- A Novel Method for identifying Exoplanetary Rings
- Authors: J. I. Zuluaga, D. M. Kipping, M. Sucerquia and J. A. Alvarado
- First Author’s Institutions: 1) Harvard-Smithsonian Center for Astrophysics, 2) FACom – Instituto de Fisica – FCEN, Universidad de Antioquia, Colombia, 3) Fulbright Visitor Scholar
Today’s question: Do you like rings?
Let’s start this Astrobite a bit different than usual. Before you read on, please click here and tell us about your favorite planet… Are you done? Good! The reason why I’m asking has to do with the ring structure around Saturn. Assuming you like Saturn’s rings, you are probably also curious whether exoplanets reveal ring structures, too, and how those can be detected. The answer to the first question is ‘Yes!’ and if you like Saturn, you probably fell in love with this planet. As Ruth told you, the authors put a lot of work into finding an explanation for the observed profile by comparing different transit profiles before they concluded that the planet hosts circumplanetary rings. That’s the point where today’s authors come into play. They present a model, which simplifies the detection of exorings.
Characteristic features of planets hosting circumplanetary rings
Figure 1 illustrates their underlying thoughts. The yellow area represents the host star and I guess it is easy to spot the planet with its circumplanetary rings moving from left to right. When the planet moves together with the rings to the right of position (when it starts its egress), one side of the rings will not hide the light of the host star anymore and thus the observed flux from the host star slowly increases. The planet itself leaves the transiting area a bit later, which then leads to a steeper increase in flux until the planet does not hide any of the light anymore. At that point the increase in flux is shallower again due to the other side of the ring structure, which still covers part of the light from the host star before this side also stops transiting at position . When the planet and its rings start transiting (the ingress), the process is reversed. The illuminating area of the star gets more hidden and the flux decreases. Principally there are two different intervals for a transit of a planet:
- The time from the entering of the planet in the transiting region until the time the planet does not cover any light of the host anymore (corresponding to or just “transit”).
- The time in which the entire planet covers the light of the host (corresponding to or “full transit”).
Considering now also the rings, four time intervals exist, namely , , and (as shown in the graph of Fig. 1). The relative flux difference during the transit of the planet with and without the rings compared to the unperturbed flux of the star is called transit depth (. In practice, the slopes corresponding to the ingress/egress of the ring and the ingress/egress of the planet are difficult to distinguish from each other. The authors stress that exoplanets with rings could be mistakenly interpreted as ringless planets. Assuming the star and the planet as being spherical and a uniform shape of the rings, the transit depth simply becomes the ratio of the area hidden by the planet including the rings and the projected surface area of the star (). If the rings’ plane is orientated perpendicular to the line of sight and the observed transit depth of the ring is interpreted as the transit depth of a ringless planet, the overestimated radius of the planet leads to an underestimation of the planetary density (as shown in Fig. 2).
Additionally, the authors repeat the derivation that stellar density is proportional to reveals another potential misinterpretation. Figure 3 illustrates the effect on stellar density of different ring inclinations. In the case of a ring plane perpendicular to the orbital direction, the stellar density would be overestimated. However, in the more common case of alignment of the rings’ plane with the orbiting plane, the increased difference leads to an underestimation of the stellar density.
A publicly available code allows for hunting
Taking into account the described phenomena of anomalous depth and the photo-ring effect to estimate probability distribution functions for the occurrence of the effects, the authors developed a computer code, which you can use to go out hunting for exoring candidates! They suggest that you focus on planets (candidates) with low densities and use their publicly available code (http://github.org/facom/exorings) to do so. But here’s a disclaimer: The code can only find candidates. To confirm their existence you still need to do a complex fit of the light curve. That’s something the code cannot do for you.