An astronomy professor at the University of Leiden, Ignas Snellen,called brown dwarfs ‘failed stars‘ because they were too heavy to be typical planets (13-75 times as heavy as Jupiter) and too light to sustain the fusion of hydrogen into helium. As a result, they exist in a limbo in astronomers’ textbooks, with the precise mechanism of their formation remaining a mystery.
However, failed or no, brown dwarfs are still massive objects and make for interesting features in the universe. One stellar example is the pithily named J1407b. It was discovered in 2012 by astronomers at the Leiden Observatory and University of Rochester, New York, orbiting a star – J1407 – about 420 light-years from Earth. J1407b is young – about 16 million years old. But it’s most striking feature is an extended ring system.
In 2012, the astronomers studying it surmised that the dwarf likely has 37 rings, altogether 120 million km in diameter (as wide as Mercury’s orbit around the Sun). Compare this to Saturn’s ring system, which is at most 300,000 km wide*. There are conspicuous gaps between these rings as well – notably at a distance of about 60 million km from the inside – indicating that there might be moons inhabiting them, formed by sweeping up the missing material.
The research team estimates that the amount of material orbiting as rings might all together weigh as much as hundred-times Earth’s moon, which is not anomalous considering J1407b is still young and likely not fully formed yet.
The way it was discovered is interesting. An exoplanet shows itself when seen through a telescope when it passes in front of its host star and casts a weak but persistent shadow on the telescope lens. When observing the J1407 system using the Super Wide Angle Search for Planets project in 2007, the astronomers found something was taking 56 days to move all the way across the face of its star. It was either an extremely large object or it had rings.
A second observation supported the rings hypothesis: the amount of starlight blocked wasn’t constant, but rose and dipped as if the amount of material passing in front of it was uneven. In fact, at one point, fully 95% of the starlight was blocked.
“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,” noted Leiden’s Matthew Kenworthy, who analyzed the data. “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.”
Estimating the mass of the ring-system would’ve required Doppler spectroscopy data as well, which wasn’t available until late 2014.
Curiously, the planet J1407b hasn’t been spotted directly yet. The astronomers are assuming it’s there simply because something like it has to hold this ring system together. In fact, its characterization as a brown dwarf is simply what it has to be at the least. The Doppler data indicates it has to weight some 10-40 times as much as Jupiter, i.e. much bigger than a gas giant, much smaller than a main-sequence star.
A paper discussing the team’s results was accepted for publication in the Astrophysical Journal on December 28, 2014. Even as studies of this giant will continue, the astronomers have called on their amateur counterparts from around the world to help them. “J1407’s eclipses will allow us to study the physical and chemical properties of satellite-spawning circumplanetary disks,” Kenworthy said of incentives.