When comets first appear near Earth, their bright tails of ionized gas stun observers, but fade with each subsequent return.
Comets are essentially balls of dirty ice. Astronomers therefore believed that these objects become fainter on their repeated return to Earth because they released too much ice and gas during their previous visits to the inner solar system. The comets are thought to have melted and shrank from the sun’s heat, so when they return, there’s less material left to release and thus a weaker coma.
But a new study by scientists at the University of Oklahoma found that even comets that only pass through the inner solar system and stay out of Saturn’s orbit fade over time. That makes no sense, because in those far reaches of the solar system, the light from the sun is so dim it shouldn’t be able to melt a comet’s ice.
In a statement (opens in new tab) Speaking of the new research, the scientists suggest there must be something going on deep in space that is altering the physical properties of those comets, leading to their blurring.
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The researchers came to this conclusion when they ran computer simulations of comets traveling through the outer solar system, near the massive massive planets Jupiter and Saturn. The models showed that the powerful gravity of these planets changes the orbits of the comets.
The comets may have begun their journey by following so-called highly eccentric elliptical orbits, approaching from the far reaches of the solar system well beyond Neptune’s orbit, then racing toward the sun before disappearing back into the outer reaches for centuries. But with each pass near Jupiter and Saturn, the comets’ orbits become more circular and don’t retreat as far from the sun, the study found.
“We can therefore expect the outer solar system to have 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 study.
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 completely absent from astronomers’ observations, and comets with larger orbits dominate our census of the outer solar system.”
To explain this strange absence, the researchers argue that the comets must have faded out, and although they are there, somewhere outside Saturn’s orbit, they are no longer visible to our telescopes.
“The fading between 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 observation campaigns have gone to great lengths over the past 20 years to build this catalog. Without them, this current work would not have been possible.”
But understanding exactly what’s happening requires more powerful telescopes than scientists can use today. Once those are available, Kaib and his colleagues say, astronomers will likely find that the outer solar system is full of faded comets.
Astronomers know of comets that orbit between Jupiter and Saturn and regularly erupt into powerful eruptions despite the cold environment, so it’s clear that “dirty snowballs” can lose their matter even far from the sun.
A study based on the research was published (opens in new tab) Wednesday (March 30) in the journal Science Advances.
If you’re looking for a binocular telescope to watch comets, check out our guide to the best binocular deals and the best telescope deals right now. Our best cameras for astrophotography and best lenses for astrophotography guides also have tips for choosing the best imaging equipment to take pictures.
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