Sunday, October 6, 2013

Rings around Warm Exoplanets

Ring systems exist around all of the giant planets in our solar system - Jupiter, Saturn, Uranus and Neptune. The rings around Saturn are the most prominent and consist primarily of centimetre-sized to meter-sized icy bodies. Since ring systems are ubiquitous around the giant planets in our solar system, they are also expected to exist around extrasolar planets.

In a paper by Schlichting & Chang (2011), the authors investigate the types of rings systems that could exist around extrasolar planets that orbit much closer to their host stars than the giant planets in our solar system. The focus is on extrasolar planets located at distances of one AU or less from their host stars. The authors coin such extrasolar planets, if they harbour ring systems, “warm Saturns,” since they exist close enough to their host stars where temperatures are too high for water and other volatiles to exist as solid ice grains. Instead, such planetary ring systems can only consist primarily of rocky materials and will differ from those in our solar system.

Artist’s impression of a gas giant planet with a ring system around it. Credit: Gabriel Gajdos.

In our solar system, Poynting-Robertson drag is not important for Saturn’s rings due to Saturn’s huge distance from the Sun, although it somewhat drives the evolution of ring particles around Jupiter. Nevertheless, Poynting-Robertson drag is expected to be significant for ring systems around warm Saturns due to the large amount of stellar insolation they receive. Poynting-Robertson drag has the effect of causing the orbit of a ring particle to decay. If mutual shadowing of ring particles can be ignored such that the orbital evolution of each ring particle can be considered independently, then ring particles need to be one meter or larger for long-lived rings (i.e. lifespan exceeding ~100 million years) to exist around warm Saturns.

However, if the ring system is optically thick, then orbital decay of ring particles due to Poynting-Robertson drag depends only on the surface mass density of the ring and significantly smaller ring particles can survive over long periods. In addition, if the ring system has a small inclination (i.e. radiation strikes the ring system at an almost edge-on angle), then the ring system will have a yet longer lifespan since a smaller effective ring surface area would be exposed to radiation from the host star.

In contrast to ring systems around the giant planets in our solar system, a ring system around a warm Saturn may appear wrapped due to competing tidal effects from the planet itself and from the host star. Observations of warped rings around a planet can reveal information about the planet’s interior structure.

Hilke E. Schlichting and Philip Chang, “Warm Saturns: On the Nature of Rings around Extrasolar Planets That Reside inside the Ice Line”, 2011 ApJ 734: 117