The planets, which scientists say are the best-studied worlds outside our solar system, “remarkably resemble Mercury, Venus, our Earth, its moon and Mars,” said Amaury Triaud, a University of Birmingham astronomer who co-authored one of the studies.
The worlds in question circle a dim star called TRAPPIST-1, which shares its name with the Belgian-operated telescope (Transiting Planets and Planetesimals Small Telescope) located in Chile. It's also a reference to the famous Trappist beer.
Astronomers peering through the scope first discovered the system two years ago and continue to uncover more details about the star and its worlds.
The new studies say that, as had been theorized, all of the planets are rocky and not gaseous.
The planets' densities, now known much more precisely than before, suggest some planets could contain up to a whopping 5% of their mass in water — 250 times more than the oceans on Earth.
According to the new research, the fourth planet in the system, known as TRAPPIST-1e, is the most Earth-like. Of the known exoplanets, or planets outside our solar system, 1-e is the one that's most similar to the Earth in size, density and the amount of radiation it receives from its star, according to researchers at the University of Bern in Switzerland.
It's also the only one of the seven planets that is somewhat denser than Earth, and it is not ruled out that liquid water exists on its surface.
Scientists are able to calculate the densities of these planets because they are lined up in such a way that when they pass in front of their star, our telescopes can detect a dimming of its light, NASA said. The amount by which the starlight dims is related to the size of the planet.
“Densities, while important clues to the planets’ compositions, do not say anything about habitability," said Brice-Olivier Demory, study co-author from the University of Bern. "However, our study is an important step forward as we continue to explore whether these planets could support life."
The star and its planets are less than 40 light-years away in the constellation Aquarius, according to NASA. Despite being so "close" to Earth, relatively speaking, the star is too dim and too red to be seen with the naked eye or even with a large amateur telescope.
The next step in exploring the TRAPPIST system will be with the help of the soon-to-be-launched James Webb Space Telescope. NASA's newest scope, scheduled for launch in 2019, will orbit the earth and be able to delve into the question of whether these planets have atmospheres and, if so, what those atmospheres are like.
The new studies were published Monday in the journals Nature Astronomy and Astronomy and Astrophysics.