mass of the earth while the largest contain more then 13 time the mass of the earth,there radius however range from 1.57 time the earth radius to as high as 3.44-3.7 time the earth radius and all of this puts them in the super-earth and mini-Neptune categories of exoplanets
all the planets in this system(the first five at least)are orbiting very close to there star and all fall inside the orbit of mercury in our solar system,and there orbits ranges from 2.3 days to the innermost planet up to over 41 days to the outermost planet,the first five planets are also in 3:2 orbital resonance with each other with distances that ranges from 0.033AU to the innermost planet up to 0.104AU to the outermost planet
the densities of those planets range from earth-like densities to neptune-like densities and that means that there core are likely rocky while there atmospheres that are composed of volatile-like gases,a team of astronomers were able to constrain the upper mass limit of planet f & g as 8.7 earth masses for planet f and 25.5 earth masses to planet g
the star itself is 0.93 time the mass and 0.86 the radius of the sun while it's surface temperature is slightly cooler then our sun at 5,378 degrees kelvin,the age of the star is younger then our sun at 2.3 billion years compare to 4.6 billion years of our sun
this picture is an artist impression of the K2-138 planetary system with all the six planet can be seen in this impression with there star in the middle,this is an much younger stellar system then our solar system(2.3 billion years compare to 4.6 billion years for our solar system)
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