When comets first appear near Earth, their bright tails of ionized gas stun observers, but on every subsequent return, they become dimmer.
Comets are essentially balls of dirty ice. Astronomers therefore believed that these objects become dimmer on their repeated returns to Earth because they released too much ice and gas during their earlier visits to the inner solar system. The comets melted and shrank because of the sun's warmth, the thinking goes, so when they return there is less material left to release and therefore a fainter coma.
But a new study by scientists from the University of Oklahoma found that even comets that only skirt the inner solar system and remain beyond the orbit of Saturn fade over time. That makes no sense because, in those far reaches of the solar system, the sun's light is so weak that it shouldn't be able to melt a comet's ice.
In a statement about the new research, the scientists suggest there must be something going on, deep in space, that changes the physical properties of those comets that leads to their fading.
The researchers came to this conclusion when they ran computer simulations of comets traveling through the outer solar system, near the giant massive planets Jupiter and Saturn. The models showed that the powerful gravity of these planets alters the comets' orbits.
The comets may have started their journey following so-called highly eccentric elliptical orbits, approaching from the farthest regions of the solar system far beyond the orbit of Neptune, then hurtling toward the sun before disappearing back into the outer reaches for centuries. But with every passage near Jupiter and Saturn, the comets' orbits become more circular and they don't retreat as far from the sun, the study found.
"We should therefore expect that the outer solar system has many more comets on these shrunken orbits compared to those on larger orbits," said Nathan Kaib, an associate professor of astronomy at the University of Oklahoma and lead author of the new research.
The only problem is that those results don't match what astronomers actually see.
"Instead, astronomers see the opposite," Kaib said. "Distant comets with shrunken orbits are almost entirely absent from astronomers' observations, and comets with larger orbits dominate our census of the outer solar system."
To explain this strange absence, the researchers posit that the comets must have faded and even though they are there, somewhere beyond the orbit of Saturn, they are no longer visible to our telescopes.
"Fading among distant comets was discovered by combining the results of computer simulations of comet production with the current catalog of known distant comets," said Kaib. "These distant comets are faint and extremely difficult to detect, and comet-observing campaigns have taken great pains to build this catalog over the past 20 years. Without it, this current work would not have been possible."
But to understand what exactly is happening would require more powerful telescopes than scientists can use today. Once those are available, Kaib and his colleagues say, astronomers will probably find out that the outer solar system is full of faded comets.
Astronomers know of comets that orbit between Jupiter and Saturn and regularly burst into powerful eruptions in spite of the cold environment, so it is clear that "dirty snowballs" can lose their matter even far away from the sun.
A study based on the research was published Wednesday (March 30) in the journal Science Advances.
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