JWST makes first detection of crucial carbon molecule in a planet-forming disk
An international team of scientists including University of Michigan astronomers have used data collected by the NASA/ESA/CSA James Webb Space Telescope to detect for the first time a molecule critical to forming planetary systems that can sustain life.
The molecule, known as the methyl cation (CH3+), was detected in the protoplanetary disk—a disk of dust rotating around a central star from which planets may be built—surrounding a young star.
This simple molecule has a unique property: it reacts relatively inefficiently with the most abundant element in our universe (hydrogen) but reacts readily with other molecules and therefore initiates the growth of more complex carbon-based molecules.
Carbon chemistry is of particular interest to astronomers because all known life is carbon-based. The vital role of CH3+ in interstellar carbon chemistry was predicted in the 1970s, but JWST’s unique capabilities have finally made observing it possible—in a region of space where planets capable of accommodating life could eventually form. Astronomers were able to detect CH3+ with a cross-disciplinary expert analysis, including key input from laboratory spectroscopists.
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. CH3+ 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.
The unique properties of JWST made it the ideal instrument to search for this crucial cation—and already, a group of international scientists have observed it with JWST for the first time.
Specifically, the international team detected a feature from some unknown molecule near 7 microns in wavelength. U-M astronomers Felipe Alarcón and Ted Bergin attempted to explore what potential molecule was emitting from a diverse assortment of previously known constituents. However, they were not able to isolate it from any known molecular emission spectrum, revealing that it was truly a new molecule that was detected: CH3+, one of the key drivers of hydrocarbon chemistry.
“CH3+ is the precursor to nearly all carbon-bearing molecules we detect in space. Molecules that formed from CH3+ are found in stars and planets that are born in distant galaxies, tracing the early stages of galactic formation in the universe. This is the very first detection of this linchpin ion which is only possible due to the incredible sensitivity of JWST,” said Bergin, chair of the U-M Department of Astronomy. “This detection validates decades-old theories about how molecules are formed in the cold, tens of degrees above absolute zero (-441 degrees Fahrenheit) reaches of interstellar space.”
The CH3+ signal was detected in the star-protoplanetary disk system known as d203-506, which is located about 1350 light years away, in the Orion Nebula. While the star in d203-506 is a small red dwarf star, 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. Scientists believe that most planet-forming protoplanetary disks go through a period of such intense ultraviolet radiation, since stars tend to form in groups that often include massive, ultraviolet-producing stars.
“While the presence of CH3+ proves previous predictions, it also pushes our understanding of interstellar chemistry by opening new avenues for research in multidisciplinary areas including the formation/composition of analogs to our solar system,” said Alarcón, a Fullbright scholar and doctoral student at U-M. “JWST spectra bear the imprint of emission from a wide variety of molecules showing its true potential in pushing the border of our current understanding of the universe.”
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.
The team that performed this research 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. For example, JWST observations of protoplanetary disks that are not subject to intense ultraviolet radiation from a nearby source show a large abundance of water—in contrast to d203-506, where the team could not detect water at all.
“This clearly shows that ultraviolet radiation can completely change the chemistry of a protoplanetary disk,” said lead author Olivier Berné of the University of Toulouse, France. “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. JWST’s MIRI and NIRSpec instruments were perfect for the job. In fact, a CH3+ detection had previously been so elusive that when the team first saw the signal in their data, they were not sure how to identify it. Remarkably, the team were able to interpret their result within four short weeks, by drawing on the expertise of an international team with a varied range of expertise.
The discovery of CH3+ was possible through a collaboration among observational astronomers, astrochemical modelers, theoreticians and experimental spectroscopists, which combined the unique capabilities of JWST in space with those of Earth-based laboratories in order to successfully investigate and interpret our local universe’s composition and evolution.”
The team’s results are published in Nature.