The case of the Centaurs that didn't bark in the night

Why does the brightness of some Centaurs suddenly increase dramatically while others do not?
Centaurs are strange objects. These strange objects are large, ice-covered bodies made of rock and ice that orbited the Sun far beyond Neptune. However, their orbits have been shifted by the gravity of the giant planet to bring them closer to the Sun. Although they can reach the Sun as close as Jupiter, their orbits are slightly elliptical and take them out to Neptune and Uranus.

This is not a safe area to be. These objects are subtly affected by the gravity of Saturn, Uranus and Neptune. They either hit one of the planets or fall further to the Sun, where Jupiter dominates their orbits. Or they are ejected entirely from the inner solar systems. These objects are transitional, and they can neither be inner solar system comets or TNOs. Chiron was the first to be discovered. He was half human and half horse Centaur according to Greek mythology. It is an appropriate name, as it is half of one thing, and not all.

Centaurs can be seen as active, which is when they actively shed gas and dust just like comets. This is usually due to comets turning water ice into gas once they reach Jupiter's orbit. However, Centaurs are too far away from the Sun to make this happen. This could also be caused by carbon monoxide (CO), or carbon dioxide (2 CO 2 )*.

Illustration of the rings around the centaur Chariklo. These are the first rings in the solar system to not circle a gas giant planet. Credit: ESO/L. Calcada/Nick Risinger (

Another possibility is that the amorphous water-ice crystallization causes it. While we are used to water ice as a crystal (like snowflakes), in extreme temperatures water ice can develop an amorphous structure. The Sun can activate objects if they are less than 2 billion kilometers from the Sun.

It might seem obvious that the best way to find out this information is to first look at active Centaurs and observe their behavior. You should also look at those that are inactive and quiet. This will allow you to see the similarities and, perhaps more importantly, the differences between them. Are their orbits different? Are active ones more likely to be closer to the Sun? If so, how much? You want to cover as many ground as possible when you are trying to find trends.

A team of astronomers studied the 13 Centaurs to find out why some Centaurs have active lives and others don't. To make it easier for faint details to be seen, the images have been reversed in color (black = brighter). None of the images showed clear evidence of activity. Credit: Lilly et al. 2021.

A team of astronomers decided that a passel (or herd) was worth a closer look. Centaurs to see what was there. The criteria for their selection were simple. They wanted to focus on those with well-defined orbits and near perihelion (closest approach the Sun), and in a position bright enough to be observed using various telescopes including the Gemini-North in Hawaii (an 8.1-meter) and the Dark Energy Camera Chile. They searched through Pan-STARRS' database and found 13 Centaurs which met their criteria.

These objects are ranging in size from 5 km to 120 meters in diameter, some as close as Jupiter to the Sun, others as close as Saturn to Uranus.

Five of the 13 were close enough to observe that amorphous ice crystallizes when it is heated.

Here's the weird part: They were all quiet. Each one.

It's possible that the closest ones were active, but at a lower level than we can see. However, if that were the case, it would mean the levels must have been very low, as they weren't expected to be active. What gives?

It is evident that being warm is not enough to cause activity. This is an interesting result. Centaurs must be active somewhere else. Is there something else?

Illustration of a comet or asteroid frozen in deep space. Credit: Getty Images/Mark Garlick/Science Photo Library

One theory is that amorphous ice crystallizes and releases heat energy, which could trigger activity. However, this doesn't appear to be the case in this instance. No activity was observed. Some research has shown that crystallization can release carbon dioxide if there is enough of it in the amorphous, but that crystallization can also absorb heat. This makes it more difficult to determine when and where a Centaur is active.

These Centaurs may have been too close to the Sun for too long that their activity has slowed down. They became active when they were first kicked into these orbits by a further out source, but they have since used up their surface ices and are now calm.

This is in line with another thought: a sudden shift in orbit could trigger activity. Perhaps the Centaur passes close enough to one the huge outer planets so that its orbit brings it closer to the Sun. This can cause landslides that expose pockets of ices which can then become gas. All of the Centaurs that were observed have been kept quite far from the large planets, so it at least makes sense. One of them, 2014 PQ70, experienced a sudden shift of orbit around 75 million kilometers approximately 50 years ago. This was determined by backtracking its position in time and its orbit now. It's still inactive like the other two, so astronomers recommend that we keep an eye out for any future outbursts.

It's not only where a Centaur lives that matters, but also how it has been changed. Orbital evolution could be the trigger that generates activity.

A different team of astronomers discovered that Chiron, first Centaur, experienced a burst in activity. It began in February 2021, and continued until June. It was interesting that this occurred when it was at its farthest point from Sun, approximately 2.8 billion km away. It is not known why.

It is obvious that these complex objects have a lot more going on than we initially thought. This happens all the time: Sometimes we think an object or a class of objects is easy to understand, but when we look deeper it turns out that they are far more complex than we originally thought.

This makes them even more fascinating. It is evident that Centaurs need to be studied more closely.