The vital role of a carbon molecule called methyl cation (CH3+) in interstellar carbon chemistry was predicted in the 1970s, but the unique capabilities of the NASA/ESA/CSA James Webb Space Telescope have finally made observing it possible — in a region of space where planets capable of accommodating life could eventually form.
Carbon compounds form the foundations of all known life, and as such are of a particular interest to scientists working to understand both how life developed on Earth, and how it could potentially develop elsewhere in our Universe.
As such, interstellar organic chemistry is an area of keen fascination to astronomers who study the places where new stars and planets form.
Molecular ions containing carbon are especially important, because they react with other small molecules to form more complex organic compounds even at low interstellar temperatures.
The methyl cation (CH3+) is one such carbon-based ion.
This ion has been posited by scientists to be of particular importance since the 1970s and 1980s.
This is due to a fascinating property of CH3+, which is that it reacts with a wide range of other molecules.
This little cation is significant enough that it has been theorized to be the cornerstone of interstellar organic chemistry, yet until now it has never been detected.
Webb’s unique properties made it the ideal instrument to search for this crucial cation.
“This detection of CH3+ not only validates the incredible sensitivity of Webb but also confirms the postulated central importance of CH3+ in interstellar chemistry,” said Dr. Marie-Aline Martin, a researcher at Paris-Saclay University.
The CH3+ signal was detected in the protoplanetary disk of the d203-506 system, which is located about 1,350 light years away, in the Orion Nebula.
Whilst the star in d203-506 is a small red dwarf, with a mass only about a tenth of the Sun’s, the system is bombarded by strong ultraviolet radiation from nearby hot, young, massive stars.
Astronomers believe that most planet-forming disks go through a period of such intense ultraviolet radiation, since stars tend to form in groups that often include massive, ultraviolet-producing stars.
Fascinatingly, evidence from meteorites suggest that the protoplanetary disk that went on to form our Solar System was also subject to a vast amount of ultraviolet radiation emitted by a stellar companion to our Sun that has long since died (massive stars burn brightly and die much faster than less massive stars).
The confounding factor in all this is that ultraviolet radiation has long been considered to be purely destructive to the formation of complex organic molecules — and yet there is clear evidence that the only life-supporting planet that we know of was born from a disk that was heavily exposed to it.
Dr. Martin and colleagues may have found the solution to this conundrum.
Their work predicts that the presence of CH3+ is in fact connected to ultraviolet radiation, which provides the necessary source of energy for CH3+ to form.
Furthermore, the period of ultraviolet radiation experienced by certain disks seems to have a profound impact on their chemistry.
“This clearly shows that ultraviolet radiation can completely change the chemistry of a protoplanetary disk,” said Dr. Olivier Berné, an astronomer at the University of Toulouse.
“It might actually play a critical role in the early chemical stages of the origins of life by helping to produce CH3+ — something that has perhaps previously been underestimated.”
Although research published as early as the 1970s predicted the importance of CH3+, it has previously been virtually impossible to detect.
Many molecules in protoplanetary disks are observed using radio telescopes.
However, for this to be possible the molecules in question need to possess what is known as a ‘permanent dipole moment,’ meaning that the molecule’s geometry is such that its electric charge is permanently off balance, giving the molecule a positive and a negative ‘end.’
CH3+ is symmetrical, and therefore its charge is balanced, and so lacks the permanent dipole moment necessary for observations with radio telescopes.
It would theoretically be possible to observe spectroscopic lines emitted by CH3+ in the infrared, but the Earth’s atmosphere makes these essentially impossible to observe from Earth.
Thus, it was necessary to use a sufficiently sensitive space-based telescope that could observe signals in the infrared.
Webb’s NIRSpec and MIRI instruments were perfect for the job.
“Our discovery was only made possible because astronomers, modelers, and laboratory spectroscopists joined forces to understand the unique features observed by Webb,” Dr. Martin said.
The discovery is reported in a paper in the journal Nature.
O. Berné et al. Formation of the Methyl Cation by Photochemistry in a Protoplanetary Disk. Nature, published online June 26, 2023; doi: 10.1038/s41586-023-06307-x