The TRAPPIST-1 system has been one of the most incredible discoveries of the last five years. Seven Earth-sized planets orbit a red dwarf star just 40 light-years away – close enough to conduct detailed investigations on these objects. The latest one delivers some very important insights into their composition.

As reported in The Planetary Science Journal, the seven planets have remarkably similar densities. That is absolutely fascinating and unlike our solar system. Obviously, if we include the gas giants, the density of our familiar planets are very different (Saturn, for example, would float on water). But even if we consider the rocky worlds exclusively, we find that Mars is significantly less dense than Venus, Mercury, and Earth.

The TRAPPIST-1 planets have all been confirmed to be terrestrial, so they are expected to contain more or less the same kind of elements, such as iron, oxygen, magnesium, and silicon. The proportions of these is certainly what’s interesting. The planets are slightly less dense than our own planet, 8 percent less to be precise, and the team put forward two scenarios to explain this.

One possibility is that the planets are water-rich. If 5 percent of their mass is water and the rest is Earth-like, the density will match. But that’s a very high water value – Earth is only 0.1 percent water by comparison, and researchers don’t expect the three innermost planets to be water-rich. It would be a weird coincidence if the four water-rich outer planets have the same density as their dry inner siblings.

A preferred explanation for the team focuses on the iron content of the seven planets. They would have the same composition as Earth but with less iron – 21 percent by mass, compared to our planet’s 32 percent. Alternatively, instead of less iron, the planets might have a higher value of iron oxide. The additional oxygen would certainly help reduce the density. Or maybe it’s both.  

“The lower density might be caused by a combination of the two scenarios – less iron overall than and some oxidized iron. they might contain less iron than Earth and some oxidized iron like Mars,” lead author Dr Eric Agol from the University of Washington, said in a statement.

This work was possible thanks to repeated observations of the system since 2016. The wealth of data collected by ground-based and space telescopes was crucial to better understand the TRAPPIST-1 system.

"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," concludes co-author Dr Caroline Dorn from the University of Zurich.