A team of astronomers led by Zachory Berta of the Harvard-Smithsonian Center for Astrophysics made the observations of the planet GJ 1214b, about 2.7 times Earth's diameter and weighing almost seven times as much.
It orbits a red-dwarf star every 38 hours at a distance of 1.2 million miles, giving it an estimated temperature of 446 degrees Fahrenheit, a release from the center reported Tuesday.
"GJ1214b is like no planet we know of," Berta said. "A huge fraction of its mass is made up of water."
The planet's density is only slighter greater than that of water, and much less than Earth's, astronomers said.
This suggests GJ 1214b has much more water than Earth does, and much less rock, they said.
The internal structure of GJ 1214b would be extraordinarily different from that of our world as a result, Berta said.
"The high temperatures and high pressures would form exotic materials like 'hot ice' or 'superfluid water,' substances that are completely alien to our everyday experience," he said.
GJ 1214b is located in the constellation of Ophiuchus, the Serpent Bearer, just 40 light-years from Earth.