TRAPPIST-1’s Seven Earth-Sized Planets Are Likely All Made of the Same Stuff

The TRAPPIST-1 system has been the most incredible discovery in the last five years. Seven Earth-sized planets orbit a red dwarf star just 40 light-years away – close enough to investigate these matters in detail. The latter provides some very important insights into their compositions. As published in the Journal of Planetary Science, these seven planets have significantly the same density. It is absolutely fascinating and in contrast to our solar system. Clearly, the densities of our known planets are very different if we include gas giants (Saturn, for example, will float in water). However, if we consider the rocky Earth exclusively, we find that Mars is significantly less dense than Venus, Mercury, and Earth.

The TRAPPIST-1 planets have been confirmed to be all terrestrial, so they are expected to contain more or less the same elements as iron, oxygen, magnesium, and silicon. The ratios of these are certainly interesting. The planets are slightly less dense than our own, 8 percent less precise, and the team put forward two scenarios to explain it. One possibility is that the planets are rich in water. If 5 percent of their mass is water and the rest is like Earth, the density will match. However, this is a very high water value – compared to Earth’s only 0.1 percent water and researchers do not expect the three underlying planets to be water-rich. It would be a strange coincidence if the four water-rich outer planets had the same density as the dry inner siblings.

A preferred explanation for the team centers on the iron content of the seven planets. They will have the same composition as Earth but with iron – 21 percent by mass compared to 32 percent of our planet. Alternatively, planets may have higher levels of iron oxide instead of less iron. Excess oxygen will certainly help reduce the concentration. Or it could be both.

Lead author Dr. Eric Agol from the University of Washington, said in a statement, “A combination of the two densities can lead to lower densities – less iron overall and some oxidized iron. They may have less iron than Earth and some oxidized iron like Mars.”

This work has made it possible to observe the repetition of this system since 2006. The resources of data collected by ground-based and space telescopes are crucial for a better understanding of the ground TRAPPIST-1 system. Co-author Dr. Caroline Dorn from the University of Zurich concludes, “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.”