Concept of the artist of the planets TRAPPIST-1, emphasizing the measurements of their diameters and masses. The TRAPPIST-1 star system is home to the largest batch of roughly terrestrial-sized planets ever found outside our solar system. It is about 40 light years away. Image via NASA / JPL-Caltech / University of Geneva.
An exciting discovery by the Spitzer Space Telescope in 2017 revealed seven Earth-sized planets orbiting the nearby star TRAPPIST-1, less than 40 light-years away. Details about these planets have been hard to come by, but astronomers have wondered, are there any like Earth? Is it like Venus? Do some have clouds or even surface water? Late last week (January 22, 2021), astronomers added another piece to the Trappist-1 planets puzzle when they published an article in the journal Journal of Planetary Sciences detailing the compositions of these worlds. The team found that they were made of things similar to each other, but unlike the planets in our solar system.
Trappist-1 is a red dwarf star, by far the most common type of star in our Milky Way galaxy. Three of the Trappist-1 planets are firmly within the habitable zone of the star, also known as the Goldilocks Zone, in which liquid water may exist on the planet’s surface. Currently, the feeling among astronomers is that water is unlikely to be found on the surfaces of Trappist-1’s three innermost planets, and if the four outermost planets have surface water, there isn’t much. The discovery contradicts the early belief among astronomers that low-density planets like these must be abundant in water. It also raises questions about how the seven Earth-like and habitable exoplanets might be.
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Astrophysicist Martin Turbet of the University of Geneva is co-author of the new study. He said in a statement:
By combining the planetary interior models of the universities of Bern and Zurich with the planetary atmosphere models we are developing at the University of Geneva, we were able to evaluate the water content of the seven TRAPPIST-1 planets with literally no accuracy. precedents for this category. of the planet.
Our models of internal and atmospheric structures show that the three inner planets of the TRAPPIST-1 system are likely to be waterless and that the four outer planets have no more than a few percent water, possibly in liquid form, on their surfaces.
Now think for a moment about the eight planets in our solar system. They vary greatly in density, with the large gas and ice giants (Jupiter, Saturn, Uranus, Neptune) being less dense and the terrestrial planets (Mercury, Venus, Earth, Mars) more dense. Because the seven bodies of TRAPPIST-1 were known to share a relatively similar density to each other, the researchers encountered a problem when they compared these exoplanets to Earth. If any of them really had surface water, astronomers ’density estimates would have to change.
That is why, at the beginning of the study, researchers worked to determine whether the lowest density of the seven exoplanets was the result of water or the inner composition, which would determine whether the planets have a nature very or only similar to the earth . According to a research statement, exoplanets are about 8% less dense than Earth. For the lowest density of the planets to be the result of surface water, there should be about 5% of the mass of each planet as surface water.
That’s a lot! In comparison, water accounts for only 0.1 percent of the Earth’s mass. So you can see right away that these worlds are, from a compositional point of view, very different from Earth.
Martin Turbet is an astronomer at the University of Geneva and co-author of the study that calculated the densities of exoplanets in TRAPPIST-1. Image through the University of Geneva.
Eric Agol is an astronomer at the University of Washington and lead author of the study that calculated the densities of exoplanets in TRAPPIST-1. Image through the University of Washington newspaper.
But as it turns out, even the potential of a few percent of the water mass is questionable. Astronomer Eric Agol of the University of Washington is the lead author of the research. He added to the statement that the planets are likely to have less than a few percent of the mass of water; otherwise, similar densities within the group would be an extraordinary coincidence.
To answer this problem, the researchers examined the composition of the seven exoplanets, looking for similarities in the group and between the group and Earth. It is believed that most rocky planets are made of metals such as magnesium, iron and nickel, and not metals such as sulfur, oxygen and silicon. Due to the density discrepancy between the group and Earth, the researchers assumed that TRAPPIST-1 exoplanets could have a similar composition to Earth, but with significantly different relationships, e.g., a lower percentage of iron. Although iron accounts for 32% of the total mass of Earth, the study indicates that iron should account for approximately 21% of the mass of each TRAPPIST-1 exoplanet.
Agol told EarthSky:
The lower density indicates that these planets have a different composition and therefore a different history, than the terrestrial planets in our solar system. Maybe the star + disc started with less iron? Or maybe the iron core never formed and the iron remained oxidized in the mantle?
Artist’s concept of the similar densities of the 7 exoplanets of the TRAPPIST-1 star system. Their similar densities suggest that they all have a similar composition. The new study has led scientists to believe that the planets have rocky surfaces and iron-rich nuclei. Their nuclei are probably smaller than those of Earth, as the planets have a mass about 8% lower than that of Earth. Earlier theories considered deep oceanic layers on the surface of each planet or planets without a core. Image via NASA / JPL-Caltech / University of Geneva.
To determine the mass of each planet, scientists measured changes in their orbital periods. They measured the time it took each planet to orbit TRAPPIST-1 and cross in front of the star as seen from Earth. This process is called transit time. Combined with radius measurements from each planet, scientists were able to calculate the densities of the planets more accurately. By a comparison with the mass of the Earth, the team was able to calculate the percentage of iron likely to be present in each of the seven exoplanets.
Using data from the already deactivated Spitzer Space Telescope, astronomers determined that the 7 planets orbiting TRAPPIST-1 have similar densities. This finding helped astronomers characterize the composition of these exoworlds compared to Earth. Although the 7 planets are considered to be approximately the size of Earth, each is approximately 8% less dense than our home planet, i.e. their makeup, although potentially similar to Earth, in terms of ‘elements like iron, is significantly different in percentages. Image via NASA / JPL-Caltech / University of Geneva.
Combining these data with measurements of the radius of each planet, the scientists calculated the densities of the planets more accurately than before. By a comparison with the mass of the Earth, the team was able to calculate the percentage of iron likely to be present in each of the seven exoplanets.
Agol said:
We have found that they need about 2/3 of the iron compared to the Earth as they are of lower density.
It is a complicated question whether the differences between Earth and the TRAPPIST planets change the life potential somewhere in the TRAPPIST-1 system. While the presence of liquid water could signal the potential for life, other factors contribute to the suitability of the planet for life to emerge, prosper, and survive. On Earth, for example, a strong magnetic field protects our planet and our life from high-energy particles in the sun. Our atmosphere is full of enough oxygen and carbon, as well as other gases necessary for animal life, photosynthesis, and habitable surface temperatures.
Despite these challenges, Agol has not yet ruled out the life potential of TRAPPIST-1 exoplanets. He said:
The connection to habitability is not yet clear. The structure of planets can affect their ability to have plate tectonics, to carry a magnetic field, and other possible implications. These aspects of the Earth affect the presence and change of life on Earth.
Therefore, many questions remain to be explored.
The study was made possible by data sets taken by the Spitzer Space Telescope since the discovery of the stellar system more than four years ago. Between 2016 and 2020, when the telescope was decommissioned and data collection ceased, Spitzer recorded 1,075 hours of observation for TRAPPIST-1. Although the dismantling of Spitzer has suspended observations, it does not mark a permanent end to the TRAPPIST-1 studies.
Agol said:
If the James Webb Space Telescope is successfully launched and put into operation later this year, we plan to continue monitoring traffic with this telescope to try to detect atmospheres with traffic transmission spectroscopy. Each transit will provide time, so we can continue to perfect the masses of the planets.
Following the discovery of exoplanets orbiting TRAPPIST-1 in 2016, the Spitzer Space Telescope recorded 1,075 hours of observation in the stellar system, generating enough data for astronomers to accurately determine the densities and composition of each. planet. The Spitzer Space Telescope was decommissioned in January 2020. Concept of the artist through NASA / JPL-Caltech.
Conclusion: A new study of the seven planet-sized exoplanets in the TRAPPIST-1 system indicates that the seven planets are extremely similar to each other in composition, but potentially very different from Earth.
Source: Refinement of transit time and photometric analysis of TRAPPIST-1: masses, radii, densities, dynamics and ephemeris
Via Unibe.ch
