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Is there a second Earth in space? Our knowledge of far-distant planetary systems continues to grow as new technologies sharpen our gaze in space. To date, 3,700 planets have already been discovered outside our solar system. The planetary masses and rays of these exoplanets can be used to derive their average density, but not their exact chemical composition or structure. The intriguing question of what these planets might look like is therefore always open.
"Theoretically, we can assume various compositions, such as a world of pure water, a world of pure rock and planets with atmospheres of hydrogen and helium, and explore what rays are expected, "explains Michael Lozovsky, doctoral candidate in Prof Ravit Helled's group at the Institute for Computational Science at the University of Zurich.
Thresholds for planetary composition
Lozovsky and his collaborators used databases and statistical tools to characterize exoplanets and their atmospheres. These are quite common and surrounded by a volatile layer of hydrogen and helium. However, directly measured data previously did not allow researchers to determine the exact structure, as different compositions could give the same mass and radius. In addition to the accuracy of the mass and radius data, the research team also investigated the assumed internal structure, temperature, and reflected radiation in 83 of the 3,700 known planets for which the masses and the rays are well determined.
"We used statistical analysis to set limits on possible compositions. Using a database of detected exoplanets, we found that each theoretical planetary structure had a "threshold radius", a radius above which no planet of this composition existed, "explains Lozovsky. The amount of gaseous layer elements heavier than helium, the percentage of hydrogen and helium, as well as the distribution of elements in the atmosphere are important factors for the determination of the threshold radius.
Super-Terres and mini-Neptunes
Researchers at the Institute for Computational Science have discovered that planets whose radius is 1.4 times larger than Earth's (6,371 km) can be of the same nature as the Earth, that is, say they have a similar composition to the Earth. Planets with a radius greater than this threshold contain a greater proportion of silicates or other light materials. Most planets whose radius is greater than 1.6 Earth's radius must have a layer of hydrogen, helium or water in addition to their rocky core, while planets larger than 2.6 radii Earth can not be aquatic worlds and could be surrounded by an atmosphere. Planets whose rays are greater than 4 rays from the Earth should be very gaseous and consist of at least 10% hydrogen and helium, as for Uranus and Neptune.
The results of the study provide new information on the development and diversity of these planets. A particularly interesting threshold is the difference between the large terrestrial planets – also called super-earths – and the small gas planets, also called mini-Neptunes. According to the researchers, this threshold is within a radius of three times that of the Earth. Below this threshold, it is therefore possible to find Earth-like planets in the vast expanse of the galaxy.
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