There is a large number of exoplanets, which have been detected during the recent two decades of the Universe’s exploration. However, only a fraction of them resemble the Earth according to their physical properties, such as mass, radius or distance to the host star. Density is another important factor influencing the potential habitability conditions on the surface of any planet. We, humans, certainly couldn’t live in a highly volatile environment, for example. So how to determine the odds that exoplanet’s terrain is solid enough to hold us above it?

Certainly, most of our readers would probably say: “Let’s get there first – we’ll surely solve other problems later!”. However, this question attracts the attention of astronomers because of the possibility to save resources needed to explore exoplanets by selecting those objects that may provide the most valuable observational data.

A study based on a similar idea has been recently presented at arXiv.org. The authors of the study analyzed 63 exoplanets, which were smaller than 4 Earth radii and had shorter than 100 days orbital periods. In this case, the duration of orbital periods was limited by the fact that planets with longer orbital periods are more difficult to detect and presently available data on such exoplanets is quite scarce. This research, conducted by scientists Lauren M. Weiss and Geoffrey W. Marcy from the University of California, Berkeley, was aimed to trace the relations between the physical parameters of potentially Earth-like exoplanets, such as mass, radius, and density, and to compare results with the previously established theories.

The authors of the study claim that, although our Solar system does not contain planets between the size of Earth and Neptune, the results of Kepler survey quite clearly indicate that such planets are common in our galaxy, occurring with orbital periods between 5 and 50 days around 24-25% of all stars. However, it is typically very difficult to measure the parameters of these planets using even the most modern equipment due to their small size. Therefore, it would seem logical to determine relations between various planetary parameters that could help making predictions based on limited observational data.

According to the scientists, exoplanet’s mass, radius and density are currently the most important parameters needed to investigate and understand the formation and evolution of these celestial bodies. A lot of other studies focused on the relationships between these parameters during the last years. The authors of the study decided to ‘update’ these empirical laws by supplementing the analysis with the masses of small planets that have been measured recently. Actually, restricting the empirical mass-radius or mass-density relations to small exoplanets could vastly improve the prediction accuracy of parameters of potential Earth-like planets.

The results of the study showed, that planet’s mass increases with radius nearly linearly for exoplanets smaller than 4 Earth radii and with orbital periods shorter than 100 days, indicating that larger planets have significantly more volatile composition compared to smaller planets. According to the determined empirical relation, the Earth-like density can be achieved for exoplanets at approximately 1.5 Earth radii. Smaller exoplanets would be more dense, likely featuring a rocky environment.

The authors of the study also provided exact empirical formulas that could be used to make predictions on mass-radius-density relations for the newly detected exoplanets.

By Alius Noreika, Source: www.technology.org