The planets in our solar system are rather neatly grouped into two broad categories: relatively small rocky terrestrial planets and large non-rocky gas giants. With the flood of extrasolar planet discoveries coming from projects like NASA’s Kepler mission, we are now finding examples of planets whose mass and radii fall between those of the Earth and Neptune. With these discoveries, astronomers are finally able to study the mass-radius relationship for super-Earths to determine how large a terrestrial planet can get before it becomes a mini-Neptune.
In my third essay for Centauri Dreams , titled “The Transition from Rocky to Non-Rocky Planets”, I discuss the latest discovery of a low-density, low-mass planet orbiting Kepler 289 called PH3 c. With a mass of four times that of the Earth but a density only slightly greater than that of water, this planet has more in common with Neptune than it does Earth. This discovery in conjunction with statistical analyses of similar planets discovered in recent years now strongly suggests that the transition from rocky to non-rocky planets occurs at about 1½ times the radius of the Earth or around 4 to 6 times its mass. This finding has profound implications for the true nature of extrasolar “super-Earths” that many have considered to be potentially habitable.
Related Reading
“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]
“The Transition from Super-Earth to Mini-Neptune”, Drew Ex Machina, March 29, 2014 [Post]