In the field of cosmology, some chunks of gas and dust are fascinating enough to be named. One in particular has been of interest to scientists since its discovery in 1986. It is an unusual comet called Machholz 1.
Unusual is Relative
Our solar system has its fair share of comets. Until recently, all of them have been classified into two categories. The majority of the comets observed in our solar system have a chemical composition that favors water ice. The estimated average H2O content of the first class of comets is 10^13 of ice. The second class of comets are distinguished by the notable presence of carbon molecule depletions, giving them the title "Carbon-Chain Depletion Comets". Why the difference? The current consensus is that comets develop different chemical composition based on where they typically reside. Class 1 comets are believed to have formed in the vicinity of our system's gas giants and subsequently traveled to the Oort Cloud where many of them remain. Class 2 comets have often been traced to the icy Kuiper Belt, specifically the much colder, farther out regions where their lack of exposure to the sun alters the rate of chemical depletion.
But Machholz 1 is a chemical oddity. While it bears many of the same carbon compounds as Class 2 comets, such as C2, C3 and cyanogen, they appear in much lower levels than those found in Carbon-Chain Depletion Comets. Recent observation has indicated that Machholz 1 isn't alone in this. A rare few identified comets share this unusual characteristic.
The Big Deal About Depletion
Scientists are interested in comets primarily for their age. It is believed that most comets formed in a primordial stage of the universe when systems like our own were just coming into existence. Their exposure to extremes in temperature and radiation make comets into valuable time capsules, hinting at conditions in our galaxy long before Earth was even a protoplanetary disk. In fact, it's highly likely that a significant portion of the water on our planet came from early comet collisions.
The low levels of carbon compounds in this new class of comet suggest a number of possibilities. Objects like Machholz 1 may be extremely old, from extremely far away, or both. In the first case, an older Class 2 comet would see a greater amount of depletion, indicating that Machholz 1 is one of the earliest objects in our solar system. Studying it would open up new avenues of thought concerning what our solar system was like in its nebular period. The other possibility is that the pending "Class 3" comets come from an alien solar system where the overall chemical content is different from ours.
Continued Opportunity for Study
Machholz 1 may be an interloper from another system or a long-lost cousin of closer comets, but these days it makes its home in an orbit around our sun. It is projected to come into view again in 2012. While this and other comets like it haven't been extensively studied over the past two decades, new data from its last appearance in 2007 has piqued the scientific community's interest in these objects of unknown origin. In three years' time, Machholz 1 is likely to get a lot more attention than it's used to.