Astronomers have spotted the first solid evidence for a planetary Trojan body forming in another system outside our own.
Can two worlds occupy the same orbit? Theory says yes: There are gravitationally stable regions leading and trailing a planet in its orbit, so it's possible a second world, with up to a super-Earth in mass, could form at the same distance from a host star as a much larger gas giant. But we haven’t seen anything quite like that — yet.
In a report to appear in Astronomy & Astrophysics, Olga Balsalobre-Ruza (Center of Astrobiology, Spain) and colleagues report the discovery of emission from a clump of dust in just such a stable region, trailing behind the already-discovered newborn planet PDS 70b.
“I was not expecting that we could detect Trojan bodies,” says exoplanet expert Sebastiaan Haffert (University of Arizona), who was not involved in the current study. “The authors of this work make a very compelling case.”
About 370 light-years away from Earth, and 20 astronomical units (a.u.) from its 5 million-year-old host star, the gas giant planet PDS 70b is still in its infancy. It’s enshrouded in dust, which glows brightly in images taken by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Applying new processing techniques to publicly available images in the ALMA archive, Balsalobre-Ruza and her team found that a second glob of hot dust appears to be following PDS 70b around its star.
That clump’s glow indicates that it holds up to two Moons worth of mass in dust grains, but there may be more than that, Balsalobre-Ruza points out. “The ALMA data we have used for this detection is only sensitive to dust particles with sizes of 2 millimeters,” she explains. “This means that more particles of different sizes are expected to be in the region.”
This clump of dust has the potential to form a planet, she notes, or it might even enshroud an already-formed world. However, Haffert adds that, since there’s so little dust mass in the region, we might be seeing the formation of a smaller body, like the largest asteroids in the main belt of the solar system. In fact, Balsalobre-Ruza prefers not to even give a name to the emission until more is known about its nature. Future observations at infrared wavelengths could detect emission directly from the forming object(s), rather than the surrounding dust, to settle the issue.
The best opportunity to confirm that the emission actually shares PDS 70b’s orbit could come as early as February 2026. That’s when the dusty masses should have gone far enough around the star to confirm their co-orbital motion.
“The PDS 70 system is really a unique system that allows us to test planet formation theories,” Haffert says. “It is really cool that we learn so much from this system alone.”