Comets and asteroids have classically been considered to be two distinctly different types of objects. Both are considered “small solar system bodies”, too small to be considered planets, but large enough to be tracked individually as they travel through the solar system. Asteroids are typically thought of as inert chunks of rock or metal that are mostly found on roughly circular, flat orbits (compared to those of the major planets) in the main asteroid belt between the orbits of Mars and Jupiter, where we believed they formed (and where we therefore believe they’ve been since the formation of the solar system). Comets, on the other hand, are thought of as “dirty snowballs” that travel along often highly elongated orbits that take them from the cold, distant outer solar system to the warm inner solar system where we usually observe them. They are believed to originate in the one of two distant reservoirs of frozen, icy bodies: the Kuiper Belt just outside the orbit of Neptune, and the far more distant Oort Cloud. Occasionally, a collision or the slight gravitational tug of a passing star sends one of these bodies Sun-ward into the inner solar system, where the object’s ice heats up and sublimates (turns from solid to gas), ejecting gas and dust which we observe as the familiar fuzzy haze that
surrounds the core, or nucleus, of the comet, and often also in the form of a cometary tail.
Comets have been able to preserve their icy content over the 4.6 billion year life of the solar system because they have spent most of their lives stored in the cold outer solar system beyond the orbit of Neptune. Meanwhile, if asteroids ever contained ice (and there is evidence that indicates some did once contain ice, albeit in the distant past), they are believed to be mostly baked dry by now by the much higher temperatures in the main asteroid belt. Recent research has been challenging this traditionally-held picture though.
The first main-belt comet, an object that had an orbit like a main-belt asteroid but had the appearance of a comet, was discovered in 1996. The thought that an object orbiting so close to the Sun could still have enough surviving ice to power cometary activity, however, was initially so disturbing to astronomers that many believed that what they had witnessed was the result of an impact tossing dust up into space. Observations six years later, however, showed that cometary activity had returned. This discovery all but ruled out the impact hypothesis for driving the activity since two random impacts on the same asteroid would be required in an
extraordinarily short period of time. Comets, however, routinely exhibit recurrent activity, as temperature changes as their orbits take them closer to and then farther away from the Sun make them warmer and then colder, turning sublimation on and off in predictable ways. Main-belt comets have much more circular orbits and as such do not go through temperature swings as severe as other comets, and so we suspect that their activity may be instead be controlled by seasonal effects caused by the tilt of their rotational poles (as compared to their orbits) in the exact same way that seasons with widely varying temperatures are caused on Earth.
Main-belt comets have much to tell us about the true composition of the asteroid belt, which in turn will help us to understand the formation of our own solar system, and therefore the conditions that might need to be present for similar solar systems to form around other stars. In the case of Earth in particular, main-belt comets may be the key to understanding a particularly vexing problem, that of discovering the origin of our water. Due to its close proximity to the Sun, the Earth is thought to have been too warm to be able to accumulate much water as it was forming, and likely accumulated most of its water from impacts from objects from colder parts of the solar system. Comets from the outer solar system were once considered good candidates for playing this role as water deliverers, but recent studies have suggested that main-belt objects may have played a much larger role than previously thought. The discovery that ice still remains in the asteroid belt in main-belt comets gives us a present-day opportunity to probe this potential ancient water source, and as such, is of great interest to astronomer.
The Main Belt Comet La Sagra imaged with PS1. studying the origin of water, and therefore of life, on Earth.
The extremely recent discovery of the main-belt comets (first discovered in 1996, but not recognized as a new class of objects until 2006 when the discoveries of two
more were announced) means that we still have much to learn about them. At the moment, just five such objects are known, meaning that at the moment, a high priority
is to discover more so that we can begin to understand the extent and diversity of the population of these strange objects. How many are there in total? Are they
confined to particular parts of the asteroid belt? Luckily for astronomers interested in these questions, this is one area where Pan-STARRS is expected to help. By
surveying the sky repeatedly and being able to detect fainter objects than previous surveys, we expect that Pan-STARRS should be able to discover many more main-belt comets. To do so, we require sophisticated techniques to sift through the mountains of Pan-STARRS data generated each night to automatically select potential comets for further inspection by humans. Since the start of the Pan-STARRS survey, these techniques have been undergoing refinements to optimize their comet-finding effectiveness and have now reached the point where Pan-STARRS has been credited with the discovery of four comets this year (three of them just in the last 2 months including C/2011 L4 (PANSTARRS) ). None of these have so far turned out to be main-belt comets, but as comet discoveries start to become more routine, we hope it’s just a matter of time!
Even before Pan-STARRS makes its first main-belt comet discovery, it is already assisting research on known main-belt comets. A new paper submitted to the Astronomical Journal earlier this week describes a worldwide observational campaign to study the most recently discovered main-belt comet named P/2010 R2 (La Sagra), or P/La Sagra for short. Pan-STARRS actually recorded the first known observations of this object, about a month before its official discovery, but unfortunately, it escaped our detection software at the time and was not found in our data until after it was discovered by others. Nonetheless, early Pan-STARRS observations of the comet played a key role in the monitoring of its activity over the year-long series of observations that we present in this new paper. In particular, these observations show the comet becoming steadily brighter over a period of months, strong evidence for ongoing dust emission, a characteristic signature of cometary activity, and confirmation that this object is indeed a true main-belt comet.
While an exciting start, we of course hope that this paper will not be the last that Pan-STARRS has to say about main-belt comets. Stay tuned…