‘Weird’ debris discs may help scientists find hidden planets in far off solar systems

Astronomers have captured some of the most detailed images of debris discs—rings of leftover dust, gas and rocks that circle a star—from fully formed, ‘teenage’ planetary systems. 

And those images, published in a suite of papers in Astronomy & Astrophysics, are expected to help astronomers locate new planets similar to the ice giants in the outer Solar System. 

When exoplanets linger in the darkest outskirts of their planetary systems, they cannot be observed directly using available technologies. But these hidden planets can distort or cause irregularities in nearby debris discs, leaving an imprint that astronomers can observe. 

“Exoplanets in the outer regions of a system are sitting in what has been a blacked-out box, so to speak,” said co-author Meredith MacGregor, an assistant professor of physics and astronomy at Johns Hopkins University. “Studying debris disc structure is like shining a flashlight and looking at a shadow cast on the wall—we can’t say for certain what the exact properties of the object are, but we can get some idea of what it might be.” 

Using the Atacama Large Millimeter/submillimeter Array (ALMA), a high-powered telescope in Chile’s Atacama Desert that detects thermal dust and gas emission, an international collaboration of researchers captured images of 24 debris discs in planetary systems between roughly 10 million and two billion years old. 

“These are the highest resolution images of these teenage systems that we’ve ever had, so suddenly we can actually see structures in amazing detail,” MacGregor said. 

Heated by a central star, particles in a debris disc give off a thermal signature that ALMA can detect. Discs without planets orbiting within them should look like symmetrical rings, with even levels of brightness and even shapes.  

Nearly all of the observed discs showed some irregularities.  

Four systems had particularly ‘weird’ discs, the researchers said. Planetary system HD121617’s disc was a standout, showing a strangely uneven brightness. Researchers believe an object, possibly a planet,created a vortex that trapped particles in parts of the disc. Areas with a higher density of particles emit more heat and, consequently, appear brighter in thermal images. 

The research builds on findings from the earlier DSHARP project with ALMA, which imaged discs in systems two million years old and younger to study the environments within which exoplanets start to form. Unlike the bright discs in newly forming systems, where more dust and gas orbit the newly formed stars, teenage discs have less mass and therefore are relatively faint and difficult to image. With this survey, however, researchers have gained a peek inside a stage of exoplanet system formation they have yet to fully explore.  

“Now, if we see 100-million-year-old discs circling their stars at around the same distance as Saturn, Uranus, Neptune and beyond, we can see the details and structures in them. That allows us to infer the presence of new planets which would otherwise remain hidden,” MacGregor said, adding that outer exoplanets in these systems have yet to be detected and catalogued because the tools available are limited.  

Astronomers have mostly used two methods to study exoplanets: the radial-velocity method detects wobbly stars that are pushed and pulled by the gravity of nearby planets; the transit method collects data as a planet passes directly in front of its star.  

Collectively, astronomers have been able to detect more than 6,000 planets using these two methods. However, researchers suspect these planets are not representative of all exoplanets. 

“If you look at those planets, most of them are very close to their stars. This means we know a lot about hot, gas giants,” MacGregor said.   

Information about icy giant planets further from their stars is almost non-existent, the researchers said, which means astronomers have limited understanding of how our solar system compares to exoplanet systems. 

“All of our outer planets—Saturn, Uranus, Neptune—have no corresponding analogs in our exoplanet catalogue; but, by looking at the debris disc shapes, we’re one step closer to finding them. It’s exciting because these debris discs should help us prioritize which systems to investigate with even higher resolution telescopes in the future,” MacGregor said. “Until we go back to confirm the existence of these exoplanets and observe them, they’ll remain a mystery.” 

---

---