Strange discovery offers ‘missing link’ in planet formation: ‘This fundamentally changes how we think about planetary systems’
Astronomers have gotten a rare glimpse of four baby planets as they grow, and it reveals something surprising: These toddler worlds are getting lighter as they get older.
The quadruplet worlds orbit in tight trajectories around the star V1298 Tau, a young system only 20 million years old (compared to our Sun’s 4.5 billion years) located about 350 light years from Earth. A new analysis, drawing on a decade of observations, shows that the planets are surprisingly light, with low densities – so bloated, in fact, that researchers compared them to Styrofoam.
These older systems are often filled with planets between Earth and Neptune in size, Mercury-like orbits. The origins of these worlds remain one of the enduring mysteries of astronomy.
“What’s so exciting is that we’re seeing a glimpse of what will become a completely normal planetary system,” said the study’s lead author. John Livingstonassistant professor at the National Astronomical Observatory of Japan, said in a statement statement. “We’ve never had a clearer picture of them in their formative years.”
Over time, the bloated worlds around V1298 Tau are expected to shrink as they shed their thick atmospheres, eventually becoming super-Earths and sub-Neptunes – planetary types absent from our own solar system but ubiquitous throughout the galaxy.
By capturing the planets at such a crucial stage in their development, the study, published January 7 in the journal Natureallows astronomers to trace the chaotic processes that sculpt planetary systems over billions of years.
“I couldn’t believe it!”
The four planets orbiting V1298 Tau were first identified In 2019 in data from NASA’s Kepler space telescope. One is about the size of Jupiter, while the other three are between the size of Neptune and Saturn.
What immediately distinguished the system was its cluttered arrangement of several oversized planets clustered in relatively narrow orbits – a configuration known in only one other system, Kepler-51, among more than 500 known multi-planetary systems.
Although the existence of planets was clear, their fundamental properties remained elusive. To pin them down, Livingston and his team embarked on a nearly decade-long observing campaign using half a dozen telescopes in space and on the ground. They followed the planets as they passed in front of their stars – events known as transits, which cause tiny dips in starlight which reveal the size and orbital period of a planet.
Importantly, small variations in the timing of transits, caused by the gravitational pull of the planets on each other, allowed the team to measure their masses. The technique is particularly powerful because it is largely immune to interference from stellar flares common around young stars, the study notes.
But the method only works if astronomers know precisely the orbital period of each planet – and for the most distant planet, V1298 Tau e, this information was missing. According to the study, only two of its transits have been observed, separated by 6.5 years by observations by Kepler and NASA’s exoplanet-hunting Transiting Exoplanet Survey Satellite (TESS) telescope, leaving astronomers unsure how many transits went unnoticed in between.
A stroke of luck occurred when the Las Cumbres ground-based observatory network – which operates telescopes in the United States, Chile and South Africa – spotted a third transit, allowing researchers to finally lock in the planet’s orbit and model the system’s full gravitational choreography.
“I couldn’t believe it!” co-author of the study Erik Petiguraassistant professor of astronomy and astrophysics at UCLA, said in the release. “The timing was so uncertain that I thought we’d have to try at least half a dozen times. It was like getting a hole-in-one in golf.”
The results showed that although they have a radius five to 10 times that of Earth, the planets have masses only five to 15 times that of Earth, making them one of the least dense planets ever discovered, Livingston said.
“By weighing these planets for the first time, we have provided the first observational evidence,” study co-author Trevor Davidan astrophysicist formerly at the Flatiron Institute in New York, who led the discovery of the system in 2019, said in the statement. “They are indeed exceptionally ‘bloated,’ giving us a crucial and long-awaited benchmark for theories of planetary evolution.”
The team then simulated the evolution of the planets and found that they had already lost much of their original atmosphere and cooled faster than standard models predicted.
“But they continue to evolve”, co-author of the study James Owenassociate professor of astrophysics at Imperial College London, said in the release. The planets are expected to continue spewing gas and contracting into super-Earths and sub-Neptunes, he said.
“Over the next billion years, they will continue to lose their atmosphere and shrink significantly, transforming into the compact systems of super-Earths and sub-Neptunes that we see throughout the galaxy,” Owen added.
