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Chasing for the Cheshire Planetary Rings: Scattered stellar light may reveal the existence of ringed

What do (hypothetical) ringed exoplanets and Cheshire, the cat of Alice’s adventures in Wonderland, have in common?

Cheshire, the magic cat in the fantabulous world of Alicia’s Wonderland, is capable of disappearing at will leaving visible its big smile only. Analogically speaking, some exoplanets, in particular those with rings around, may unexpectedly vanish and appear again on a different point of their orbit. How this would be possible is explained by Mario Sucerquia in his most recent publication from the Núcleo de Formación Planetaria (NPF) in Chile.


Even though no Saturn-like exoplanets have been detected to date, for many years a considerable theoretical effort has been put to get the grounds ready for such a discovery. Firstly, when a planet passes in front of its host star the stellar brightness is diminished, and if this event is aligned to an observer on the Earth sometimes the signal will be strong enough for our telescopes to spot the planet. Secondly, such a change in the stellar brightness is proportional to the planet’s size, so if the orbiting planet bears rings around itself the brightness drop will be much more evident, as the rings will occupy a bigger area of the stellar disc.


In this new publication titled “Scattered light may reveal the existence of ringed exoplanets”, published in the scientific journal Monthly Notices of the Royal Astronomical Society (MNRAS), researchers from the NPF: Mario Sucerquia, Matías Montesinos, and Amelia Bayo; in collaboration with Jaime A. Alvarado-Montes from Macquarie University (Australia) and Jorge I. Zuluaga from Universidad de Antioquia (Colombia) propose a complementary method for detecting and characterizing the orbits of planets with rings. Such a method does not require planetary transits, favouring the detection of face-on-oriented planetary systems concerning observers on (and around) the Earth.


The detectability of planetary rings, at least for the present research stage, requires them to be slightly tilted with respect to the parent planet’s orbit and relatively close to the host star. The first condition is nothing new. For instance, planetary rings here in our Solar System have inclinations of about 27 and 96 degrees as in the case of Saturn and Uranus, respectively. The second condition is more hypothetical, as it is thought that close-in giant planets may bear complex rings-systems orbiting around them.


How can those ringed planets from the aforementioned publication appear/reappear out of nowhere? The answer lies on the fact that the rings preserve their inclination while the planet orbits around the star. This makes the planet on its passing by the periastron (point q) reflect a significant fraction of stellar light towards a distant observer, while the scattering occurs in the opposite direction to the observer when the planet passes by the apoastron (point Q), as shown in the next figure. On its own, the planet keeps constantly reflecting a small fraction of stellar light towards the observer as it completes its orbit. The intensity of reflected light will depend on different physical and orbital parameters of the planet+rings system, including but not limited to: the planet’s and rings’ size and albedo; the orbital eccentricity, and the mean distance from the planet to the star.



How is this effect perceived from the Earth?


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As seen from the Earth, planetary transits produce a dip in the stellar brightness, whereas the systems proposed by Sucerquia et al. would give rise to an unusual phenomenon that could be detected as a gradual increase/decrease in the intensity of stellar light (i.e. due to the high reflection produced by the rings). After seeing the upper panel of the next figure, we may think of this effect as a kind of an upside-down transit.



High-precision/stability photometric observations may discern a very interesting and peculiar event: they would witness how along its orbit, a tenuous point (the planet) would unexpectedly increase its brightness, to gradually disappear afterwards and leave a subtle trace only, corresponding to the reflection of stellar light owing to the planetary atmosphere.





Furthermore, by using the measured change in the stellar brightness it is possible to infer and constrain some orbital and physical characteristics for these ringed exoplanets, such as the orbital period, the mean distance from the planet to the star, the orbital eccentricity, and the size of both the planet and its rings. The researchers state that, in the future, it will also be feasible to determine some exotic properties like planetary magnetic fields using the light polarized by the rings.

Detecting this type of systems requires high-sensitivity instruments. However, space-based telescopes such as TESS or CHEOPS (already in orbit) are sensitive enough to detect them. Also, future telescopes like the James Webb Space Telescope (JWST), whose sensitivity is estimated to be of a few parts per million (ppm), portend a significant amount of discoveries including moons or rings, like those described so far.


Have wee sen this before?


Despite how exotic this phenomenon may look, there exists some historical data that show similar occurrences when Galileo Galilei used his telescope in 1610 to point at Saturn, describing what seemed to be a “planet with ears” that faded as time went by. This event can be explained by using two phenomena: the Saturn rings’ inclination and their projected area towards the Earth.


Galileo's Observations Créditos: Getty images

Scheme that shows how Saturn’s appearance changes due to the changing positions of the Earth (E) and Saturn while they orbit the Sun (G), according to Christiaan Huygens. Image credits: Public Domain.

To summarize, according to Carroll in his novel, the cat named Cheshire can gradually disappear until leaving his big smile only, making Alice notice that many times she could have seen a cat without a smile but never a smile without a cat. Likewise are the Cheshire rings: we may have seen thousands of planets without rings, but with the newly proposed method it will be possible to first see the rings before the planet.

Preprint publication: https://arxiv.org/abs/2004.14121

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