As comets approach the Sun, they release gas and dust known to astronomers as comet activity. For comets passing near or within Earth’s orbit, this activity slows down on successive orbits. University of Oklahoma astronomer Nathan Kaib discovered that this same phenomenon of comet fading occurs when comets make repeated passes through the farthest region beyond Saturn.
Kaib, an associate professor in the Homer L. Dodge Department of Physics and Astronomy at OU’s Dodge Family College of Arts and Sciences, is the senior author of the article “Comet Fading Begins Before Saturn,” published in Scientists progress.
“Long-period comets, those that take at least hundreds of years to circle the Sun once, spend most of their lives thousands of times farther from the Sun than Earth,” Kaib said. “However, they sometimes develop very elliptical orbits and, in turn, make regular incursions towards the Sun and its nearby planets. When these comets approach the Sun, its intense heat turns their icy surfaces into gas.
This cometary activity is what gives comets their striking appearance in the sky and makes them relatively easy for astronomers to find. While the Sun’s extreme heating steadily depletes their supply of surface ice, the activity of comets passing near Earth decreases or fades over time.
In this study, Kaib found that this fading phenomenon also occurs among comets passing through the outer solar system near or beyond Saturn’s orbit. What makes his findings surprising is that these comets experience much lower heating from the Sun than those closer to Earth. In fact, unlike closer comets, the Sun’s heating is so weak that water-based ice cannot evaporate on these comets.
By running computer simulations of comets traveling near giant planets in the outer solar system, Kaib showed that the gravity of giant planets rapidly shrinks the orbits of distant comets so that they make smaller excursions away from the Sun between passes through. the outer solar system.
“We should therefore expect the outer solar system to have many more comets in these reduced orbits compared to those in larger orbits,” he said. “Instead, astronomers see the opposite; distant comets with reduced orbits are almost entirely absent from astronomers’ observations, and comets with larger orbits dominate our census of the outer solar system. The rapid fading of comets that occurs during this orbital shrinkage explains this paradox, as it will effectively render older comets invisible to astronomers’ searches.
Since distant comets are difficult to study due to their remoteness, astronomers’ understanding of comets is primarily based on studying those in near-Earth orbits. Kaib’s discovery suggests that passages through the outer solar system can alter the physical properties of many near-Earth comets before they are discovered.
“Fading among distant comets was discovered by combining the results of computer simulations of comet production with the current catalog of known distant comets,” Kaib said. “These distant comets are faint and extremely difficult to detect, and comet-watching campaigns have gone to great lengths to build this catalog over the past 20 years. Without it, this current work would not have been possible.
Kaib expects the Legacy Survey of Space and Time, a 10-year mission to survey the southern sky at the Vera C. Rubin Observatory in Chile, to rapidly increase comet discoveries.
“The fading of comets characterized in my work will be key to fully understanding and interpreting this impending flood of newly discovered comets,” he said.
The computer simulations for this work were performed at the OU’s Supercomputing Center for Education and Research. Kaib is currently on sabbatical at Case Western Reserve University in Cleveland, Ohio.
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Fading of comets begins beyond Saturn
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