Just 40 light-years from Earth is a planetary system that has intrigued scientists since its discovery in 2016.
While some of the planets in this system will be observed by the upcoming James Webb Space Telescope, scheduled to launch in October, researchers continue to uncover new details that make TRAPPIST-1 even more distinct.
The planets, which are too small and faint to be seen by any current ground or space-based telescopes, have such similar densities that they may have equally similar compositions, new research has suggested.
Given that these planets are about 8% less dense than Earth, researchers have taken a stab at trying to understand how much iron, oxygen, magnesium and silicon these planets contain.
The study published last week in The Planetary Science Journal.
The TRAPPIST-1 system includes seven planets, all rocky and similar in size to Earth — and the most planets like this found orbiting one star. In contrast, the planets in our solar system vary both in size and composition, including smaller rocky planets like Venus, Mercury and Earth and giant gaseous ones such as Jupiter and Saturn.
The TRAPPIST-1 system is a reminder that no two planetary systems are alike.
Astronomers announced in February 2017 their discovery of the seven Earth-size planets orbiting a star 40 light-years from Earth. Initially, the researchers only identified three planets around the star in 2016, but they used multiple telescopes and discovered the other four planets using the transit method — measuring dips in starlight as planets passed in front of the star.
The seven exoplanets were all found in tight formation around an ultracool dwarf star called TRAPPIST-1, and the planets bear the TRAPPIST name — which the researchers borrowed from their favorite beer.
These planets are so close to their star that they would all fit within the orbit of Mercury around the sun, which is the closest planet in our solar system to our host star. This proximity means that if you were to stand on the surface of one of these planets, the other planets in the system would appear like large moons in the sky.
Astronomers were able to obtain precise measurements of the masses and diameters of the TRAPPIST planets using the transit method, which also enabled them to determine the planets’ densities.
Their densities are so close that there is no more than a 3% difference between them, the researchers concluded.
Even though these exoplanets are only 8% different in density from Earth and Venus, this difference suggests that the TRAPPIST planets are made differently. The researchers determined that they could be similar to Earth in every element with the exception that the composition of the seven planets could be 21% iron while Earth is 32% iron.
The iron in the TRAPPIST planets could contain more oxygen, which would actually form iron oxide — otherwise known as rust.
In our solar system, Mars gets its telltale red hue from iron oxide on the surface. Despite this, Mars has an iron planetary core, much like the other rocky planets in our solar system of Venus, Mercury and Earth.
If the TRAPPIST planets contain more of this oxidized iron, they would not include solid iron cores, which could account for the density difference.
Researchers believe that a combination of lower iron content, as well as some iron oxide, accounts for the difference between the TRAPPIST planets and Earth.
The TRAPPIST system has intrigued scientists for many reasons, and this suggestion about their composition is one of the latest findings.
Astronomers previously have suggested that some of the planets orbiting the TRAPPIST-1 star are in the star’s habitable zone, meaning some of the middle planets could be at the right surface temperature to support liquid water, and perhaps life, on their surfaces.
The closest planets would be too hot and the furthest planet from the star may be covered in ice.
Astronomers have estimated that TRAPPIST-1 is between 5.4 billion and 9.8 billion years old, meaning it could be nearly twice as old as our own solar system, which formed 4.5 billion years ago.
When some of the TRAPPIST planets are observed by the James Webb Space Telescope in the future, astronomers could learn if these planets have atmospheres and if so, what those atmospheres contain.
“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,” said study coauthor Caroline Dorn, an astrophysicist at the University of Zurich, in a statement.
“The TRAPPIST-1 system is fascinating because around this one star we can learn about the diversity of rocky planets within a single system. And we can actually learn more about a planet by studying its neighbors as well, so this system is perfect for that.”