spaceSpace and Physics

One Of The Trappist-1 Planets Might Have An Iron Core Like Earth


Jonathan O'Callaghan

Senior Staff Writer

An artist's impression of the Trappist-1 system. ESO/M. Kornmesser

The Trappist-1 system is, to put it bluntly, pretty damn awesome. It’s close, it’s got seven rocky planets, and some of them might be habitable. Now, the system just got a little bit awesomer.

Astronomers Gabrielle Englemann-Suissa and David Kipping from Columbia University’s Cool Worlds Laboratory have suggested that one of the planets in the system may have an iron core, making it even more enticing.


“The TRAPPIST-1 system provides an exquisite laboratory for understanding exoplanetary atmospheres and interiors,” they note in their study, available on arXiv and submitted to Research Notes of the American Astronomical Society.

The particular benefit of the system, 40 light-years from Earth, is that the planets are relatively close to each other, meaning their comparative masses can easily be measured. The system has seven planets, three of which are in the star’s habitable zone, but all orbit far closer to their star than Mercury does to our Sun.

And that star itself is useful, as it’s a small M-dwarf that’s about an eighth the size of the Sun. With masses around that of Earth, each planet produces a very large dip in the star’s light from our point of view – known as the transit method – making the planets relatively easy to see and study when they orbit every few days.

The planets orbit very close to their host star. NASA/JPL-Caltech

All of that combined means it’s possible to work out the precise masses and radii of the planets. And from that, you can even work out what they might be like inside.


“If you know the mass and radius very precisely, like the TRAPPIST-1 system, you can compare them to that predicted from theoretical interior structure models,” Kipping told Universe Today.

And the results? Well, they found that the fourth planet from the star, Trappist-1e, had a 99.3 percent chance of having a silicate-iron interior, “indicating strong evidence for an iron core,” they wrote. The core is thought to make up at least half of the planet. Earth’s core, by comparison, is just 15 percent of the planet.

Trappist-1e is about 0.9 times the size of Earth, and orbits well within the star's habitable zone, with a decent chance of liquid water on its surface. Having an iron core means Trappist-1e may well have a protective magnetosphere. On Earth our magnetosphere is pretty crucial to life, fending off radiation from the Sun and elsewhere in the universe.

Of the seven planets in the system, Trappist-1e is thought to be the “rockiest”. And it's not clear that the other planets have iron cores at all, meaning Trappist-1e might just be the most Earth-like world in the system.


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