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Artist’s concept of the TRAPPIST-1 planets, highlighting the measurements of their diameters and masses. The TRAPPIST-1 star system is home to the largest cluster of roughly Earth-sized planets ever found outside of our solar system. It’s 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 nearby star TRAPPIST-1, less than 40 light years away. It was difficult to find details of these planets, but astronomers wondered if it looked like Earth? Are some like Venus? Are there clouds or even surface water? At the end of last week (January 22, 2021), astronomers added another piece to the puzzle of the planets of Trappist-1, when they published an article in the magazine Journal of Planetary Sciences detailing the compositions of these worlds. The team discovered that they were made of materials similar to each other, but quite different from 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 lie firmly within the star’s habitable zone – aka the Goldilocks zone – where liquid water can exist on a planet’s surface. At the moment, the feeling among astronomers is that water is unlikely to be on the surfaces of the three innermost planets of Trappist-1, and, if the four outermost planets have any surface water is not a lot. The discovery contradicts an early belief among astronomers that low density planets like these must be abundant in water. It also raises questions about how the seven exoplanets might be Earth-like and habitable.
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Astrophysicist Martin Turbet from the University of Geneva is co-author of the new study. He said in a statement:
By combining the planetary interior models from the Universities of Bern and Zurich with the models of the planetary atmosphere that we are developing at the University of Geneva, we were able to assess the water content of the seven TRAPPIST-1 planets with literally unprecedented precision. for this category. of the planet.
Our internal and atmospheric structure models 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 surface.
Now let’s think about the eight planets in our solar system. Their density varies considerably, the large gas and ice giants (Jupiter, Saturn, Uranus, Neptune) being less dense and the terrestrial planets (Mercury, Venus, Earth, Mars) more dense. Since the seven bodies of TRAPPIST-1 were known to share a relatively similar density with each other, the researchers were faced with a dilemma when comparing these exoplanets to Earth. If any of them actually had surface water, astronomers’ density estimates would have to change.
That is why, at the start of the study, the researchers set out to determine whether the lower density of the seven exoplanets was the result of water or their inner composition, both of which would determine whether the planets are very or only quite similar to Earth. . According to a research statement, exoplanets are about 8% less dense than Earth. For the lower density of the planets to be the result of surface water, approximately 5% of each planet’s mass would have to exist as surface water.
It’s a lot! In comparison, water represents only 0.1% of the land mass. So you can see right away that these worlds are very different in composition from Earth.
Martin Turbet is an astronomer at the University of Geneva and co-author of the study which calculated the densities of exoplanets in TRAPPIST-1. Image via 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 via The Daily at the University of Washington.
But, in the end, even the potential of a few percent of the water body is questionable. University of Washington astronomer Eric Agol is the lead author of the research. He added in the statement that the planets are likely to have less than a few percent of water mass; otherwise, the similar densities within the group would be an extraordinary coincidence.
To answer this dilemma, the researchers looked at the makeup of the seven exoplanets, looking for similarities within the group and between the group and Earth. It is believed that most rocky planets are made up of metals like magnesium, iron, and nickel, and non-metals like sulfur, oxygen, and silicon. Due to the density gap between the group and Earth, the researchers speculated that the TRAPPIST-1 exoplanets might have a similar composition to Earth but with significantly different ratios, for example a lower percentage of iron. While iron accounts for 32% of the total mass of Earth, the study indicates that iron should represent about 21% of the mass of each TRAPPIST-1 exoplanet.
Agol told EarthSky:
The lower density indicates that these planets have a different makeup, 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 got oxidized in the mantle?
Artist’s concept of the similar densities of the 7 exoplanets in 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 planets have rocky surfaces and cores rich in iron. Their cores are probably smaller than those of Earth, as the planets each have a mass about 8% less than that of Earth. Previous theories considered the deep ocean layers on the surface of each planet, or the coreless planets. 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 for each planet to orbit TRAPPIST-1 and to cross in front of the star as seen from Earth. This process is called transit time. Combined with measurements of the radius of each planet, scientists were able to calculate the densities of the planets with more precision. Using a comparison to 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 now decommissioned Spitzer Space Telescope, astronomers determined that the 7 planets orbiting TRAPPIST-1 had similar densities. This discovery has helped astronomers characterize the makeup of these exoworlds in relation to Earth. Although the 7 planets are considered to be roughly the size of Earth, they are each about 8% less dense than our home planet, which means that their makeup, while potentially similar to Earth, in terms of elements like iron, is significantly different in percentages. Image via NASA / JPL-Caltech / University of Geneva.
By combining this data with measurements of the radius of each planet, scientists calculated the densities of the planets more accurately than before. Using a comparison to 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 about 2/3 of the iron is needed relative to Earth because it is a lower density.
Whether the differences between Earth and the TRAPPIST planets alter the potential for life somewhere in the TRAPPIST-1 system is a complicated question. While the presence of liquid water might signal the potential for life, other factors contribute to a planet’s ability for life to arise, thrive, and survive. On Earth, for example, a strong magnetic field protects our planet and life from the high-energy particles of the sun. Our atmosphere is filled with sufficient oxygen and carbon as well as other gases necessary for animal life, photosynthesis, and living surface temperatures.
Despite these challenges, Agol has not yet ruled out the life potential of TRAPPIST-1 exoplanets. He said:
The link with livability is not yet clear. The structure of the planets could affect their ability to have plate tectonics, carry a magnetic field, and other possible implications. These aspects of Earth affect the presence and change of life on Earth.
So there are still many questions to explore.
The study was made possible by datasets taken by the Spitzer Space Telescope since the discovery of the star system more than four years ago. Between 2016 and 2020, when the telescope was decommissioned and stopped collecting data, Spitzer logged 1,075 hours of observation for TRAPPIST-1. Although the downgrading of Spitzer put a brake on observations, it does not mark the final end of the TRAPPIST-1 studies.
Agol said:
If the James Webb Space Telescope is successfully launched and put into service later this year, we plan to continue monitoring transits with this telescope to try and detect atmospheres with transit transmission spectroscopy. Each transit will provide timing, so that we can continue to refine the masses of the planets.
After the discovery of exoplanets orbiting TRAPPIST-1 in 2016, the Spitzer Space Telescope recorded 1,075 hours of observation on the star system, generating enough data for astronomers to accurately determine the densities and composition of every planet. The Spitzer Space Telescope was decommissioned in January 2020. Artist’s concept via NASA / JPL-Caltech.
Bottom line: A new study of the seven Earth-sized exoplanets in the TRAPPIST-1 system indicates that the seven planets are extremely similar to each other in their makeup, but potentially quite different from Earth.
Source: Refinement of the photometric and temporal analysis of the transit of TRAPPIST-1: masses, radii, densities, dynamics and ephemeris
Via Unibe.ch
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