from SPACE

The Family Tree of Exoplanets Has Just Divided Into Two Branches

By Elizabeth Howell, Seeker

This sketch illustrates a family tree of exoplanets. Planets are born out of swirling disks of gas and dust called protoplanetary disks. The disks give rise to giant planets like Jupiter as well as smaller planets mostly between the sizes of Earth and Neptune. Researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission discovered that the smaller planets can be cleanly divided into two size groups: the rocky Earth-like planets and super-Earths, and the gaseous mini-Neptunes. / Credit: NASA/Kepler/Caltech (T. Pyle)

Scientists have reorganized the exoplanetary tree of life into two distinct branches. Most exoplanets discovered so far are close in size to Earth or either Neptune, according to a new study led by the California Institute of Technology. But astronomers are puzzled as to why there is a gap between these two planetary sizes.

The work, which is based on an analysis of thousands of known exoplanets, shows that planets in our galaxy overwhelmingly fall into two groups. The first includes rocky planets up to 1.75 times the size of Earth, and the second group is made up of gaseous Neptune-like worlds between 2 to 3.5 times the size of Earth. (Neptune, by comparison, is roughly 4 times the size of Earth.)

The work includes data from NASA’s Kepler space telescope, which searches for Earth-like worlds in the habitable zones of their stars, and the W. M. Keck Observatory, which detects planets using the High Resolution Echelle Spectrometer (HIRES) on the Keck I telescope. The researchers attempted to classify these planets similarly to how biologists classify animal species.

[ click to continue reading at SPACE.com ]