Hubble research suggests Earth could once have been a 'Hot Neptune'.

Hubble research suggests Earth could once have been a 'Hot Neptune'.This artist's illustration shows a giant cloud of hydrogen streaming off a warm, Neptune-sized planet just 97 light-years from Earth. The exoplanet is tiny compared to its star, a red dwarf named GJ 3470. These discoveries reinforce the idea that the hotter version of these distant worlds may be a class of transitory planet whose ultimate fate is to shrink down to the most common type of known exoplanet, mini-Neptunes — planets with heavy, hydrogen-dominated atmospheres that are larger than Earth but smaller than Neptune. Eventually, these planets may downsize even further to become super-Earths, more massive, rocky versions of Earth - Picture by Atlas Photo Archive/NASA/ESA/D Player (STScI)