The concept of the planet artist TRAPPIST-1, highlighting the measurements of their diameters and masses. The TRAPPIST-1 star system is home to the largest batch of Earth-like planets ever found outside our solar system. It is about 40 light years away. Image by NASA / JPL-Caltech / University of Geneva.
A captivating discovery of the 2017 Spitzer Space Telescope revealed seven Earth-sized planets orbiting the nearby star TRAPPIST-1, less than 40 light-years away. Details about these planets were hard to find, but astronomers wondered, are they Earth-like? Are there people like Venus? Do they 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 a paper in the journal Journal of Planetary Sciences detailing the compositions of these worlds. The team found them 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 planets Trappist-1 are firmly in the habitable zone of the star – aka the Goldilocks Zone – where there may be liquid water on the planet’s surface. Currently, the feeling among astronomers is that water is unlikely to be found on the surfaces of the three innermost planets of Trappist-1, and if the four outer planets have surface water, it is not much. 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 similar and habitable the seven exoplanets could be to Earth.
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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 from 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 unprecedented accuracy for this category. of the planet.
Our models of internal and atmospheric structure show that the three inner planets of the TRAPPIST-1 system are probably waterless and that the four outer planets have no more than a few percent water, possibly in liquid form, on their surfaces.
Now let’s think for a moment about the eight planets in our solar system. They vary greatly in density, the large gas and ice giants (Jupiter, Saturn, Uranus, Neptune) being less dense, and the terrestrial planets (Mercury, Venus, Earth, Mars) being denser. Since it was known that the seven bodies in TRAPPIST-1 have 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.
Therefore, at the beginning of the study, the researchers worked 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 were very or just somewhat Earth-like in nature. 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, about 5% of the mass of each planet should exist as surface water.
That’s a lot! In comparison, water represents only 0.1% of the Earth’s mass. So you can immediately see that these worlds are compositionally quite 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 via the University of Geneva.
Eric Agol is an astronomer at the University of Washington and the lead author of the study that calculated the densities of exoplanets in TRAPPIST-1. Image from the University of Washington Daily.
But, as it turns out, even the potential for 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 in the statement that the planets will probably have less than a few percent of the mass of water; otherwise, the similar densities within the group would be an extraordinary coincidence.
To answer this dilemma, 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 non-metals such as sulfur, oxygen and silicon. Due to the density discrepancy between the group and Earth, the researchers assumed that TRAPPIST-1 exoplanets may have a structure similar to Earth, but with significantly different ratios, for example, a lower percentage of iron. While iron represents 32% of the total mass of the 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 appearance and therefore a different history from the terrestrial planets of our solar system. Maybe the star + disk started with less iron? Or maybe the iron core never formed and the iron remained oxidized in the mantle?
The artist’s concept regarding 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 led scientists to believe that the planets have rocky surfaces and iron-rich cores. Their cores are probably smaller than Earth’s, because the planets each have a mass about 8% smaller than Earth’s. Previous theories considered deep oceanic layers on the surface of each planet or planet without a nucleus. Image by NASA / JPL-Caltech / University of Geneva.
To determine the mass of each planet, scientists measured changes in their orbital periods. They measured how long it took each planet to orbit TRAPPIST-1 and pass in front of the star, as seen from Earth. This process is called transit synchronization. Combined with measurements of the radius of each planet, scientists were able to calculate the densities of the planets more accurately. Using a comparison with Earth’s mass, the team was able to calculate the percentage of iron that could be present in each of the seven exoplanets.
Using data from the decommissioned Spitzer Space Telescope, astronomers have determined that all 7 planets orbiting TRAPPIST-1 have similar densities. This discovery helped astronomers characterize the composition of these exow worlds compared to Earth. While all 7 planets are considered to be about the size of Earth, each of them is about 8% less dense than our home planet, which means that their makeup, although similar to Earth, in terms of elements such as iron, is significantly different in percentage. Image by NASA / JPL-Caltech / University of Geneva.
By combining these data with measurements of the radius of each planet, scientists calculated the densities of the planets more accurately than before. Using a comparison with Earth’s mass, the team was able to calculate the percentage of iron that could be present in each of the seven exoplanets.
Agol said:
I found that about 2/3 of the iron is needed compared to the Earth because they are of lower density.
Whether the differences between Earth and the TRAPPIST planets change the potential for life somewhere in the TRAPPIST-1 system is a complicated question. While the presence of liquid water could signal the potential for life, other factors contribute to the adequacy of a planet for life to appear, thrive and survive. On Earth, for example, a strong magnetic field protects our planet and life from high-energy particles from the sun. Our atmosphere is filled with enough oxygen and carbon, as well as other gases necessary for animal life, photosynthesis and living surface temperatures.
Despite these challenges, Agol has not yet ignored the life potential of TRAPPIST-1 exoplanets. He said:
The connection with habitability is not yet clear. The structure of the planets could 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.
So, 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 no longer collected data, Spitzer recorded 1,075 hours of observation for TRAPPIST-1. Although the decommissioning of Spitzer has taken its toll on observations, it does not mark a permanent end to TRAPPIST-1 studies.
Agol said:
If the James Webb Space Telescope is successfully launched and commissioned later this year, then we intend to continue monitoring transit with that telescope to try to detect atmospheres with transit transmission spectroscopy. Each transit will give us time 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 stellar system, generating enough data for astronomers to accurately determine the densities and structure of each planet. The Spitzer Space Telescope was decommissioned in January 2020. The artist’s concept through NASA / JPL-Caltech.
Conclusion: A new study of the seven Earth-sized exoplanets in the TRAPPIST-1 system indicates that all seven planets are extremely similar to each other in makeup, but potentially quite different from Earth.
Source: Refinement of transit time and photometric analysis of TRAPPIST-1: masses, radii, densities, dynamics and ephemeris
Via Unibe.ch
