Astronomers think they’ve spotted the formation of one or more planets around a star 5,000 light-years away, in the constellation Monoceros. In a recent study, a group of researchers discovered lumps of dust 3–10 times as massive as Earth nestled in a series of spiral arms around the star.
The researchers, led by astrophysicist Philipp Weber from the University of Santiago in Chile, speculated that the dust clumps could be the seeds of gas giant planets.
When the lumps become massive enough, they will have sufficient gravitational attraction to rapidly absorb gas from their surroundings, growing to Jupiter-mass in just a few thousand years. This way of forming planets has long been hypothesized, but the discovery is the first direct evidence of the process. The researchers published their findings in the Astrophysical Journal Letters.
A Young Planet-Forming Star
Astronomers have been monitoring the young star, called V960 Mon, since 2014, when it suddenly increased in brightness twentyfold. Data from the Very Large Telescope (VLT) in Chile revealed that the star is surrounded by a disk of gas and dust that spirals outward in a series of arms. Researchers concluded that the flash was caused by material falling into the star from the disk, hinting that the star could be forming planets.
Weber and his colleagues compared the VLT images with observations of the same star gathered by the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope, also located in Chile. ALMA’s sensitive antennas can detect long-wavelength emissions coming from lukewarm dust in the system’s spiral arms, piercing through the otherwise opaque material, and revealing dense blobs growing inside.
Astronomers first identified an exoplanet 30 years ago and now think that most Sun-like stars in the Milky Way host planets. “Nature is telling us that it’s very easy for her to form planets, and that’s a slap in the face for astronomers because we still don’t know how they form,” said astrophysicist Sebastián Pérez, also from the University of Santiago in Chile, a coauthor of the study. “With this study we have advanced a bit further.”
Astronomers debate between two leading theories of planet formation. In the classical approach, called core accretion, rocks and asteroid-sized objects crash into each other, forming increasingly larger bodies that eventually become massive enough to absorb gas from their surroundings. The alternative theory is called gravitational instability, in which clouds of pebble-sized particles within a planetary disk become so dense that they collapse under their own gravity, growing to gas-absorbing mass much faster than under the core accretion scenario.
Until now, direct observations supporting the gravitational instability theory have been lacking.
The new observations finally pointed to a system where the conditions seem right for gravitational instability to occur. “Around V960 Mon we found exactly what predictions said gravitational instability in process should look like,” Pérez said. “We see the lumps of dust that are massive enough, and the VLT images reveal that there’s enough gas that could be absorbed, forming the giant planets.”
V960 Mon is a bit farther away than most planet-forming stars that have been studied so far, most of them within 1,000 light-years from Earth. “We weren’t really expecting to see anything because it’s at the limit of our telescopes’ power,” Pérez said.
But by looking at distant V960 Mon, astronomers got to see more of its surroundings. “In other systems that are closer by, we have looked closer to the star, and we haven’t found this evidence,” Pérez said. “So maybe the gravitational instability process maybe only works at large scales.”
The age of the star also might play a role. Computer simulations show that gravitational instability would produce objects hundreds of kilometers across in just 1,000 years or less. Speed matters, because when stars reach adulthood, they produce powerful stellar winds that disperse the dust and gas around them. If planets haven’t formed by that time, they likely won’t.
Astronomers previously looked for evidence of gravitational instability in older planet-forming systems. “These [planetary formation] processes are very quick for astronomical timescales,” Pérez said. “Maybe what these observations are telling us is that this phase of gravitational instability happens earlier in the evolution of the star.”
More Work Ahead
“This is all good news, consistent with the theoretical predictions,” said astrophysicist Roman Rafikov from the University of Cambridge in the United Kingdom, who has produced computer simulations of the gravitational instability process. “Planet formation by the gravitational instability [process] should be very challenging close to the star, but it becomes very likely far out in the disk,” Rafikov said, agreeing that the new observations seem to match what the models predicted.
“Nailing down the precise nature of these clumps observationally is what I would focus on next, as this might help us understand the actual dynamics of planet formation by the gravitational instability [process],” Rafikov said. Confirming the authors’ interpretation would most certainly require additional, more sensitive ALMA observations, he added. “I find this interpretation plausible but not yet at the smoking gun level.”
Pérez said he and his colleagues plan to continue observing the system in more depth and conduct computer simulations that could re-create the findings.
—Javier Barbuzano (@javibarbuzano), Science Writer