Studying moons in Solar System as analogs for compact exoplanetary systems

Posted on September 12, 2013

Currently there are more than 800 known exoplanets, but this number may expand greatly according to the trends in the field of astronomy. The data from the Kepler space observatory (before the experienced technical problems) has already provided information about potential exoplanets, including a number of planetary systems with more than one planet.

Astronomers admit, that it is rather hard to research these planets due to the limited precision of the observations, although there are many astrophysical parameters which are particularly interesting to scientists: the orbital structure, dynamical stability, chemical composition of exoplanets.

A group of scientists from San Francisco State University, NASA Exoplanet Science Institute at Caltech, and L’Université de Bordeaux (France) performed a study to compensate the technical limitations of the equipment. They analyzed similarities between the Solar System moons and so-called compact multi-exoplanetary systems. The results of this research were published at arXiv.org.

What is a compact exoplanetary system? Its main feature is the fact that most of those planets (at least the ones that have been identified) are located in close proximity to the host star. For example, one such system – Kepler-11 – contains six detected planets and all of them are within 0.5 AU (astronomical unit) of their mother-star. The authors claim that this is not the only example, and that there are identified exoplanets which are so close to their stars that their orbital period is considerably shorter compared to the Mercury’s period.

The scientists proposed an idea in their study, according to which the compact exoplanetary systems should be somewhat similar to the moons in the Solar System, or at least this assumption should be valid for the largest moons orbiting the ‘local’ gas giants. They analyzed data collected during the Kepler mission to prove this hypothesis.

A visualization of the radii and semi-major axes of the Solar System moons and Kepler planets discussed in this study. The radii are all scaled in units of the host, whether the host be a planet or a star. Similarly, the separations of the objects f rom their host are scaled in units of the host radii. In proportionality to the h ost, the largest moon is Triton and the largest planet is Kepler-30c.

A visualization of the radii and semi-major axes of the Solar System moons and Kepler planets discussed in this study. The radii are all scaled in units of the host, whether the host be a planet or a star. Similarly, the separations of the objects from their host are scaled in units of the host radii. In proportionality to the host, the largest moon is Triton and the largest planet is Kepler-30c. Credit: Courtesy of the researchers.

The analysis was performed by scaling the radii and semi-major axis of each body to the radius of the host: a giant planet in the case of the Solar System and a star in the case of the exoplanets identified by the Kepler survey. The scaled data was compared statistically, and also some additional parameters were calculated for the accuracy purposes, including the determination of the tidal dissipation timescales for the moons and exoplanets.

The study results showed that there is a correlation between the scaled radial and axial properties of exoplanets in compact multi-planet systems, and that there is a significant similarity to the analogous correlation for the moons of the Solar System gas giants. According to the authors, the slight differences in the correlation gradients could be explained by (faster) inward migration of the moons towards their host planet.

The team hopes that it will be possible to make additional comparisons between the densities of Solar System moons and exomoons in the future research. This could provide further insights on the similarity of these celestial bodies.

Certainly, a small distance from the star is not a very practical trait from human point of view, since it precludes the exoplanet’s potential to support life. However, compact exoplanetary systems could provide a lot of useful data about formation and evolution of planets in star systems with different configurations. Further research based on this data could be useful for better understanding of our own Solar System’s history.

By Alius Noreika, Source: Technology.org