Might there be gas giant planets somewhere with moons as large as the Earth, or at least Mars? Projects like the Hunt for Exomoons with Kepler (HEK) are on the prowl for exomoons, and the possibility of large moons leads to astrobiological speculation when a gas giant is in its star’s habitable zone. Interestingly, we may be looking at evidence of an extremely young — and very large — moon in formation around a planet that circles the young star J1407.

That would be intriguing in itself, but what researchers at Leiden Observatory (The Netherlands) and the University of Rochester have found is an enormous ring structure that eclipses the young star in an epic way. The diameter of the ring system, based on the lightcurve the astronomers are getting, is nearly 120 million kilometers, which makes it more than two hundred times larger than the rings of Saturn. This is a ring system that contains about an Earth’s mass of dust particles, with a marked gap that signals the possibility of the large moon.


Image: Artist’s conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b. The rings are shown eclipsing the young sun-like star J1407, as they would have appeared in early 2007. Credit: Ron Miller.

The ring system itself was discovered in 2012 by Eric Mamajek (University of Rochester) and team, with Leiden’s Matthew Kenworthy and Mamajek now refining the observations and working out the details. What emerges is a ring system with over thirty separate rings. And you need to see the lightcurve, which is available below. Kenworthy’s enthusiasm about the find is evident:

“The details that we see in the light curve are incredible. The eclipse lasted for several weeks, but you see rapid changes on time scales of tens of minutes as a result of fine structures in the rings. The star is much too far away to observe the rings directly, but we could make a detailed model based on the rapid brightness variations in the star light passing through the ring system. If we could replace Saturn’s rings with the rings around J1407b, they would be easily visible at night and be many times larger than the full moon.”

Exoring model for J1407b from Matthew Kenworthy on Vimeo.

I love the many worlds presented to us in science fiction, but I’m hard pressed to come up with a depiction of anything quite like this. Says Mamajek:

“If you were to grind up the four large Galilean moons of Jupiter into dust and ice and spread out the material over their orbits in a ring around Jupiter, the ring would be so opaque to light that a distant observer that saw the ring pass in front of the sun would see a very deep, multi-day eclipse. In the case of J1407, we see the rings blocking as much as 95 percent of the light of this young Sun-like star for days, so there is a lot of material there that could then form satellites.”

The figure below, from the paper, gives a static view of the same data:


Image: From the paper. The caption reads: “Model ring fit to J1407 data. The image of the ring system around J1407b is shown as a series of nested red rings. The intensity of the colour corresponds to the transmission of the ring. The green line shows the path and diameter of the star J1407 behind the ring system. The grey rings denote where no photometric data constrain the model fit. The lower graph shows the model transmitted intensity I(t) as a function of HJD. The red points are the binned measured flux from J1407 normalised to unity outside the eclipse. Error bars in the photometry are shown as vertical red bars.” Credit: Matthew Kenworthy/Eric Mamajek.

As to J1407b, the planet these rings surround, the astronomers estimate that it has an orbital period of about a decade, with a mass most likely in the range of between ten and forty Jupiter masses. The gap in the ring structure points to a satellite in formation that has an orbital period of approximately two years around the gas giant. It becomes clear that if we can find more instances of early disks, we can begin to study comparative satellite formation around exoplanets. From the paper:

J1407 is currently being monitored both photometrically and spectroscopically for the start of the next transit. A second transit will enable a wide range of exo-ring science to be carried out, from transmission spectroscopy of the material, through to Doppler tomography that can resolve ring structure and stellar spot structure significantly smaller than that of the diameter of the star. The orbital period of J1407b is on the order of a decade or possibly longer. Searches for other occultation events are now being carried out (Quillen et al. 2014) and searches through archival photographic plates (e.g. DASCH; Grindlay et al. 2012), may well yield several more transiting ring system candidates.

The paper also points out possible ring structures around Fomalhaut b (anomalous bright flux in optical images) and Beta Pictoris b (anomalous photometry), though neither of these has been confirmed. The scientists involved are encouraging amateur astronomers to help monitor J1407 as the attempt to constrain the mass and period of the ringed planet J1407b continues. Observations can be reported to the American Association of Variable Star Observers (AAVSO).

The paper is “Modeling giant extrasolar ring systems in eclipse and the case of J1407b: sculpting by exomoons?” accepted for publication by the Astrophysical Journal (preprint).