Even beyond the orbit of Saturn, previously thought to be part of their safe zone, comets lose volatile ice and become fainter over millions of years. What causes this change remains a mystery, but the solution may explain some other puzzling observations of icy objects in the outer Solar System.
Every few years, Earth is dazzled by a comet visiting the inner Solar System, making its first pass – or first for tens of thousands of years – close to the Sun. Meanwhile, many comets pass close to the Earth much more frequently, barely noticed. We pay them little attention, unless visiting via spacecraft, because they've spent so much time in proximity to the Sun that most of their frozen material has been vaporized away. The displays they put on today have lost their sparkle.
Out past Saturn, however, sunlight is weak – so weak that ices of methane and ammonia that turn to gas at temperatures of −182.456 °C (−296.421 °F) and −77.73 °C (−107.91 °F) respectively stay solid. Therefore, it was assumed a comet could spend as long as it liked in such space without losing material, or brightness. In the journal Science Advances the University of Oklahoma's Dr Nathan Kaib provides evidence this isn't true.
“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,” Kaib in a statement. “However, sometimes they develop highly elliptical orbits and, in turn, make regular incursions toward the Sun and its nearby planets.”
Jupiter and Saturn can dramatically change the orbit of a comet that makes a close approach to them, but the two giant planets can also have a slower effect on much more distant comets. Kaib modeled how the orbits of comets would change as a result of this slow tug, finding that many would shrink, so even their maximum distance from the Sun would be much greater when first formed.
“We should therefore expect that the outer solar system has many more comets on these shrunken orbits compared to those on larger orbits,” 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.”
Kaib explains this paradox by concluding the comets with shrunken orbits have, like those in the inner Solar System, faded to the point where they are hard for us to detect. The claim would only work if comets really are stable beyond some as-yet-undetermined greater distance.
It is of course possible Kaib's modeling is wrong, and the reason we are finding fewer comets with shrunken orbits is that there are just not that many there to find. However, several recent discoveries provide support for his claims.
Cometary activity was spotted on the so-called “mega comet” C/2014 UN271 (Bernardinelli-Bernstein) when it was not much closer than Neptune. This follows observations by Hubble of cometary activity by C/2017 K2 Panstars at 2.4 billion kilometers (80 percent as far from the Sun as Uranus). Meanwhile the deeply strange object Comet 29P/Schwassmann-Wachmann keeps erupting despite being in a fairly circular orbit beyond Jupiter where its ices should be stable.
We don't know what is driving the release of gasses in these cases, but they show icy objects can lose material at great distances from the Sun. Although slow, given enough time, such processes may cause comets to fade in the manner Kaib proposes.