Dusty belts of debris from the birth of stars are wide and dynamic, fed by frequent collisions between exocomets and stirred by the gravity of nearby planets, according to a recent study published in Astronomy & Astrophysics. The findings offer new insights into the planet-building process.
The belts are analogous to the solar system’s Kuiper Belt, a doughnut-shaped zone beyond the orbit of Neptune that holds hundreds of millions of icy bodies. The exocomet belts analyzed in the new study show a wide range of characteristics, including different widths, masses, and brightnesses. The belts were probably sculpted by unseen exoplanets, the authors conclude.
“What I find the most exciting is that this study shows again that planets are everywhere. Even if we can’t see them directly, we see their signposts in those disks.”
“We find that each belt looks unique, so each planetary system is different,” said study member Steve Ertel, an astronomer at Steward Observatory and lead scientist for the Large Binocular Telescope Observatory, both at the University of Arizona. “But what I find the most exciting is that this study shows again that planets are everywhere. Even if we can’t see them directly, we see their signposts in those disks.”
Researchers with the REASONS (Resolved ALMA and SMA Observations of Nearby Stars) project produced high-resolution images of the belt systems around 74 stars within about 500 light-years of Earth—the largest sample to date.
The team made new observations of some of the systems with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the Submillimeter Array (SMA) in Hawaii, which are sensitive to the glow of the dust and small pebbles that form the belts. Researchers combined those results with earlier observations of other systems made with ALMA to complete the sample set.
Exocomet Breakups
The belts are found at distances of 10–100 astronomical units (1 AU equals the average distance from Earth to the Sun) from their central stars, comparable to the 30 AU from the Sun to the inner edge of the Kuiper Belt. They are created by objects up to about 1 kilometer in diameter that are similar to bodies in the Kuiper Belt and comets that occasionally visit the inner solar system—hence the name “exocomets.”
Such bodies may be leftover building blocks from the birth of planets and moons. In the case of the Kuiper Belt, many were hurled away from the Sun by the gravity of those newly forming planets.
“Where we observed these cold rings, the bodies are thought to be composed of large amounts of ice in addition to the rocky material or dust,” Ertel said. “When those bodies collide, they break up into smaller and smaller fragments, and this is what we see as dust.”
The dust provides “important insights into the underlying planetary systems,” Ertel said, because, as with the Kuiper Belt and asteroid belt in our own solar system, belt properties relate to the orbits and masses of planets.
Some systems have more than one ring or band, suggesting they might contain multiple planets, while the thickness of some rings suggests they could contain bodies from about 140 kilometers in diameter to the size of the Moon (which has a diameter of about 3,500 kilometers)—too small to be seen in the REASONS observations but large enough to influence a ring’s internal dynamics.
“The main surprise probably was the fact that broad belts are likely more common than narrow rings,” said Luca Matrà, a physicist at Trinity College Dublin and the study’s lead author. “Many of us hold dear the image of the beautiful Fomalhaut ring, probably the most famous exocomet belt. However, we were very surprised to learn that such rings are rare.”
Several factors may be responsible for the rings’ shapes and sizes, Matrà said, including collisions between objects within the belts, the initial conditions in which they formed, and interactions between material in the belts and nearby planets, perhaps as the result of planetary migrations.
Initial conditions include the amount of material available to form the belts, the luminosity of the star, and the nearby stellar environment. A brighter, hotter star should evaporate ices at greater distances within the disk of material from which the building blocks of planets, known as planetesimals, are born. More material in the initial disk could spread out and better shield itself from the star’s radiation, preventing the loss of dust to interstellar space. If a star formed in a tight cluster, on the other hand, interactions with other stars might limit the growth of planet-forming disks.
Stirring Up Some Excitement
Migrations, in which gravitational interactions cause planets to move toward or away from their star, could whip up the objects in narrow rings, which are common in young star systems where new planets are being born, like ice chips in a blender. That stirring motion could cause the rings to spread out to form the wider belts seen today.
“There was lots of excitement in the early solar system, and now we’re seeing the same sorts of things happening elsewhere. I find that really, really fascinating.”
“In our own solar system, Uranus and Neptune probably originally weren’t as far from the Sun as they are today, but they got pushed outward by Jupiter and Saturn,” said Sharon Montgomery, a physics professor at Pennsylvania Western University in Clarion who was not involved in the new study. “Eventually, Neptune created all kinds of stirring in the Kuiper Belt. So there was lots of excitement in the early solar system, and now we’re seeing the same sorts of things happening elsewhere. I find that really, really fascinating.”
The new study also indicates that the dust structures lose both mass and surface area as they age, with smaller rings and belts depleted more quickly than wider ones. Both findings are consistent with models of planetary formation and disk evolution, according to the researchers.
Matrà said the team will expand its work through a more detailed study of some of the REASONS targets. “We took 18 of these belts and pushed to the limit of the ALMA resolution, going to the maximum possible resolution to ask pressing new questions,” Matrà said. The answers should provide even deeper insights into these intriguing bands of exocomets.
—Damond Benningfield, Science Writer