This Star Is Home to the Largest Group of Earth-sized Planets Ever Found

Scientists have revealed intriguing new details about the largest group of roughly Earth-sized exoplanets known to science.

A study published in the Planetary Science Journal on Friday demonstrates that the seven planets orbiting the TRAPPIST-1 star—located around 40 light-years from the sun—have remarkably similar densities, providing insights into their composition.

The star system, which was discovered in 2016, is one of the most promising targets in the search for life outside our solar system given that it is relatively close—thus fairly easy to observe—and the fact that at least three of the planets are considered to lie in the star's habitable zone.

This is the region around a star where conditions are such that liquid water—crucial to life as we know it—could exist on the surface of these planets.

A previous study published in 2018, led by an author of the latest paper, provided the most accurate calculations to date of the TRAPPIST-1 planets, finding that they all had roughly similar masses and sizes to Earth—indicating they are rocky worlds.

But the latest study—based on four years of observations by NASA's now-defunct Spitzer Space Telescope in addition to ground-based measurements—has further refined our understanding of the TRAPPIST-1 planets.

"The new observations allowed us to use transit data from a much longer time span than was available to us for the 2018 calculations," Simon Grimm, an author of the latest study from the Center for Space and Habitability and NCCR PlanetS, University of Bern, Switzerland, said in a statement.

"With the new data, we were able to refine the mass and density determinations of all seven planets, and it turned out that the derived densities of the planets are even more similar than we had previously expected."

The TRAPPIST-1 system appears to be quite different to our own solar system, where the densities of the eight planets vary significantly. The gas giants—Jupiter, Saturn, Uranus and Neptune—are much larger but far less dense than the four rocky worlds, Earth, Venus, Mars, and Mercury.

The fact that the TRAPPIST-1 planets are similar in density indicates that they may have a similar material composition, the authors of the study said. In addition, the team's measurements suggest that the planets are about eight percent less dense than Earth.

The authors of the study say the slightly lower densities of the planets compared to Earth might be the result of different scenarios.

One is that the planets have a similar composition to Earth, but with a lower percentage of iron—around 21 percent compared to 32 percent. Another possibility is that the iron in the planets is infused with high levels of oxygen, forming iron oxide or rust, the study found.

"The night sky is full of planets, and it's only been within the last 30 years that we've been able to start unraveling their mysteries, also for determining the habitability of these planets," another author of the study, Caroline Dorn from the University of Zürich, said in a statement.

The TRAPPIST-1 planets
Artist's illustration of the TRAPPIST-1 planets. University of Bern