Astronomers have gotten a rare glimpse of four baby planets in the process of growing. And something surprising was revealed. It’s that these toddlers’ worlds are getting brighter with age.
The quadruplets orbit in a tightly packed orbit around the young star V1298 Tau, which is only 20 million years old (compared to the Sun’s 4.5 billion years), located about 350 light-years from Earth. A new analysis based on 10 years of observations shows that the planet is surprisingly light and sparse. In fact, it was so swollen that researchers likened the planet to Styrofoam.
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These old star systems often have planets the size of Earth and Neptune clustered together in tight orbits similar to Mercury. The origin of such a world remains one of the eternal mysteries of astronomy.
“What’s really exciting is that we’re seeing a preview of what will likely be a very normal planetary system,” study lead author John Livingston, an assistant professor at the National Astronomical Observatory of Japan, said in a statement. “We have never had such a clear picture of their formative years.”
Over time, the expanded world around V1298 Tau is expected to shrink as it sheds its thick atmosphere, eventually becoming a super-Earth or sub-Neptunian, a planet type that is absent from our solar system but ubiquitous throughout the galaxy.
By capturing planets at critical stages of development, the study, published in the journal Nature on January 7, allows astronomers to trace the chaotic process that forms 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 roughly the size of Jupiter, while the other three are between Neptune and Saturn.
What immediately made this system stand out was its dense layout of multiple oversized planets packed into a relatively tight orbit. This configuration is known from only one of the more than 500 known multiplanetary systems, the Kepler-51 system.
Although the planet’s existence was clear, its fundamental nature remained elusive. To locate them, Livingston and his team embarked on a nearly 10-year observation campaign using six telescopes in space and on the ground. They tracked planets as they passed in front of stars. This phenomenon, known as a transit, causes a small dip in the star’s light, revealing the planet’s size and orbital period.
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Importantly, small variations in the timing of the transits caused by the planets’ gravitational pull together allowed the researchers to measure the planets’ masses. The technique is particularly powerful, the study notes, because it is largely uninterrupted by stellar flares, which are common around young stars.
But this method only works if astronomers know exactly each planet’s orbital period. And for the outermost planet, V1298 Tau e, that information was missing. According to the study, observations by Kepler and NASA’s Transiting Exoplanet Survey Satellite (TESS) telescope have only been observed twice in the six-and-a-half year gap, and astronomers remain unsure how many transits were missed in that time.
Luck struck when the ground-based Las Cumbres Observatory Network, which operates telescopes in the United States, Chile, and South Africa, spotted the third passage, allowing researchers to finally fix the planet’s orbit and model the system’s full gravitational motion.
“I couldn’t believe it!” study co-author Eric Pettigra, an assistant professor of astronomy and astrophysics at UCLA, said in a statement. “The timing was so uncertain that I thought I would have to try at least six times. It was like getting a hole-in-one in golf.”
The researchers found that although these planets have radii five to 10 times larger than Earth’s, they have masses only five to 15 times Earth’s, making them the least dense planets ever discovered, Livingstone said.
“By weighing these planets for the first time, we have provided the first observational evidence,” study co-author Trevor David, a former astrophysicist at the Flatiron Institute in New York who led the discovery of the system in 2019, said in a statement. “They are indeed very ‘puffy’ and provide a much-needed and important benchmark for theories of planetary evolution.”
The team then simulated the planet’s evolution and found that it had already lost much of its original atmosphere and was cooling faster than standard models predicted.
“But they are still evolving,” study co-author James Owen, associate professor of astrophysics at Imperial College London, said in a statement. He said these planets will continue to emit gas and shrink to super-Earths or sub-Neptunes.
“Over the next billion years, they will continue to lose their atmospheres and shrink significantly, transforming into a compact system of super-Earths and sub-Neptunes found throughout the galaxy,” Owen added.
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