Small planets , 1–4x the size of Earth , are extremely common around Sun-like stars , and surprisingly so , as they are missing in our solar system .
Recent detections have yielded enough information about this class of exoplanets to begin characterizing their occurrence rates , orbits , masses , densities , and internal structures .
The Kepler mission finds the smallest planets to be most common , as 26 % of Sun-like stars have small , 1-2 R _ { \oplus } planets with orbital periods under 100 days , and 11 % have 1–2 R _ { \oplus } planets that receive 1-4x the incident stellar flux that warms our Earth .
These Earth-size planets are sprinkled uniformly with orbital distance ( logarithmically ) out to 0.4 AU , and probably beyond .
Mass measurements for 33 transiting planets of 1–4 R _ { \oplus } show that the smallest of them , R < 1.5 R _ { \oplus } , have the density expected for rocky planets .
Their densities increase with increasing radius , likely caused by gravitational compression .
Including solar system planets yields a relation : \rho = 2.32 + 3.19 R / R _ { \oplus } [ g cm ^ { -3 } ] .
Larger planets , in the radius range 1.5–4.0 R _ { \oplus } , have densities that decline with increasing radius , revealing increasing amounts of low-density material ( H and He or ices ) in an envelope surrounding a rocky core , befitting the appellation ‘ ‘ mini-Neptunes. ’ ’ Planets of \sim 1.5 R _ { \oplus } have the highest densities , averaging near 10 g cm ^ { -3 } .
The gas giant planets occur preferentially around stars that are rich in heavy elements , while rocky planets occur around stars having a range of heavy element abundances .
One explanation is that the fast formation of rocky cores in protoplanetary disks enriched in heavy elements permits the gravitational accumulation of gas before it vanishes , forming giant planets .
But models of the formation of 1–4 R _ { \oplus } planets remain uncertain .
Defining habitable zones remains difficult , without benefit of either detections of life elsewhere or an understanding of life ’ s biochemical origins .