The seven planets orbiting the ultracold dwarf Trappist-1 are mostly rocky, and some of them may have more water than Earth.
A new study published in the journal Astronomy & Astrophysics, which focuses on the density of worlds in the Trappist-1 system, has shown the most accurate results to date. So, it was found that some of the planets are 5% covered with water - this is 250 times more than water on Earth.
“All Trappist-1 planets are very similar to Earth: they have a solid core surrounded by an atmosphere,” says Simon Grimm, an exoplanetologist at the University of Bern, in a letter to Space.com. "Trappist-1 is the planet most similar to Earth in terms of mass, radius and energy from a star."
Grimm and his colleagues became interested in the system after its discovery in 2016 and decided to study it using the transit time variation (TTV) method. By observing small variations in the periods in which the planet passed in front of the star from our point of view, this method allows researchers to conduct perhaps the most accurate studies of planetary masses and densities.
“TTV is now the only method for determining the masses, and therefore the density of planets like Trappist-1,” says Grimm.
Scientists used data from the Spitzer space telescope and several spacecraft from the European Southern Observatory in Chile to make detailed observations that would help study variations in planetary orbits.
A graph of the masses and radii of the Trappist-1 planets, Earth and Venus. Curves trace idealized compositions of rocky and water-rich environments (surface temperature fixed at 200 K) / University of Bern.
If the planet revolved around its star alone, then it would be exposed only to the gravitational effect of the star. But if there are two or more worlds in the system, the planets interact gravitationally, acting on each other with a force corresponding to their masses. These changes depend on planetary masses, distance and other orbital parameters.
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At the same time, "overcrowded systems" like Trappist-1 make it difficult to determine the effects of individual planets, since each of them affects its neighbors. It is easier to measure the planets of this system directly, since they rotate synchronously. Together, the seven exoplanets form a resonant chain that connects them all and suggests a slow, calm evolution.
“The Trappist-1 system is special because all its planets are in resonance,” explains Grimm.
The scientist used a simulation that he previously used to calculate planetary orbits and adapted it to the TTV analysis. Using more than 200 transits, his team simulated the masses and densities of the planets, simulating their orbits until the time the simulated transits matched the observations.
The researchers found that planetary densities range from 0.6 to 1.0 Earth's. Seven of them are rich in water, and on some it takes as much as 5% of the total mass. For comparison: water is only 0.02% of the Earth's mass.
Trappist-1b and c - the closest to the star - most likely have rocky cores and are surrounded by dense atmospheres.
Trappist-1d is the lightest of the seven planets, with a mass of about 30% of the mass of the Earth. Its low mass can be due to the expanded atmosphere, the ocean, or a layer of ice.
Trappist-1f, g, and h are far enough away from their star that the water on their entire surface is completely frozen. Thin atmospheres are unlikely to be able to contain heavier molecules like Earth.
In addition, there is Trappist-1e, which is the most Earth-like of the group. It is somewhat denser than our planet and, most likely, has a denser iron core. It may also lack a dense atmosphere, ocean, or ice sheet.
The researchers warned that these results say nothing about planetary habitability. However, the work can help scientists better understand the conditions at work in crowded systems and determine whether life can exist on the worlds of the Trappist-1 system.
Vladimir Guillen
