Understanding the ring system of J1407b.

J1407 (or to give it its full name, 1SWASP J140747.93-394542.6), is a 16 million-year-old K-type orange dwarf star 420 light years from Earth in the Scorpius-Centaurus OB association, in the constellation of Centaurus. In 2007 this star underwent a series of complex eclipses over a period of 56 days, which astronomers eventually concluded were most likely to have been caused by a large planetary companion with an extended ring system and an eccentric orbit, named J1407b, passing in front of the star. Ring systems around planets are thought to be product of the way in which the planets form. As a young planet grows in mass material is pulled from the circumstellar disk (disk of material surrounding the young star from which the planets form) into a circumplanetary disk (disk around the planet). Most of this material eventually accretes onto the planet of is lost back into space, but some can go on to form a system of moons or rings around the planet, the most notable example of this in our own Solar System being the ring system seen around the planet Saturn).

  

In a paper published on the arXiv database at Cornell University Library on 27 September 2016 and accepted for publication in the journal Astronomy & Astrophysics, Steven Rieder of the RIKEN Advanced Institute for Computational Science and Sterrewacht Leiden at Leiden University, and Matthew Kenworthy, also of Sterrewacht Leiden at Leiden University, describe a series of models of the J1407 system and the conclusions about the system drawn from these.

Rieder and Kenworthy assumed that the ring system orbited the planet in the same plane as the planet orbited the star, and that the planet had an average distance from the star of 5AU (i.e. five times the distance at which Earth orbits the Sun), giving it an orbital period of 11 years. The Star was given a mass equivalent to 0.9 times that of the Sun, while the planet was modelled at a series of increments at 20, 40, 60, 80 and 100 times that of Jupiter. Since the planet has never been directly detected it is assumed that the long access of the orbit is directed towards the Earth (eccentric orbits are essentially egg-shaped), which it the most likely explanation of a large companion body escaping detection in the system, with the eclipses occurring at or very close to the planetary perihelion (i.e. the closest point on the orbit to the star, where the planet is moving fastest). The obit of the planet was modelled at eccentricities of between 0.6 and 0.7, meaning that at perihelion it would be between 1.5 and 2.0 AU from the star and moving at a rate of between 27 and 33 kilometers per second.

 The orbit of J1407b model B80, with J1407b located at pericentre. The J1407b system (red) is shown to scale for the initial size of the model. The size of the star (orange) is exaggerated by a factor 20. Grey circles indicate the distance to the star in AU, while the black ellipse shows the orbit. Rieder & Kenworthy (2016).

Each model planet was surrounded by a series of 50 rings with an inner edge ranging from 0.26 AU to 0.66 AU. Particles were assumed to start equidistant from each other within each ring, but the radial distance of each particle was then changed by a random amount. Each ring system generated in this way was run through the simulation twice, once with a prograde orbit (i.e. in the same direction as the orbit of the planet) and once in a retrograde orbit (i.e.. in the opposite direction to the orbit of the planer. Particles that travelled beyond 2AU from the planet were assumed to have been lost from the system. The simulation was run for 9000 orbits, equivalent to 500 000 years.

Rieder and Kenworthy found that rings with prograde orbits tended to be disrupted easily in the simulations, with the largest surviving ring system in a prograde orbit being capable of producing an eclipse only 40 days long, far shorter than the observed phenomenon. Ring systems with retrograde orbits, however, fared better, and several simulations were capable of producing eclipses 56 days in length or even longer. This suggests that the eclipses could well be caused by a planet, J1407b surrounded by a series of rings with a retrograde orbit. This is not an unreasonable requirement, as in our own Solar System the panets Venus and Uranus have retrograde rotations, and this has also been obeserved in exoplanet such as WASP-17b. The simulations also suggest that the planet is likely to be large, closer in size to 100 times as massive as Jupiter than 20 times as massive. Such an object would be more likely to be a Brown Dwarf than a planet (Brown Dwarfs are objects to large to be considered planets, but to small to be considered stars; they are thought to be able to fuse deuterium in their cores, but not hydrogen). 

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Faint companions discovered to two planet-hosting stars.                                               Almost all planets orbiting other stars have been discovered by one of two planetary detection techniques: occlusion, on which the planet passes in front of the star, causing the amount of light reaching us from that star to dim ever so slightly on a regular timescale, or radial velocity, in which the gravity of the planet causes the host star to wobble back and...

The possibility of an Earth-mass planet in the habitable zone of the Kepler-68 system.      The Kepler Space Telescope has located many multi-planet systems since its inception, which combined with discoveries made by other planet-hunting missions has enabled scientists to begin to construct models of planetary systems orbiting other stars. This is particularly complicated where not all planets are visible to the space telescope, which is only...

Generating free oxygen in the atmosphere of exoplanets without the presence of life.           In the past two decades over a thousand planets have been found orbiting stars other than our own, many of which appear to be small rocky planets in the habitable zones of their stars (i.e. the zone in which such a...

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Deciphering the rings of 10199 Chariklo.

10199 Chariklo is a Centuar (asteroid or comet-like body found between the orbits of Jupiter and Neptune), which was first discovered in 1997 and subsequently found to be the largest such body, with an estimated diameter of about 300 km. Like other Centaurs it is thought to have originally been a Kuiper Belt Object, i.e. a body orbiting outside Neptune, that was pushed further into the Solar System by an encounter with the planet. In 2014 10199 was discovered to have a system of rings, the first non-planetary mass object to have such a system, though a similar ring system has since been found around another Centaur, Chiron.

In a paper published on the online arXiv database at Cornell University Library on 4 February 2016, Margaret Pan and Yanqin Wu of the Department of Astronomy & Astrophysics at the University of Toronto examine the rings of 10199 Chariklo, in order to determine their properties and try tounderstand their origins.

Artist's impression of 10199 Chariklo. Institute of Astrophysics of Andalusia.

Pan and Wu calculate that 10199 Chariklo has an oblateness of 0.213 (i.e. its equatorial radius is 21.3% larger than its polar radius) and that its equatorial radius is 144.9 km. The body has two known rings, an inner ring with a radius of 390.6 km and a width that varies between 5.5 and 7.1 km, while the outer ring has a radius of 404.8 km and has a width of 3.6 km. Using these measurements combined with the optical depth of the rings (i.e. their transparency, the amount of light they absorb as it passes through them), Pan and Wu calculate that the rings have a total mass of about 10 billion tonnes.

Next Pan and Wu considered possible scenarios for the formation of the rings of 10199 Chariklo. In order to do this it was assumed that the average residency in a Centaur-type orbit is about five million years; Centaurs orbit in an area of the Solar System where they are perturbed by the gravitational influences of the giant planets, and ate therefore thought likely to be disrupted from these orbits in a relatively short time (astronomically speaking). Prior to becoming a Centaur 10199 Chariklo is assumed to have been a Kuiper Belt Object, orbiting outside the orbit of Neptune for about five billion years (i.e. the age of the Solar System). Furthermore Pan and Wu calculate that given the week gravitational pull of 10199 Chariklo, the rings should disperse to the point of no longer being visible within about 500 000 years of their formation, unless they are being constantly replenished from some source. This lifetime could be lengthened if the rings are accompanied by several shepherding satellites (i.e. larger bodies orbiting 10199 Chariklo that excerpt gravitational influences on the rings), however this would require several kilometre sized objects, which are unlikely to have remained undetected.

Firstly Pan and Wu examined the possibility that the rings might have originated while 10199 Chariklo was still within the Kuiper Belt. The most likely cause of ring-formation around a body in the Kuiper Belt would be a collision with a smaller body. Impact events are thought to be extremely rare in the Kuiper Belt (where matter is extremely thinly spread), but over the course of five billion years an object the size of 10199 Chariklo might be expected to undergo several such collisions. However the likelihood of such an event having happened within the last 500 000 years is extremely low, and the likelihood of our having observed two bodies (Chariklo and Chiron) with ring systems caused by collisions in the Kuiper Belt within the last 500 000 years is negligible.

An alternative scenario is that matter could have been removed from the surface of 10199 Chariklo by rotational disruption, however the body currently only rotates on its access roughly once every seven hours, not fast enough to cause such disruption, and there is no reason to believe that it would have rotated significantly faster within the last 500 000 years.

A third possible scenario is that the rings began as a small satellite orbiting 10199 Chariklo while it was in the Kuiper Belt, which was broken up by tidal forces during the encounter with Neptune that pushed it into a Centaur-type orbit. Such small moons are thought likely to be quite common around Kuiper Belt Objects, so this scenario has some plausibility, however in order for rings generated by such an encounter to survive, the object would have to be shifted from a Kuiper Belt orbit to a Centaur orbit in a single encounter with Neptune. Models of such processes suggest that most Kuiper Belt Objects take several close encounters with Neptune to disrupt them into the Solar System, with only a very small percentage being shifted in a single encounter. Again this is just about plausible for a single body, but the presence of two ringed Centaurs suggests that some other factor is at play.

As an alternative Pan and Wu suggest that the rings could be the result of dust being carried from the surface of the body during outgassing events. For the purpose of the study they model outgassing of carbon monoxide, but note that essentially the same processes could apply to outgassing of carbon dioxide, nitrogen, cyanide or other volatile compounds,

Carbon monoxide has a very low sublimation point (point at which it turns directly from a solid to a gas), so that it can be removed from a body even by the gentle warming the Sun provides in the Kuiper Belt. This means that a Kuiper Belt Object orbiting at about 20 AU (20 times as far from the Sun as the Earth) would have lost all of its carbon monoxide over the five billion year history if the Solar System. However a body at 40 AU would only have lost carbon monoxide down from its surface down to a depth of about 1 km,  with more deeply buried carbon monoxide protected from the Sun's heat.

Were such a body moved into a Centaur-type orbit by an encounter with Neptune, then such buried carbon monoxide would be heated to above its sublimation point and degas into space, in the same way as comets (thought to be Kuiper Belt Objects knocked into the Inner Solar System) emit gas and dust to form a coma and tail as they approach the Sun. 10199 Chariklo currently orbits at an average distance of 15.8 AU, at which distance Pan and Wu calculate the body would be warmed through sufficiently for all carbon monoxide to be lost from its interior within 500 000 years. However the body has a highly eccentric orbit, varying from 13.1 to 18.5 AU over an orbital period of 22 915 days, sufficient to provoke distinct seasonality on the surface. This means that even though the interior of the body had warmed sufficiently for carbon monoxide to sublimate, temperature variations at the surface could lead to the gas resolidifying close to the surface during the 'winter' season when it is furthest from the Sun.

 The calculated orbit of 10199 Chariklo. JPL Small Body Database Browser.

This seasonality could lead to a spring on 10199 Chariklo (possibly quite close to perihelion, when the body is at its nearest to the Sun), when the surface could reach a critical temperature and large quantities of accumulated subsurface carbon monoxide could degas over a very short period of time, carrying considerable volumes of dust from the surface. Even if the majority of this dust fell back to the surface a sufficient amount could reach an altitude high enough to remain in orbit to create at least a temporary orbit, creating at least a temporary ring system. Crucially such a series of events would slow down the rate at which carbon monoxide is lost from 10199 Chariklo, so that instead of a slow degasing over 500 000 years, a series of interrupted degasing events could go on for several million years.

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Molecular oxygen in the coma of Comet 67P/Churymov-Gerasimenko.                             In recent years studies of the comas of numerous active comets (the coma of an active comet is the gas and dust envelope which surrounds it; this is not a true atmosphere as it is continuously replenished by...

Sinkholes on Comet 67P/Churyumov-Gerasimenko.

Many comets have been observed to have pitted surfaces. Initially these pits were thought to be the result of collisions with smaller bodies, as with craters on planets and moons, but they have been shown to be far to numerous for this to be the case, as...

Emissions from Comet C/2002 VQ94 (LINEAR).                                                C/2002 VQ94 (LINEAR) was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) team at the...

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