If our solar system is any indication, planets with rings around them are quite common, but finding extrasolar ringed planets has proven difficult. Now astronomers have developed a surprisingly simple method for detecting these distant "exorings." It'll only be a matter of time before we find the next Saturn.
Despite the hundreds of exoplanetary discoveries, astronomers still haven't been able to definitely prove that any of them are ringed. Recently, a team of astronomers announced the discovery of a massive disc orbiting Super Jupiter J1407b, but the exact nature of this object is under contention. It's possible, for example, that the planet is actually a brown dwarf and its apparent "rings" a miniature version of a protoplanetary disc.
In our solar system, Saturn, Jupiter, Uranus, and Neptune all have rings. It's fair to assume, therefore, that other gaseous planets also have rings; we just have to find a way to "see" them.
To do so, astronomer Jorge Zuluaga, an associate professor at the Institute of Physics of the University of Antioquia, along with the help from his colleagues at the Harvard-Smithsonian Center for Astrophysics, has devised a novel two-step method to quickly analyze the existing database to create a short-list of exoring candidates worthy of further analysis.
Schematic representation of the transit of a ringed planet in front of its star. When compared with the light curve of an non-ringed analogue (dashed line) the transit of a ringed planet is deeper (the relative flux diminish by a larger fraction) and longer. Caption and image credit: Zuluaga/University of Antioquia.
According to Zuluaga, a ringed-planet should produce a "deeper" and longer transit than a non-ringed object. A transit happens when astronomers on Earth happen to catch a glimpse of an exoplanet traveling in front of its host star, which is detected as a slight dimming.
For instance, if a Jupiter-like planet had a ringed system like Saturn's, it would appear to astronomers as being much larger than it really is. Objects that appear bigger than expected are tagged as "false positives." Zuluaga says it's here where astronomers should begin their search.
Magnitude of the so-called Photo-ring effect predicted at different projected inclinations and tilts (small "saturns"). ZuluagaCaption and image credit: Zuluaga/University of Antioquia.
Secondly, astronomers can exploit what's known as the "astereodensity-profiling effect." A release from the University of Antioquia explains:
Planetary transits have a wealth of information, not only about the planet, but about the star itself. If we combine the transit depth (that depends on the size of the star) and the duration of the transit (that depends on orbital velocity and hence on the stellar mass) we can estimate the density of the star. This transit-based stellar density could be then compared with the density measured independently with another method (asteroseismology for example). If they do not coincide, something is really wrong with our assumptions about the planet or its orbit. Zuluaga, Kipping et al. have demonstrated that the presence of Rings leads to a systematic underestimation of stellar density. This effect is called the "Photo-ring effect".
Taken together, these two methods are still not enough to identify remote exorings. Once a list of candidates is created, however, astronomers can use more powerful and efficient methods to confirm the existence of rings around those planets.
This study has been accepted for publication in Astrophysical Journal Letters: "A Novel Method for Identifying Exoplanetary Rings".