Rings in Super Saturn

Astronomers have discovered an enormous ring system and it is possible that a moon around a planet orbiting another sun

The article was published with the approval of Scientific American Israel and the Ort Israel Network,

Models of the rings raise the possibility that a Mars-sized moon resides among them. If this possibility is recovered, this moon will be the first moon detected outside our solar system, and will teach about the possibility that more moons are expected to be discovered. Further studies of this unique system promise to reveal unprecedented new details about how planets and moons form around other stars.

Most of the astronomical activity is done in the offices and observatories where the scientists work. But if you

It's no coincidence that one of the best pubs in Tucson, Arizona (called 1702, after the street address it's located on) is located near the University of Arizona's Steward Observatory. This was where my colleague, Eric Mamajek from the University of Rochester, showed me something that sent us both on a quest for the first ringed planet outside our solar system, a quest that included both the world's most modern telescopes and century-old astronomical observations. Along the way, we not only found a ring system larger than Saturn's, but also something that looked like a newborn moon.

Identify the rings

The story begins in 2011, when Mamajek and his then research student at the University of Rochester, Mark Pico, compiled a catalog of very young stars located near Earth. To estimate the age of the candidate stars, Mamajek checked and extracted the rotation rates of the stars around their axis. Young stars spin around themselves faster than older stars, and the spin can be timed by watching the starspots (darker and colder areas on the surface of the star) entering and leaving the field of view.

One of the candidates to be included in the survey had no name, only a code based on the instruments that observed it and its position in the sky, in the constellation Centaurus: 1SWASP J140747.93-394542.6. Now it is called J1407 in our language. This star and others considered were too far away to see their spots directly, so Mamajek and Pico instead looked at J1407's "light curve": a graph of its brightness over time. They looked for small dips in the graph, which appear when spots enter the field of view and reduce the star's light. Planets can also create such dips when they cross the line of sight between their star and Earth (an event known as a transit). Mamajek and Pico found the curve of J1407 in the Planetary Survey database called Super WASP, who as of today, after monitoring about 31 million stars, has found with a camera more than 100 planets passing by their star.

The light curve did show that J1407 is a rapidly spinning young star, but it also contained some other intriguing information. A random peek at J1407's light curve, measured by SuperWASP's camera, found that this seemingly featureless star flickered in 2007 in an unpredictable pattern for many nights, then dimmed repeatedly until it nearly disappeared for more than a week, finally returning to its normal brightness. Data from other years did not show any such varied behavior of this star. In 2007, this strange event did not leave a special impression, and this curve gathered dust in the archives without anyone paying attention to it. But after Mamajek saw her in 2011, he couldn't forget her.

"I printed the light curve, hung it on the wall of my office and looked at it for weeks," he told me in a pub in Taxon. “Her crazy build and details were extraordinary. What could have caused these rapid changes in the star's brightness?”

Some time after this discussion we started working together to solve the mystery. We soon ruled out some obvious suspects such as problems with the SuperWASP cameras or poor viewing conditions. We have therefore concluded that the source of J1407's mysterious dimming is not an Earth-based factor.

We soon concluded that something very fast and very large had covered the surface of J1407 and caused it to flicker in a process analogous to a solar eclipse. The rate of brightness fluctuations taught that the object that caused the eclipse raced across the star at a speed of 30 kilometers per second, and yet, the eclipse itself lasted 56 days! This long period of time means that the size of the object is at least 145 million kilometers.

There are not too many plausible explanations as to the possible identity of such a large structure. We tested them all and eliminated them one by one. Could it be a dust belt surrounding the star in a close orbit? No, we did not find infrared glow around J1407, which we would expect hot dust to emit and give away the existence of the cloud. Could it be a binary system of two stars orbiting each other, with a red giant shadowing its smaller counterpart, such as a white dwarf, neutron star, or black hole? No, such systems tend to emit much more X-rays than we've seen, and also J1407 doesn't look like a giant star. Could the flickering be nothing more than a coincidence caused by the shadow of something floating in deep interstellar space, between Earth and the star, or is J1407 a complicated triple star system, with a companion 180 million kilometers across that casts its shadow? No, none of these options fit the data either. In the end, the simplest explanation that was consistent with all the observations was something very strange: the dips in the light curve may be the result of a giant ring system, 200 times larger than Saturn's, that has orbited and passed through an unseen planet orbiting J1407 and between Earth in 2007.

But why do we think it was a ring system? The characteristic that most captured the attention of the light curve was the level of detail that could be seen there in all orders of magnitude of time: the eclipses lasted 56 days, but it was possible to see rapid changes that occur even in periods as short as 20 minutes. These velocities hinted to us that the object that cast the giant shadow had many substructures, and the more or less symmetrical shape of the light curve showed that the object had a circular or elliptical geometry, very similar to Saturn's familiar ring system. If we're right, these massive planetary rings are the first to be found outside our solar system.

Hunt the planet

If it was indeed a giant ring system, then there must be a giant planet orbiting J1407, holding the rings in place. We therefore set out to search for this planet, which we named J1407b, with the help of advanced instruments on top of two of the largest observatories on Earth: Keck II telescope In Hawaii, which are 10 meters in diameter, and"The Very Large Telescope" (VLT) in Chile, which are 8.2 meters in diameter.

Even the biggest and brightest planets are much dimmer than the suns around them, and are difficult to see. But J1407 is a comparatively very young sun. Given the young age of the star, no more than 16 million years, any gas giant (such as Saturn) that orbits it will still emit a bright glow of infrared light, resulting from the heat of its formation. Based on J1407's distance from us, its putative companion would appear through the lens of a powerful telescope only 50 milliseconds of arc away, about the distance between the goalposts on a football field as seen from the surface of the Moon. But even though the observation is challenging, it is still just within the limits of what is possible.

A mystery of light and shadow: a confusing pattern

A light curve, a graph depicting the variation in the brightness of a star over time, is a basic tool in star research. A brief spike in brightness can be caused by stellar eruptions, while momentary dips can signal sunspots or the shadow of the holiday planet around it. But the wildly swinging light curve produced by star J1407 in 2007 was unlike anything astronomers had seen before. Something strange caused the star to flicker and dim for months on end.
(Source: “Fine Structure Analysis of the Eclipse of 1SWASP J140747.93−394542.6 Hints at the Existence of Exomoons,” by T. A. M. Van Verkehoven, M. A. Kenworthy, and E. I. Mamjek, Monthly Notices of the Royal Astronomical Society, Volume 441, Issue 4, 11 July 2014.)

For two years we tried to photograph the planet, while at the same time we were looking for cyclic shifts in the movement of the star, which would betray the existence of the invisible world that surrounds it and causes it to oscillate back and forth due to the pull of gravity. We also recruited a network of amateur and professional astronomers (including the American Association of Variable Star Observers) to monitor J1407's brightness each night and look for dimming that would signal the beginning of another eclipse.

We found nothing. However, this result does not mean that the planet does not exist; Even if its mass was 12 times that of Jupiter, we could easily miss it. We could also look at the wrong time, when the planet is behind its star and therefore invisible to us. However, the failure allowed us to rule out some variants of low-mass stars that might have contributed to J1407's dimming if they were companions.

The rings: removing the lot

Despite the uncertainty, we rowed on, trying to guess the architecture of the rings that we suspect are swirling around J1407b. For months, our team developed computer models that would be able to extract from J1407's light curve information about the composition and the three-dimensional structure of the rings.

As we stood and discussed the problem with several colleagues in front of the board, we suddenly received an epiphany: although we do not know the exact number and location of each ring, the steepness of the inclined segments of the light curve could give us clues about the overall geometry of the system, such as its angle of inclination relative to to the star With the help of this additional piece of information we were able to complete the computer model of the rings and produce artificial light curves based on various assumed tilts and rotations of the rings. Indeed, one of the configurations we tested matched the dips and jumps we saw in the J1407 data!

Armed with this new knowledge, we mapped the ring system, fitting each part of the light curve to a different ring distance from planet J1407b. Whenever the slope of the light curve changes, this difference marks the start or end of a ring transit across the star. By counting all these points on the light curve, we saw at least 24 rings. But if we take into account gaps in the data created from time to time due to poor observation conditions, it is more likely to estimate that there are at least 100 rings in the system.

We were lucky to be able to see J1407b's ring system at this stage of its evolution. To understand why, you can think about our familiar Saturn, and how its ring system developed over time. The solid appearance of his rings is an illusion. They consist of ice particles moving in circular orbits around the planet. These particles, as a whole, are shaped by tiny moons orbiting within or right next to their outer boundary. Saturn is thought to have had larger rings in the past, but the small particles at the outer limits of the system stuck together through their mutual gravitational attraction in a "snowball" process that eventually formed some of Saturn's moons as we know them today. This fleeting view must have been beautiful. If they were spectators of that cosmic time period, who had the privilege of seeing this spectacle, they would surely have congratulated themselves on their good fortune.

Like Saturn's ring system eons ago, J1407b's system also appears to be undergoing a change today. Our model raises the possibility that there is a large gap in the system between the rings, likely created by something astronomers have never seen until now: a newly born moon in another solar system (exo-moon) orbiting J1407b. According to our calculations, this moon takes nearly two years to orbit J1407b and may have a mass similar to that of Mars. Although the large gap between the rings cannot be considered an unequivocal identification of an exomoon, still, if we confirm the existence of J1407b and the existence of its ring system, the gap will be the best evidence yet for the existence of those elusive moons that we have been searching for for a long time so much

The emerging picture of this dynamic and exotic system is nothing less than spectacular. If you fly into this solar system from interstellar space, you'll see the sun's glow nearly overshadowing the dim glow of its cooler (but still hot enough to glow red) planet. If you approach the planet from above, you can see the rings, like bright ripples spreading out against the dark velvet background of space. Decorated with a garland of fans made of fragments of rock formed by collisions, the plane of the rings will be washed in undulating waves of clumps of material. Some of these waves will break on the large gap created by the largest mass of material that resides between two rings, a moon the size of Mars.

If you stand in front of this moon when its orbit slightly deviates from the plane of the rings, you can see the encircling rings arching against the sky all around you. And if the moon has an atmosphere, stray ring particles will burn from the heat created by friction as they pass through it, filling the sky with majestic showers of roaring meteors. Above you, you can see the planet J1407b embedded in the sky like a small pebble in the glare of light from its rings that extend in all directions, streaked with bands of dark clouds and glowing like a burning ember.

But this system offers astronomers much more than the possibility of a beautiful view. Gas giant planets orbiting close to their star are among the easiest worlds to detect outside our solar system, but since they have no solid surfaces, the chances of finding life of the kind we know are small. A large holly moon around such a planet would be an entirely different story, as it could provide a relatively life-friendly, rocky, water-bearing surface. If our solar system is a typical example, then our galaxy could be filled to the brim with billions of large moons orbiting giant planets. Proving that moons exist around gas giants outside our solar system would greatly expand the possibilities of places where life could exist.

For years, a small group of researchers has been diligently searching for exomoons, mainly by the indirect effects they can have on the motion of the planets they orbit. Planets transiting their parent star provide a precise, periodic dimming of its light, but the mass of a large, invisible exomoon further shifts the eclipse schedule and disrupts its regularity. Astronomers such as David Kipping of Columbia University have conducted vigorous searches for exomoons, by trying to locate this signature in the light curves of the worlds passing by their star, found by NASA's Kepler satellite designed to locate planets in the universe. To date they have not found any exomoons. But J1407b's possible moon suggests that these ongoing searches may soon bear fruit.

However, so far both the planet and its moon are speculative. The largest telescopes and the most sensitive instruments on Earth have yet to find conclusive evidence that unequivocally confirms their existence. But it is possible that this evidence will come from archival data collected using much cruder technology in years past, such as a collection foundat the Harvard-Smithsonian Center for Astrophysics.

back to the Future

Many researchers work in this center, and its offices and corridors are bustling with people delving into data from space telescopes, writing articles, running simulations and participating in lectures. But a few meters from this bustling building are Harvard's board warehouses, housed in a quiet brick structure next to the main building, which few tread. There, in one of the wings, piles of paper envelopes on long shelves fill the walls of three floors, from floor to ceiling. You would think it was a second-hand record store, but instead of vinyl records, these envelopes have more than half a million photographic plates from several observatories, a quarter of all astronomical photographic plates in the world. They document a hundred years of observations of the night sky.

These photo plates are now being scanned In Harvard's Digital Access to One Hundred Years of Heaven project, whose goal is to take all the information stored on these fragile glass slides, digitize it and upload it to the web. We were able to determine that J1407 appears on about 700 of these panels, in images taken between 1901-1984. With the help of the data from these boards, we can look for more eclipses and thus we may be able to know when the next eclipse will occur.

Meanwhile, our best guess is that the next eclipse will occur sometime in the next decade. Until then, we continue to search for conclusive proof of the existence of the planet and its rings, and dedicated astronomers monitor J1407 almost nightly. They look for the dip in starlight caused when the outermost ring begins to pass by. When this happens, we will be able to make many observations to study the rings in more detail. As the rings move past the star, we can use spectrographs on the world's largest telescopes to collect some of the star's glow shining through and around the rings to determine their chemical composition and how that composition changes with distance from J1407. The most exciting detail is that J1407 is a relatively bright star that can be seen from the southern hemisphere and is easy to spot by observation. Astronomers with small telescopes can track the star's brightness fluctuations in real time and provide a continuous 24-hour overview from around the world.

Our deep dive into the giant rings of J1407b will be only the beginning of a more comprehensive series of studies concerning the question of how solar systems form. The conventional wisdom is that newborn gas giants form disks around the planet that crystallize into moons and rings, and we expect to soon spot more such systems by the shadows they must cast far across the galaxy. Now that we know what we're looking for, the race is on to find more giant ring systems and exomoons like the ones we think are around J1407b. My colleagues and I are already scouring new databases for telltale signals of other ring-bearing planets in other systems. Saturn's wonderful ring system may soon face serious competition from systems around other stars.

To watch a video about the process of mapping the rings:

Exoring model for J1407b from Matthew Kenworthy on Vimeo

Matthew Kenworthy is Associate Professor of Astronomy at the Leiden Observatory in the Netherlands. He studies the holiday planets around other stars and manufactures coronagraphs (telescopes that can block the direct light of a star and thus identify objects in its vicinity) to help take pictures of them. In his spare time, he bakes bread and goes on bicycle trips with his family in the Dutch countryside.