In the last five years, researchers have unexpectedly discovered the most common type of exoplanet seen to date is one missing from the solar system — a world between the sizes of Earth and Neptune. Planets like these apparently substantially outnumber larger, Jupiter-size planets, at least relatively close to their stars.
"These are sometimes called 'super-Earths,'" study author Andrew Howard, an astronomer at the University of Hawaii at Manoa, told SPACE.com.
Super-Earths are part of the most commonly seen class of exoplanetary systems. These involve one or more planets between one to three times the size of Earth orbiting much closer to their stars than one astronomical unit, the distance from Earth to the sun.
The composition of the exoplanetary systems that researchers have observed so far may solve a key mystery regarding which of two competing models of how planets form is correct. The core accretion model of planetary formation suggests worlds grow when gas accumulates relatively quickly onto a solid planetary core, while the mechanism known as gravitational instability has a planet's interior and atmosphere born simultaneously.
The core accretion model predicts that giant planets should be more common around massive stars that are rich in "metals" — that is, elements heavier than helium — since these stars have discs surrounding them dense in the dust and ice that could go into a planet's core. So far, it looks as if metal-rich stars are in fact more likely to host giant planets within five astronomical units of them, suggesting the core accretion model is how planets are generally created.