The discovery probability of long-period comets passing near the Sun is highest during their first passage and then declines, or fades, during subsequent return passages. Comet fading is largely attributed to devolatilization and fragmentation via thermal processing within 2 to 3 AU (astronomical unit) of the Sun (1 AU being the Earth-Sun distance). New numerical simulations by Dr. Nathan Kaib, an astronomer in the Department of Physics and Astronomy at the University of Oklahoma, show that comet-observing campaigns miss vast numbers of long-period comets making returning passages through the Saturn region (near 10 AU) because these comets fade during prior, even more distant passages exterior to Saturn and thus elude detection.
This image, taken with the 2.5-m Nordic Optical Telescope, shows the long-period comet C/2014 B1 (Scwartz). The orbit of this comet lies exterior to Saturn’s, yet it exhibited cometary activity during its most recent passage through the Solar System’s giant planet region. Image credit: David Jewitt.
“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,” Dr. Kaib said.
“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.”
“This cometary activity is what gives comets their striking appearance in the sky and makes them relatively easy for astronomers to find,”
“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 the new study, Dr. Kaib discovered that this fading phenomenon also occurs among comets passing through the outer Solar System near or beyond the orbit of Saturn.
What makes his findings surprising is that such comets experience much weaker heating from the Sun compared to those nearer Earth.
In fact, unlike nearer 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 the outer Solar System’s giant planets, Dr. Kaib showed the gravity of the giant planets quickly shrinks the orbits of distant comets so they make smaller excursions away from the Sun in between passages through the outer Solar System.
“We should therefore expect that the outer Solar System has many more comets on these shrunken orbits compared to those on larger orbits,” Dr. Kaib said.
“Instead, astronomers see the opposite; 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.”
“Rapid comet fading that occurs during this orbit-shrinking explains this paradox, since it will effectively make older comets invisible to astronomers’ searches.”
Given that distant comets are hard to study due to their remoteness, astronomers’ understanding of comets is mostly based on studying the ones on orbits near Earth.
The new finding suggests that passages through the outer Solar System may 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,” Dr. Kaib said.
“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 study was published in the journal Science Advances.
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Nathan A. Kaib. 2022. Comet fading begins beyond Saturn. Science Advances 8 (13); doi: 10.1126/sciadv.abm9130
Source link: https://www.sci.news/astronomy/long-period-comets-10668.html
