It is unusual in astronomy to catch a planet in the act of birth, and rarer to watch one creating its own cosmic nursery. But that is exactly what astronomers did with WISPIT 2b a giant, still-forming gas giant, five times the size of Jupiter, surrounded by a huge, multi-ringed protoplanetary disk around a young sun-like star. The finding, using the European Southern Observatory’s Very Large Telescope (VLT) in Chile’s Atacama Desert, is the first unequivocal detection of a planet creating a gap in a disk as complicated as this.
The planet, which is about 5 million years old, is estimated to be orbiting its parent star at a distance of approximately 56 astronomical units — more than Pluto’s average distance from the Sun by over two times. Its host system is part of a small-studied category of young stars that provide an exceptionally clean laboratory for investigating planet disk interactions. “We did not expect to find such a spectacular system,” stated Richelle van Capelleveen, a Leiden University doctoral student and author of the study. “This system will likely be a benchmark for years to come.”
WISPIT 2b was detected in the first few years of a five-year survey to see if wide-orbit gas giants are more common around young or old stars. The preliminary “snapshot” exposures, which lasted only minutes per target, indicated that the dust disk is surprisingly complex, with multiple rings in concentric lanes a characteristic that, a recent ALMA studies, often indicates the gravitational influence of embedded planets. “When we saw this multi-ringed disk for the first time, we knew we had to try and see if we could detect a planet within it,” said University of Galway astronomer Christian Ginski. Near-infrared follow-up imaging picked up the glowing WISPIT 2b sitting in a gap in the disk.
The near-infrared detection of the residual heat from formation was supplemented by visible-light imaging from the University of Arizona with a custom high-contrast instrument. The optical data identified a thin wavelength signature suggesting that the planet is actively accreting gas, constructing its atmosphere in real time. Such detections are technically challenging: the host star is thousands of times as bright as the planet, so adaptive optics systems must be used to counteract atmospheric turbulence and coronagraphs to mask starlight.
The circumstellar protoplanetary disk is a whopping 380 astronomical units in size and made of gas and dust — planet-building raw materials. ALMA observations and simulations with software such as PlanetaLP have revealed that giant planets are able to shape such disks by forming deep gaps and pressure ridges, confining millimeter-sized dust grains at their margins. Not only do these processes define the disk’s structure, but they also affect the migration and final orbital arrangement of planets. The ODISEA project’s five-stage evolutionary model suggests that WISPIT 2b’s system sits in an intermediate phase, where a young giant has already begun to reorganize its natal environment.
Direct imaging of exoplanets at this stage is exceptionally rare. The only comparable case, discovered in 2018, also involved Ginski’s team. In most systems, planets are inferred indirectly from the gaps and spirals they leave behind. Here, the pairing of disk substructure and validated planetary companion is an unusual calibration opportunity for theories of planet formation, such as rapid growth processes like pebble accretion, which can form giant planets in a million years at large orbital radii.
The technical success relies heavily on the adaptive optics and infrared facility of the VLT, able to resolve fine structure in disks hundreds of light-years distant. The same methods have been employed to take the very first direct spectra of giant exoplanets and demonstrate atmospheric content, which calls into question current models. With WISPIT 2b, upcoming spectroscopic research might identify the chemical composition of its atmosphere during its formative phase a dataset that could shed light on the disk material to planetary envelope transition.
“Capturing an image of these forming planets has proven extremely challenging,” Ginski said. “It gives us a real chance to understand why the many thousands of older exoplanet systems out there look so diverse and so different from our own solar system.” With the next generation of instruments on the Extremely Large Telescope, astronomers anticipate probing even tinier, colder planets in comparable disks, getting closer to an overall picture of how planetary systems form from whirling clouds of dust and gas.
