What is TRAPPIST 1

The TRAPPIST-1 planets could be made of similar material

The seven exoplanets at the star Trappist-1, which were discovered in 2016 at a distance of about 40 light years from Earth, offer an insight into the enormous diversity of the planetary systems in our universe. As NASA's Jet Propulsion Laboratory JPL reported in a press release today, a current study that has just been carried out in the Planetary Science Journal it was published that the planets have remarkably similar densities. Researchers from the Universities of Bern, Geneva and Zurich, who are all members of the National Research Center NCCR PlanetS, are also involved in the study.

An ideal system for observation

In 2018, researchers led by Simon Grimm from the University of Bern, who is also involved in the current study, provided the most accurate calculation of the masses of the seven planets at the star TRAPPIST-1. These calculations had shown that the planets at TRAPPIST-1 are roughly the size and mass of the earth and are also rock planets - in contrast to planets that are dominated by gas, such as Jupiter and Saturn.

The planet family has been studied with several space- and ground-based telescopes. For example, the Spitzer Space Telescope, operated by NASA's JPL in Southern California, provided over 1,000 hours of targeted observations of the system before it was taken out of service in January 2020. “The targeted observations allowed us to use transit data for a much longer period of time than we had for the calculations from 2018. With the new data, we were able to refine the determination of the mass and density of all seven planets, and it turned out that the derived densities of the planets are even more similar than we had previously expected, ”says Simon Grimm. This result also shows how important it is to observe such exoplanetary systems over several years.

“The TRAPPIST-1 system is fascinating because we can learn about the diversity of rocky planets within a single system. And we can also learn more about a single planet by studying its neighbors, so this system is perfect for that, ”explains Caroline Dorn, astrophysicist at the University of Zurich and co-author of the current study.

Seven planets of similar density

In our own solar system, the densities of the eight planets are very different. The gas-dominated giants Jupiter, Saturn, Uranus and Neptune are larger but much less dense than the four rocky planets Earth, Venus, Mars and Mercury. The seven TRAPPIST-1 planets, on the other hand, all have a similar density and size, which makes the system very different from ours. That the planets around TRAPPIST-1 are all of similar density could mean that they all contain roughly the same composition of materials (such as iron, oxygen, magnesium and silicon). Since the planets are about 8% less dense than Earth, it can be assumed that their composition differs from that of our home planet. Based on this conclusion, the authors of the study hypothesized which mixture of components could give the TRAPPIST-1 planets this specific density.

Water and iron as possible explanations

The team investigated, among other things, whether the surface of the planets could be covered with water, which would change the overall density. "We have combined the models of the University of Bern for the inner workings of planets with the models of the planetary atmosphere that we are developing at the University of Geneva, and have thus been able to determine the water content of the seven TRAPPIST-1 planets with a precision that is literally unprecedented for this planetary category." says Martin Turbet, astrophysicist at the University of Geneva and co-author of the study. If the lower density could be explained by the occurrence of water, it would have to make up about 5% of the total mass of the four outer planets. For comparison: water makes up less than 0.1% of the total mass of the earth. "However, our models of the inner and atmospheric structure show that the three inner planets of the TRAPPIST-1 system are probably waterless and that the four outer planets do not have more than a few percent water, possibly in liquid form, on their surfaces" , said Turbet. As Eric Agol, astrophysicist at the University of Washington and lead author of the new study, says, this explanation is also unlikely because it would be an extreme coincidence if all seven planets contain exactly enough water that their densities are so similar.

Another way to explain the lower density is that the TRAPPIST-1 planets are similar in composition to Earth, but with a lower percentage of iron - about 21% compared to 32% on Earth, according to the study . Alternatively, the iron in the TRAPPIST-1 planets could be infused with a high proportion of oxygen, causing iron oxide or rust to form. The extra oxygen would reduce the density of the planets. The surface of Mars gets its red color from iron oxide, but like its three earthly siblings it has a core made of unoxidized iron. If the lower density of the TRAPPIST-1 planets were exclusively due to oxidized iron, then the planets would have to be rusty throughout and could not have any iron cores.

Eric Agol explains: "The lower density could be attributed to a combination of the two scenarios - that they contain less iron than Earth and some oxidized iron like that of Mars."

“The night sky is full of planets, and it is only in the last 30 years that we have been able to unravel their secrets - also to determine the habitability of planets, among other things,” says Caroline Dorn in conclusion.

The TRAPPIST-1 system

The first two confirmed planets in the TRAPPIST-1 system were identified in 2016 by the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. Subsequent observations by Spitzer and ground-based telescopes showed that the system has seven planets.

The planets were found using the transit method: researchers cannot see the planets directly (they are too small and weak), so they use telescopes to look for dips in the star's brightness that occur when the planets pass in front of the star.

Repeated observations of the star's dips in brightness and precise measurements of the orbital times of the planets enabled astronomers to make precise measurements of the planets' masses and diameters, which in turn were used to calculate their densities.