Just 40 light years away, 7 Earth-like planets with similar compositions

The search for another habitable location in the vastness of the cosmos has been one of the most intriguing explorations of all times, bringing us close to a set of stars and exoplanets. The most fascinating among them is the TRAPPIST-1 system, which consists of the most Earth-sized planets found in the habitable zone of a single star.

A new research published in The Planetary Science Journal has now evoked a renewed interest in the system, with astronomers finding that the exoplanets have remarkably similar densities.

This system of seven rocky worlds – all with the potential for water on their surface – is an exciting discovery in the search for life in other worlds. The discovery of the system was first announced by the US National Aeronautics and Space Administration (Nasa) in 2017. In 2018, a closer study of the seven planets suggested that some could harbour far more water than the oceans on Earth, in the form of atmospheric water vapour for the planets closest to their star, liquid water for others, and ice for those farthest away.

The measured density

According to the study, "they all contain about the same ratio of materials thought to compose most rocky planets, like iron, oxygen, magnesium, and silicon. But if this is the case, that ratio must be notably different than Earth’s. The TRAPPIST-1 planets are about 8 per cent less dense than they would be if they had the same makeup as our home planet." The researchers now believe that a few different mixtures of ingredients could give the TRAPPIST-1 planets the measured density.

The understanding of a planet's density is critical in establishing the chemical composition of the planet. Planets in our own solar system have a varied density, the gas-dominated giants – Jupiter, Saturn, Uranus, and Neptune – are larger but much less dense than the four terrestrial worlds because they’re composed mostly of lighter elements like hydrogen and helium. Even the four terrestrial worlds show some variety in their densities, which are determined by both a planet’s composition and compression due to the gravity of the planet itself. By subtracting the effect of gravity, scientists can calculate what’s known as a planet’s uncompressed density and potentially learn more about a planet’s composition, Nasa said in a report.

Understanding Iron on TRAPPIST-1

According to the research, the seven planets on the TRAPPIST system have similar densities, with a difference of just 3 per cent. However, the difference of density between the TRAPPIST planets and Earth is 8 per cent. TRAPPIST-1 planets are less dense is that they have a composition similar to the Earth’s, but with a lower percentage of iron – about 21% compared to the Earth’s 32%, according to the study. The iron in the TRAPPIST-1 planets might be infused with high levels of oxygen, forming iron oxide, or rust. The additional oxygen would decrease the planets’ densities. The surface of Mars gets its red tint from iron oxide, but like its three terrestrial siblings, it has a core composed of non-oxidised iron.

Nasa, quoting Eric Agol, an astrophysicist at the University of Washington and lead author of the new study, said the answer might be a combination of the two scenarios – less iron overall and some oxidised iron. The team also looked into whether the surface of each planet could be covered with water, which is even lighter than rust and which would change the planet’s overall density. If that were the case, water would have to account for about 5 per cent of the total mass of the four outer planets.

Trappist-1 star compared to our Sun. (NASA)

“The night sky is full of planets, and it’s only been within the last 30 years that we’ve been able to start unravelling their mysteries,” said Caroline Dorn, an astrophysicist at the University of Zurich and a co-author of the paper. “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 neighbours as well, so this system is perfect for that.”

The discovery

The first three planets of the system were discovered in 2016 using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. The scientists used the transit method to establish the presence of planets around the star. The method is to look for dips in the star’s brightness created when the planets cross in front of it.

An artist's concept of TRAPPIST-1. (Nasa)

Repeated observations of the starlight dips combined with measurements of the timing of the planets’ orbits enabled astronomers to estimate the planets’ masses and diameters. Previous studies have established that the planets in the system have roughly the size and mass of the Earth and thus must also be rocky, or terrestrial.