Astronomers have confirmed that the star J1407 seems to have a companion with a gigantic ring system, inside which an “exomoon” might be forming.

In the early 1600s, when Galileo first pointed his telescope at Saturn, he described the planet's sweeping rings as “ears.” Since that time, astronomers have closely studied Saturn’s ring system, revealing an intricate system composed of a vast number of ice and rock particles, ranging in size from 1 centimeter to 10 meters, or about the size of the tip of your pinky finger to the size of a small house. These particles are not evenly spread out near Saturn, either; instead, there are large gaps in the rings that have been cleared out by the moons of Saturn.

Ring formation is thought to be fairly common for larger planets, as rings are found around all of the outer solar system planets. (We’ve even seen them around an asteroid.) Furthermore, astronomers think that the giant planets’ major moons formed in souped-up versions of the ring systems we see today, like the planet-forming disks observed around some young stars. But they’ve only observed one good candidate for such a circumplanetary disk, around a companion to a star called 1SWASP J140747.93-394542.6 (hereafter J1407).

The most detailed glimpse of that ring system, or of any ring system outside of our own solar system, was just obtained by Matt Kenworthy (Leiden Observatory, The Netherlands) and Eric Mamajek (University of Rochester).

Kenworthy and Mamajek, in a paper recently accepted by the Astrophysical Journal, describe observations of the nearby, 16-million-year-old Sun-like star called J1407. In 2012, Mamajek, Kenworthy, and their colleagues discovered a large planetary companion to the star by identifying an eclipse of the star in its light curve. Yet, as the companion, named J1407b, passed between Earth and its host star, a rich and intricate eclipse pattern occurred, in deep contrast to the typical dips seen for a single companion. (Watch a video of the complicated dips at the bottom of this page, or on the University of Rochester's news cite.) The team attributed this behavior to the presence of a four-ring system around J1407b with at least two gaps.

The pair has performed a new analysis of the data, obtained by the SuperWASP planet search. This analysis shows a rich structure in the rings, with at least 30 separate ring structures. The ring structure itself is gigantic by our own solar system’s standards, with a diameter of 1.2 Earth-Sun distances and a mass on the order of 100 Moons, or about the same mass as Earth (Saturn’s rings total roughly one-thousandth the Moon’s mass). If these rings were around Saturn, they would have a diameter nearly 14 times larger than the full Moon and would appear bright enough to be seen in the daytime sky.

“This planet is much larger than Jupiter or Saturn, and its ring system is roughly 200 times larger than Saturn’s rings are today,” says coauthor Mamajek. “You could think of it as kind of a super Saturn.”

An Exomoon in the Making?

Perhaps even more exciting is Kenworthy and Mamajek’s confirmation of one large, clear gap in the ring around J1407b. From our understanding of Saturn’s ring structure, we know that moons can carve out gaps between rings. “One obvious explanation is that a satellite formed and carved out this gap,” says Kenworthy.

Astronomers can use the size of a gap to infer the mass of the moon responsible; the gap in J1407b’s disk implies a satellite mass between that of Earth and Mars, with an orbital period of approximately two years.

This makes the companion to J1407b a very exciting first in the field of exomoons. Despite some tantalizing results, Kepler observations have produced no definitive identification of an exomoon. The search for a direct detection of an exomoon is ongoing, with high-precision observations from the Kepler spacecraft fueling the search.

Astronomers estimate that the orbital period of J1407b is about ten years, but unfortunately they don’t exactly when the next eclipse will happen. “It could be tomorrow, next year, a few years, we don’t know,” says Mamajek. Amateur observations reported to the American Association of Variable Star Observers (AAVSO) have already ruled out some orbital periods for J1407b, and his team encourages amateur astronomers to continue to help them monitor J1407. Such additional observations would help detect the next eclipse by the rings and also constrain the companion’s period, size, and mass, as well as the size and mass of the rings. Observations of J1407 can be reported to the American Association of Variable Star Observers (AAVSO).

This glimpse of a ring around a planet in a young solar system gives astronomers a valuable glimpse back in time. They can use these observations to constrain models of planet formation and perhaps better understand how our own solar system evolved with time.

In the meantime, the astronomers are searching other photometric surveys, looking for eclipses by yet-undiscovered ring systems. Kenworthy explains that finding eclipses from more objects like J1407’s companion “is the only feasible way we have of observing the early conditions of satellite formation for the near future. J1407’s eclipses will allow us to study the physical and chemical properties of satellite-spawning circumplanetary disks.”

Reference: M. A. Kenworthy and E. E. Mamajek. "Modeling Giant Extrasolar Ring Systems in Eclipse and the Case of J1407b: Sculpting by Exomoons?" Accepted to Astrophysical Journal.

Below, you'll find a simulated video showing the complicated light pattern the team observed as J1407b's rings passed in front of the star. Credit: University of Rochester