Researchers are trying to figure out what the planets are made of, which are increasingly found outside our solar system, writes eurekalert.org.
Is there a second Earth in space? Our knowledge of planetary systems is constantly growing thanks to new technologies. To date, 3,700 planets have been discovered outside our solar system. The planetary masses and radii of these exoplanets can be used to determine their average density, but not their exact chemical composition and structure. Therefore, an intriguing question still remains open: what these planets look like.
“In theory, we can assume all sorts of things, such as a world of pure water, absolute mountains or a planet with an atmosphere of hydrogen and helium …” explains Michael Lozovsky, a doctoral student with Professor Ravit Helling's group at the Institute of Computational Sciences at the University of Zurich.
Lozovsky and coworkers used databases and statistical tools to characterize exoplanets and their atmospheres. They are usually surrounded by a volatile layer of hydrogen and helium. However, the available data does not allow researchers to determine the exact structure of the planet, since planets of the same mass and radius can have different compositions. In addition to the accuracy of the data related to mass and radius, the research team also investigated the putative internal structure, temperature and reflected radiation from 83 of the 3,700 known planets for which their masses and radii are accurately determined.
“We used statistical analysis to determine the limits of possible compositions. Using a database of detected exoplanets, we found that every theoretical planetary structure has a 'threshold radius' - a radius above which a planet cannot have a specific composition,”explains Michael Lozovsky. An important factor in determining the threshold radius is the number of elements in the gaseous layer that are heavier than helium, the percentage of hydrogen and helium, and the distribution of elements in the atmosphere.
Researchers from the Institute of Computational Science have found that planets with a radius of up to 1.4 times that of Earth (6371 km) can have a composition similar to Earth. Planets with radii above this threshold have a higher proportion of silicates or other lightweight materials. Most planets with a radius greater than 1.6 Earth's radius must have a layer of hydrogen or water in addition to their rocky core, while radii larger than 2.6 Earth's mean that planets cannot be water worlds and must therefore be surrounded atmosphere. Planets with radii greater than 4 Earth radii are expected to be very gaseous and consist of at least 10 percent hydrogen and helium, similar to Uranus and Neptune.
The research results provide new insight into the development and diversity of the planets. One particularly interesting threshold concerns the difference between large Earth-like planets, otherwise called super-Earths, and small gas planets, so-called mini-Neptunes. According to researchers, this threshold is in a radius three times the radius of the Earth. Thus, below this threshold, Earth-like planets can be found in the vast expanse of the galaxy.