How do you weigh a planet you can’t see from many light-years away? Astronomers may have the answer — and it involves “reading between the rings,” aka the bright beautiful dusty structures that newborn exoplanets create around their young stars.
Planets in general are born from the dust, gas and tiny fragments called “planetesimals,” that surround young stars. As a result, in their relative youth, these worlds are found still embedded in this natal-material swirling around in plate-like structures called protoplanetary disks. However, recent observations have revealed that as these infant exoplanets orbit their parent stars, they also carve lanes in this disk of gas and dust.
While such rings have been used to determine the presence of exoplanets around stars, this new research suggests a way to use those grooves to actually assess the characteristics of exoplanets, too.
“We’ve long understood that the rings could be created from concentrated dust that piles up just beyond the orbit of young, embedded planets, but we’ve been so far unable to link features of these rings to planet masses,” team leader Amena Faruqi of the Astronomy and Astrophysics Group at the University of Warwick in the U.K, said in a statement. “By reading ‘between the rings,’ we have now found a way to reconstruct the masses of the planets that create the rings, even when those planets are too faint or too embedded to observe directly.
“These bright rings are not just beautiful structures — they are essentially planetary fingerprints.”
Investigating a dusty star system
The first step taken by Faruqi and colleagues involved using computer simulations to assess how the masses of exoplanets would create distinct shapes for the rings in protoplanetary disks. They discovered that the width of dust rings and the location of the brightest point in that ring are key in assessing the characteristics of cloaked exoplanets.
Excitingly, the relationship between a planet’s mass and the peak brightness of the dust ring it creates holds regardless of what wavelength of light the system is imaged in — as well as regardless of the size of the dust grains in the ring. That means astronomers don’t need to know the exact conditions around an infant star to assess the mass of its exoplanets.
The scientists tested their new technique by applying it to a planetary system located around 370 light-years away called PDS 70, which astronomers have been studying with the Atacama Large Millimeter/submillimeter Array (ALMA), an array of 66 radio antennas located in northern Chile.
“One of the strengths of this work is that it doesn’t stay in the realm of theory — we’ve been able to take these simulation results and apply them directly to real observed systems,” Jessica Speedie of Massachusetts Institute of Technology (MIT) said in the statement. “Using the PDS 70 system as an observational laboratory in particular enabled a real verification of the approach, giving us confidence that these methods are genuinely ready to be applied widely as soon as possible.”
PDS 70 was a useful test subject for the team because it possesses at least two exoplanets, PDS 70 b and PDS 70 c, and has been directly imaged. The technique delivered an estimated mass for PDS 70 c in line with current estimates of around 7.5 times the mass of Jupiter. The team’s results also delivered some surprising insights into the processes that surround planet-formation as well as raising questions that astronomers will be keen to answer.
“Another striking result of the simulations is that, in typical discs, more massive forming planets can trap as much as 20 times the mass of Earth of dust within these rings,” Ralph Pudritz of the Department of Physics and Astronomy at McMaster University said in the statement. “This confirms ALMA observations — but raises the question of why new planets have not been detected in the trapped dust and pebbles of the ring. “Our results suggest that the dust is sufficiently abundant and concentrated enough to potentially kick-off planet formation. This is an important insight that will initiate further observations and theory.”
Ultimately, this new technique and its power to study infant planetary systems could also aid our understanding of how our own planetary system took shape around 4.6 billion years ago.
“What excites me most is the timing. With ALMA delivering increasingly detailed disk images, and future facilities on the horizon, there has never been a better moment to develop these methods,” team member Farzana Meru of the Department of Physics at the University of Warwick said. “Combining our dust-based diagnostics with gas pressure observations will open up a powerful new window onto the hidden planets shaping these disks and the diverse planetary systems they will go on to form.”
The team’s research was published on Thursday (May 28) in The Astrophysical Journal.