Earlier this year the comet C/2016 R2 (PanSTARRS) made its closest approach to the Sun in more than 20,000 years. It never got close enough to Earth to be seen with binoculars, but astronomers were intrigued to witness an object with a different composition from any previously measured ones. An analysis of the gases C/2016 R2 threw off has raised two theories about why it is so different, including the possibility it was once part of a planet beyond Neptune.
We find several dozen new comets each year, only a few of which get bright enough to create much interest. As C/2016 R2 got closer it stood out, with comet-watchers noting first its unusually complex tail, and then that it was bluer than any comet seen for more than 50 years.
The color was attributed to a large amount of carbon monoxide (CO), and very little dust, in the tail.
In a forthcoming paper in Astronomy and Astrophysics (preprint on arXiv), Dr Nicolas Biver of Sorbonne University describes using the Institut de radioastronomie millimetrique's 30-meter telescope to compare the composition of gases coming from C/2016 R2 to those from other comets.
The CO signal was 10 times stronger than normal, and the molecular nitrogen concentration was higher still. Meanwhile, common comet gases such as water vapor, methanol, and cyanide were rare or almost non-existent. Only two comets have previously been seen to produce this much carbon monoxide, and both were very large and producing other gases and dust in quantities up to 100 times greater than C/2016 R2.
Two previous comets have looked a little like this, but both were observed so long ago that we lacked the capacity to study them with the advanced equipment we have today, so their make-up has remained largely mysterious.
Although comets vary in their composition, something this far from the norm raises big questions about how it formed.
The capacity of water ice to trap nitrogen depends strongly on temperature, so it is likely that any comet with so much nitrogen formed in very cold conditions far from the Sun. Even if C/2016 R2 formed from icy grains in the outer Solar System, however, the lack of dust is puzzling.
Biver posits a more intriguing scenario, where C/2016 R2 was once part of a large Kuiper Belt Object (KBO) like Pluto. Radioactive decay would have warmed the KBO's innards causing ices with low boiling points to migrate towards the surface, producing an outer shell of carbon monoxide and nitrogen. If a collision with another large object knocked a chunk off the KBO's surface it would probably have formed something with a composition like C/2016 R2, making this a possible remnant of Planet X.