Space Telescope Science Institute
Our solar system contains two major classes of planets. Earth is a rocky terrestrial planet, as are Mercury, Venus, and Mars. At about the distance of the asteroid belt, there is a "frost line" where space is so cold more volatile material, like water, can remain frozen. Out here live the gas giants–Jupiter, Saturn, Uranus, and Neptune–which have bulked up on hydrogen and helium and other volatiles.
Astronomers are curious about a new class of planet not found in the Solar System. Weighing in at 12.6 Earth masses the planet is more massive than Earth, but less massive than Neptune (hence, intermediate between the rocky and gaseous planets in the Solar System). What's more, the planet, GJ 3470 b, is so close to its red dwarf star that it completes one orbit in just three days! As odd as it seems, planets in this mass range are likely the most abundant throughout the galaxy, based on surveys by NASA's Kepler space telescope. But they are not found in our own solar system.
Astronomers enlisted the combined multi-wavelength capabilities of NASA's Hubble and Spitzer space telescopes to assemble for the first time a "fingerprint" of the chemical composition of GJ 3470 b's atmosphere, which turns out to be mostly hydrogen and helium, and surprisingly, largely lacking heavier elements. One possible explanation is that the planet formed as a 10-Earth-mass rocky core that then accumulated hydrogen very close to its star, rather than migrated in which is the conventional wisdom for star-hugging planets.