Distant long-period comets quickly fade away, study

Comet properties substantially evolve at solar distances much larger than previously considered.


Long-period comets take more than 200 years to complete an orbit around the Sun. As they approach Sun, they emit gas and dust. This cometary activity is what gives comets their striking appearance in the sky and makes them relatively easy for astronomers to find.

This activity slows over successive orbits for comets passing near or inside Earth’s orbit. University of Oklahoma astronomer Nathan Kaib has found this same comet-fading phenomenon occurs as comets make repeated passages through the more distant region beyond Saturn.

University of Oklahoma astronomer Nathan Kaib said, “Long-period comets, those that take at least hundreds of years to go around the Sun once, spend most of their lives thousands of times further from the Sun than the Earth is. However, sometimes they develop highly elliptical orbits and, in turn, make regular incursions toward the Sun and its nearby planets. As these comets approach the Sun, its intense heat turns their icy surfaces into gas.”

“As extreme heating from the Sun steadily depletes their surface ice supply, the activity of comets passing near Earth diminishes, or fades, over time.”

In this study, Kaib found that this fading phenomenon also happens among comets passing through the outer solar system near or past Saturn’s orbit. The fascinating fact is that such comets experience much weaker heating from the Sun than those nearer Earth. 

Kaib ran computer simulations of comets traveling near the outer solar system’s giant planets. He showed that the gravity of the giant planets quickly shrinks the orbits of distant comets. This makes smaller excursions away from the Sun in between passages through the outer solar system.

Kaib said“We should therefore expect the outer solar system to have many more comets on these shrunken orbits than those on larger orbits. Instead, astronomers see the opposite; distant comets with shrunken orbits are almost absent from astronomers’ observations, and comets with larger orbits dominate our census of the outer solar system. Rapid comet fading during this orbit-shrinking explains this paradox since it will effectively make older comets invisible to astronomers’ searches.”

“Fading among distant comets was discovered by combining the results of computer simulations of comet production with the current catalog of known distant comets. 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.”

“The comet fading characterized in my work will be critical to properly understanding and interpreting this imminent deluge of newly discovered comets.”

Journal Reference:

  1. Nathan A. Kaib et al. Comet fading begins beyond Saturn. DOI: 10.1126/sciadv.abm9130
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