Since their discovery, the planets of the Trappist-1 system - about 40 light years from our sun - have been hailed by scientists as our best hope of finding alien life outside our solar system.
A series of new studies by an international team of researchers has now revealed fascinating details about these promising distant worlds.
One team, led by University of Birmingham astronomer Amaury Triaud, found that all seven of the Earth-sized planets in the Trappist-1 system consist mostly of rock, with up to 5% of their mass accounted for by water – a significant amount. By comparison, Earth's oceans account for only 0.02% of the planet's mass.
It appears that five of the planets probably do not have an atmosphere filled with hydrogen and helium, suggesting that they are more similar to the rocky worlds of the solar system than, say, gas giants such as Uranus or Neptune.
The data is consistent with the planets having atmospheres that are similar to either Venus or Earth. More observations will be needed to confirm both the existence of atmospheres and their composition.
Up to six of the planets could be included in the optimistic habitable zone - the region in which liquid water, crucial to life as we know it, could remain on the surface given the right conditions. However, it is not known what form water takes on the Trappist planets as the amount of heat they receive from the star – which is just 9% as massive as our sun - is unknown.
While researchers are trying to understand which of the planets are most likely to be habitable, it seems that Trappist-1e – the fourth planet from the star – most closely resembles Earth, although we know little about its surface conditions and atmosphere.
"When we combine our new masses with our improved radii measurements and our improved knowledge of the star, we obtain precise densities for each of the seven worlds and reach information on their internal composition," said Triaud. "All seven planets remarkably resemble Mercury, Venus, our Earth, its moon, and Mars."
In an attempt to catch the starlight interacting with the planetary atmospheres, the scientists observed the planets passing in front of their star through the Hubble Space Telescope. This enabled astronomers to deduce their mass, radius and density.
The Trappist-1 planet orbits are all packed tightly together and are closer to their host star than Mercury is to the sun. The fact that the star is a red dwarf (and therefore much cooler than the sun) means the planets may still fall within a habitable temperature range. They may also be tidally locked, meaning that the same side is always facing the star.
The next step will be to explore the Trappist system with Nasa's James Webb Telescope, which will be able to determine for certain whether the planets have atmospheres and, if so, if they can provide the right conditions for liquid water to exist on the surface.
"Hubble is doing the preliminary reconnaissance work so that astronomers using Webb know where to start," said Nikole Lewis from the Space Telescope Science Institute in Baltimore.
"Eliminating one possible scenario for the makeup of these atmospheres allows the Webb telescope astronomers to plan their observation programmes to look for other possible scenarios for the composition of these atmospheres."