Astronomers call them super-Earths, and they are abundant outside our solar system. But the more experts learn about them, the weirder our own planet seems in comparison.
Planets the size of Earth and up to four times larger are believed to make up about three-quarters of the planet candidates discovered by NASA's Kepler spacecraft.
Astronomers have eagerly catalogued some 3,000 of these planets in the hopes that they may point to the existence of life elsewhere in the galaxy.
But experts told a meeting of the American Astronomical Society outside the US capital on Monday that while super-Earths and mini-Neptunes are common, they bear little resemblance to the planet we call home.
"Our solar system seems to be different. All these planets that Kepler has found, they are strange," said Yoram Lithwick of Northwestern University.
"Twenty to 30 percent of all stars have these crazy planets."
Super-Earths and mini-Neptunes that are more than two and a half times the radius of Earth "must be covered with lots and lots of gas, which is the most surprising result," said Lithwick.
He studied about 60 such planets and found that they likely formed "very quickly after the birth of their star, while there was still a gaseous disk around the star."
"By contrast, Earth is thought to have formed much later, after the gas disk disappeared," he said.
Not only are many of these planets hotter than Earth, having a huge amount of gas covering their rocky core would result in extreme atmospheric pressure.
"It would be like being below 10 oceans here on Earth," said Geoff Marcy of the University of California, Berkeley.
Asked if life could exist under such conditions, Marcy told reporters he had asked some of his friends who are biology experts the same question. In short, they were not sure.
"It is not impossible," he said. "We know very little about how life got started and in what environments it might flourish."
Since Kepler cannot return any data about mass, astronomers have learned to study it through alternate methods, like making Doppler measurements of the planets' host stars, seeing how they wobble as a result of the gravitational tug from the orbiting planet.
Planets with higher mass make for more intense wobbling because they exert a greater gravitational tug on their stars.
David Kipping, of the Harvard-Smithsonian Center for Astrophysics, described his team's latest discovery of a planet called KOI-314C in a presentation called "An Earth-mass world nothing like home."
Located some 200 light years away, "a stone's throw by Kepler's standard," the planet orbits its star every 23 days.
The planet's temperature is about 220 degrees Fahrenheit (104 Celsius), and it is coated in an atmosphere of hydrogen and helium hundreds of miles thick.
The planet is one of three in a mini solar system, in which the cohabitants "kick each other, they perturb each other frequently," he told reporters.
Since it is relatively close, Kipping said he hopes further study with the Hubble space telescope or its successor, the James Webb Space Telescope to be launched in 2018, could shed more light on its characteristics.
Another prospect for further research is the super-Earth exoplanet GJ 1214b, some 40 light years away, which is believed to be covered with clouds, according to researcher Laura Kreidberg at the University of Chicago.
Its atmosphere lacks water, methane or carbon dioxide, and its clouds could be made of zinc sulfide and potassium chloride, she said.
At the conference, astronomers announced 70 new planet confirmations, 16 mass determinations from Doppler follow-up observations and five new rocky planets.
NASA's Kepler space telescope launched in 2009 on a mission to find Earth-like planets by observing transits, or dimmings in light, as they passed in front of their stars.
It is no longer fully operational, having lost traction in the second of its four orienting wheels last year, but astronomers hope it will be able to continue offering limited observations of distant worlds.