In 1986, while watching a star some 63 light-years away called Beta Pictoris, French astronomer Anne-Marie Lagrange and her colleagues noticed something deeply strange. They were watching because, two years earlier, other researchers observing the young, 23-million-year-old star had viewed edge-on the infrared glow of what seemed to be a giant spinning disk of dust and gas, similar to that from which our own solar system was born long ago. Beta Pictoris appeared to be in the latter stages of assembling its own planetary system, and astronomers essentially had a front-row seat.
Studying the starlight shining through the disk Lagrange spied unexpected hints of motion coming and going over hours and days, almost as if some shadowy light-absorbing structures were every now and then swirling into view. For months Lagrange and her colleagues struggled to explain the observations; they considered stellar pulsations, drifting dust grains and other phenomena, but none closely matched the data.
Grasping at straws, in 1987 they offered up one last, wild explanation, later proved to be true: They were seeing starlight shining through giant plumes of gas pouring off icy objects plunging through the disk toward the star. That is, they were seeing star-grazing comets—aka exocomets—years before the first discoveries of exoplanets. Lagrange would go on to devote her PhD work to Beta Pictoris under the tutelage of fellow French astronomer Alfred Vidal-Madjar of the Paris Institute of Astrophysics (IAP), and in 2008 helmed a team that discovered and imaged a giant planet, Beta Pictoris b, freshly formed around the star.
Nearly 30 years after the discovery of Beta Pictoris’s disk and comets, the system is one of the most-monitored objects in the sky. Today, Lagrange and a team of other French astronomers add one more facet to astronomers’ understanding of the embryonic planetary system, announcing the most complete census of its exocomets ever created. Their findings are published in Nature. (Scientific American is part of Nature Publishing Group.)
Using eight years of archival data from the European Southern Observatory’s HARPS planet-finding spectrograph, the team catalogued an unprecedented number of star-grazing comets around Beta Pictoris, detecting nearly 500 by the telltale absorption of starlight from their gassy tails passing in front of the star as seen from Earth. A few other stars are also known to harbor exocomets but never before have astronomers mapped such great numbers of these small, icy bodies so far beyond our solar system. “This is a laudable study, and the determination of these researchers is remarkable,” says Aki Roberge, an astronomer at NASA Goddard Space Flight Center who wrote a commentary to accompany the paper. “On one hand, star-grazing comets were discovered around Beta Pictoris a long time ago but on the other hand this study is only possible through sustained, dedicated monitoring over many years.”
By carefully analyzing the speeds and estimated sizes of each detected cometary gas cloud selected from more than 1,000 HARPS observations, the team discovered that the comets are divided into two distinct families—an outer family sedately circling the star at distances comparable with the separation of Mercury from the sun and an inner family exhibiting a wide range of velocities, orbiting even closer in. Curiously, the family farther out from the star seems to be producing far more gas than the closer-in comets—the exact opposite of what would be expected, given that comets in our solar system tend to grow more active the closer they come to the intense heat of our sun.
According to the study’s lead author, Flavien Kiefer, an astronomer at the IAP, the likely explanation is that the inner family consists of older comets that have nearly depleted their reservoirs of gas and dust, whereas the outer family is composed of fresher or bigger comets produced from the recent fragmentation of a larger parent body. Based on the orientations of their scattered, close-in orbits, the inner cometary family also appears to be trapped in an orbital resonance, herded around the star by the gravitational influence of a nearby massive planet—perhaps Beta Pictoris b, or maybe another world as yet unseen. “This resonance is very similar to the influence of Jupiter in our own solar system, which produces most of the short-period comets around the sun,” Kiefer says. “We could be seeing some of the ejected remnants from the formation of Beta Pictoris b…. It’s like we are observing a much younger version of our sun, just after it formed its planets.”
One mystery still unsolved is the nature of the parent body that produced the outer belt of comets around the star. Kiefer says the parent body might have been an extra-large comet that came from the inner belt, something trapped in resonance with Beta Pictoris b. If the giant comet passed too close to the planet, gravitational forces could have pulled the comet apart, exposing fresh material to evaporate in starlight. But Roberge notes that the outer belt’s progenitor could have been planetary in size.
In 2013 she was part of a team that used the Atacama Large Millimeter/submillimeter Array of radio telescopes to discover two giant clouds of carbon monoxide at the outer fringes of the Beta Pictoris system. One possible explanation for the positioning and shape of the clouds was the gravitational sculpting from a giant, unseen planet far from the star but another was the recent, destructive collision of two Mars-mass icy worlds. Fragments from such a collision could have cascaded down into the inner parts of the system as a swarm of massive comets. “If the carbon monoxide clumps were caused by this putative massive collision, then that could just possibly be connected to these fragments we’re now seeing,” Roberge says. “Whether that’s actually the case, I don’t know, but it would make me very happy if this all hung together like that.” Pinning down the plausibility of this alternate formation scenario will depend on dynamical modeling of the fragments produced by such a cataclysmic collision as well as on future observations of the orbiting carbon monoxide clouds.
Beyond the comets’ source, the greatest mystery concerning them is: Where is their water? Astronomers have yet to see any indication of it, despite years of searching. “If these are icy, water-rich comets like we expect them to be, we’d expect to see the water photoevaporating and getting broken up into daughter products like hydrogen and oxygen—and we haven’t really seen that yet,” Roberge says. “No one has made a hard prediction for how much water should be there and whether we could’ve seen it or not but we all have this burning question in the back of our minds. Someone will probably stick their neck out on this soon.”
If deeper investigations fail to show any sign of water in Beta Pictoris’s comets, Kiefer says, the solution may be that they are actually quite different from our own. They may perhaps being made mostly of carbon monoxide or carbon dioxide ices rather than water—an unsettling prospect for astrobiologists hoping that most stars will harbor water-rich habitable worlds. He and his collaborators are already planning more in-depth studies of the star’s comets as well as those of one of its siblings born from the same stellar nursery—a star called HD 172555 that has already been revealed to have a few exocomets of its own. Roberge is also studying a handful of exocomets recently found around another star, 49 Ceti.
Looking back on the recent history of a star far away but close to her heart, Lagrange feels vindicated by the ongoing waves of discovery. “In the 1980s I didn’t expect to still be working on Beta Pictoris 30 years later,” she says, adding that she had been discouraged from studying the star for her PhD. “Many people were very skeptical about the comet scenario, and did not believe that one could detect comets outside the solar system; it was barely known that solar system comets sometimes grazed the sun and evaporated. I’m glad that this comet scenario has survived all these tests throughout the years.”