With the recent close flyby of comet Siding Spring from the surface of Mars and the upcoming landing of Rosetta’s Philae lander on 67P/Churyumov–Gerasimenko, comets have taken center stage during the last few months. Yet not all of the action surrounding these small cosmic “dirty snowballs” is limited to our side of the galaxy only. Akin to the discovery of exoplanets, exocomets have also been detected orbiting other stars as well, like 49 Ceti, Eta Corvi, and HD 100546. Around one of these stars, called Beta Pictoris, hundreds of exocomets are constantly producing large amounts of gas and dust through a perpetual process of collision and evaporation, providing astronomers with a valuable insight into the similar processes that have taken place during the early days of our own Solar System.
Located 63.4 light-years away in the direction of the constellation Pictor, Beta Pictoris has been the subject of extensive study by astronomers for decades, in their search for planets around other stars. Being a member of the young Beta Pictoris Moving Group, which is between 12 and 20 million years old, Beta Pictoris is an A-type, main sequence bluish-white dwarf star with 1.75 times the mass and 8.7 times the luminosity of the Sun that is most famous for its large 1,450-Astronomical Unit-wide structurally complex circumstellar disc of dust and gas, seen edge-on from our vantage point here on Earth (1 Astronomical Unit is the average Earth-Sun distance). Staying true to its reputation as a potential planet-making factory, Beta Pictoris was eventually found in 2008 to harbor a gas giant planet, called Beta Pictoris b, with a mass 12 times that of Jupiter and orbiting its star at a distance of approximately 8-10 Astronomical Units away—an orbit comparable to that of Saturn in our Solar System.
Detailed spectroscopic studies of the circumstellar disk around Beta Pictoris had also identified a series of narrow redshifted absorption lines which exhibited a strong, short-term time-dependent variability, that had been attributed to the presence of infalling comet-like material that was absorbed by the star. Further observations had helped to establish the disk’s complex structure as well, revealing the existence of several distinct rings of hot silicate dust and cold volatile organic compounds like carbon monoxide within the disk’s inner and outer regions respectively. Theoretical models had shown that these rings should have been cleared away on a very short amount of time following the formation of the Beta Pictoris system, destroyed by the star’s intense ultraviolet radiation pressure. The fact that they have remained stable means that they must be continuously replenished, either through the collisions of kilometer-sized or larger asteroid- and comet-like objects within the disk, or evaporation as the former approach close to and are absorbed by their host star.
A new study, which was published on the 23 October issue of the journal Nature by a French team of astronomers, gives more credence to this hypothesis by presenting strong evidence for the existence of two distinct families of comets around Beta Pictoris. To that end, the researchers analyzed a total of 1,106 spectroscopic observations of Beta Pictoris that had been obtained between 2003 and 2011 with the High Accuracy Radial velocity Planet Searcher, or HARPS, a high-resolution spectrograph which is mounted on ESO’s 3.6-meter telescope at the La Silla Observatory in Chile. Hidden in these archival data were approximately 6,000 transient absorption features which varied on timescales of one to six hours and exhibited high radial velocities with respect to Beta Pictoris itself. In order to better characterize the nature of these absorption features and exclude those that were caused by noise and other artifacts in the data, the researchers conducted a careful systematic analysis, eventually coming up with a sample of 493 different transiting signatures that had an adequately high signal-to-noise ratio. The results of their analysis showed that each one of these signatures corresponded to a transiting exocomet in the Beta Pictoris system. “The young planetary system surrounding the star Beta Pictoris harbours active minor bodies,” write the researchers in their study. “These asteroids and comets produce a large amount of dust and gas through collisions and evaporation, as happened early in the history of the Solar System. Spectroscopic observations of Beta Pictoris reveal a high rate of transits of small evaporating bodies, that is, exocomets … Our results show that the evaporating bodies observed for decades in the Beta Pictoris system, are analogous to the comets in our own Solar System.”
Further study of the speeds, sizes, and orbital orientations of these excocomets revealed that the latter weren’t scattered in random orbits around their host star but were divided in two distinct populations instead, each with its own orbital and physical properties. One of these populations displays relatively shallow absorption lines, which indicates that it consisted of relatively old cometary bodies that had depleted their reservoirs of gas and dust, lacking any significant outgassing activity and were found to orbit closer to Beta Pictoris, while appearing to be trapped in a mean motion resonance possibly with Beta Pictoris b or another hypothetical as yet unseen massive planet which, according to the researchers, causes them to fall toward their star. The second population features much deeper absorption lines, indicating that its members are much younger and more active, possibly produced by the recent fragmentation of a larger body and orbiting further out on similar paths that resemble those of the Kreutz family of comets in our own Solar System and those of the Shoemaker-Levy 9 cometary fragments which impacted Jupiter in July 1994. “Beta Pictoris is a very exciting target!” says Flavien Kiefer, a postdoctoral researcher at the Pierre and Marie Curie University in Paris, France, and lead author of the study. “The detailed observations of its exocomets give us clues to help understand what processes occur in this kind of young planetary system.”
Besides being the biggest census ever of exocomets around another star, the study by Kiefer’s team might also help to answer a series of puzzling observations of the Beta Pictoris system that were made by a research team in 2012 with the Atacama Large Millimeter/submillimeter Array, or ALMA, in Chile. These observations had revealed the presence of a vast belt of carbon monoxide gas that was asymmetrically distributed around Beta Pictoris, ranging in distance between 50 and 160 AU with a large portion concentrated in a massive clump that was located 85 AU from the star—equivalent to approximately three times the distance between Neptune and the Sun in our Solar System. The presence of this vast, gaseous belt around Beta Pictoris was particularly puzzling, due to the fact that all carbon monoxide should be destroyed by the star’s ultraviolet radiation in timescales of less than a century. The amounts that have been detected around Beta Pictoris, which have been calculated to be more than 200 million billion tons, or about one-sixth the mass of Earth’s oceans, indicate that the gas should be constantly replenished. “Although toxic to us, carbon monoxide is one of many gases found in comets and other icy bodies,” says Aki Roberge, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and member of the ALMA research team that conducted the observations. “In the rough-and-tumble environment around a young star, these objects frequently collide and generate fragments that release dust, icy grains and stored gases.”
One of the proposed explanations for this observed overabundance of carbon monoxide has been that two Mars-sized planetary bodies within the star’s circumstellar disk have collided not more than a million years ago, producing a flux of asteroid and comet-sized debris which through subsequent ongoing collisions have been providing the carbon monoxide belt with fresh material ever since. Nevertheless, the ALMA observations have also hinted at the possibility of the belt ultimately consisting of not one but two distinct clumps located on opposite sides of the star, with the second one escaping detection due to the circumstellar disk’s edge-on orientation relative to our line of sight. If future observations of the gas’ orbital motion were to confirm this latter hypothesis, the most probable cause for the formation of the two clumps, according to scientists, would be the ongoing collisions between cometary bodies in two huge comet swarms that would be trapped in mean motion resonances on opposite sides of Beta Pictoris by the massive gravity of a hypothetical and as yet undetected Saturn-type planet. Simulations have shown that such a planet, if it indeed exists, would be located in the inner region of the carbon monoxide belt. “Detailed dynamical studies are now under way, but at the moment we think this shepherding planet would be around Saturn’s mass and positioned near the inner edge of the CO belt,” says Mark Wyatt, an astronomer at the University of Cambridge in England. In either case, scientists have calculated that in order for the carbon monoxide belt to have remained in its present state since the formation of Beta Pictoris, an unusually high amount of cometary mass must have been orbiting the star, exhibiting a high collision rate, with a large comet being completely destroyed every five minutes.
The study of exocomets around Beta Pictoris helps to showcase the fact that the processes that have led to the formation and evolution of our own Solar System seem to be very common throughout the rest of the Universe as well. “For the first time a statistical study has determined the physics and orbits for a large number of exocomets,” says Kiefer. “This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
Indeed, as the history of astronomical study has repeatedly shown, learning more about the Universe means to learn more about ourselves.