Objects that look like asteroids can still become active for a number of reasons. These objects are known as centaurs and can have points of activity and produce tails. **pamela l gay/psi。
According to an article by Eva Lilly, a senior scientist at the Institute for Planetary Science, a close encounter with Jupiter or Saturn leads to a rapid reshaping of the orbit that could cause the centaur to exhibit comet-like activity.
Centauri are small celestial bodies, similar in size to asteroids but similar in composition to comets, revolving around the Sun in the outer solar system, mainly between the orbits of Jupiter and Neptune.
We've found answers to some long-standing mysteries as to why some centaurs are as active as comets while the rest look like ordinary quiet asteroids. No one knows why they behave the way they do. It doesn't make any sense.
There is no correlation with size, color, or even orbital type," says Lilly, lead author of "Activity Triggers in the Jupiter Family of Comets in the Centauri and High Perihelion Families," which appeared in the Astrophysical Journal Letters. PSI scientists Kat Volk, Jordan Steckloff and Henry Hsieh are co-authors.
In our work, we studied all known dynamical histories of Centauri, both active and inactive, and combined our findings with thermal modeling. We are interested in finding some pattern that is common to active Centauri and lacking in inactive celestial bodies in the population. Mapping the dynamical history of the constellation Centauri is a tricky feat – they operate in the realm of giant planets, and their orbital evolution is controlled by the chaotic effects of the giants' gravity, "says Lilly.
We used a numerical integrator – one that could **how the orbits of celestial bodies evolved**, but for centaurs, this can only be known in a very short time - usually a few hundred years, after which chaos makes **inaccurate. We found that all active centaurs experienced a close encounter with Jupiter or Saturn, and this encounter resulted in a huge orbital change that we call a "jump".
The a- jump is primarily a reduction of the semi-major axis of the orbit of the Centauri, while reshaping it from an elliptical to a more circular and lower perihelion orbit. The change is very rapid - about a few months, the semi-major axis can be reduced by a few astronomical units, "said Lilly.
The A-Jump then effectively places the affected Centaurs in orbit, where their surfaces can be heated for a longer period of time, so that heat waves can reach the ice inside, which then sublimates and effectively animates the Centauri. Centaurs are cold by nature;They originate in the Kuiper Belt beyond Neptune, but most of their dynamic lifespans are spent in distant regions of the solar system, where the environment is too cold for water and other ice to sublimate.
They are actually stored in the refrigerator," Lilly said. "A-jumps simply move some of them quickly closer to the Sun, where it's warm enough for the ice to undergo a phase transition, like sublimation, turning the constellation Centauri into a comet. This is confirmed by our thermal model. Our findings suggest that each Centauri has the ability to become active by nature, but it all depends on how its orbit evolves.
Activity has been observed in about 10% of centaurs, and the reason for this has not been known until now. All comets of the Jupiter family are periodically active and behave like ordinary comets in most cases.
Therefore, we believe that the A-jump may be the main trigger for comet activity in the constellations Centauri and JFC. Our findings further suggest that the analysis of recent dynamic history can be used to identify objects that are currently active or likely to become active soon, and we have identified three such centaurs that have recently made jumps and should be considered high-priority targets for observational monitoring in search of activity," Lilly said.
More information: Eva Lilly et al., Semi-major axis jump as an activity trigger for comets of the Jupiter family in the constellation Centauri and high perihelion, Astrophysical Journal Letters (2024). doi： 10.3847/2041-8213/ad1606