Astronomers confirm infant planets are 'puffy' and shrink with age

Astronomers uncovered that the most common type of planets in our galaxy grow, confirming theories that these worlds begin as ‘bloated’ babies before losing their dense atmosphere, according to a new study published in Nature.

The study examined four infant exoplanets in the V1298 Tau system, which orbits a star that is just 20 million years old. These planets have radii that are five to ten times larger than Earth’s but possess only five to fifteen times its mass. As a result, their density is similar to that of polystyrene foam.

Researchers explain that the intense heat and light from the young star cause the atmospheres of these planets to expand significantly. As a consequence, these “puffed-up” worlds lose substantial amounts of gas into space. Eventually, they will contract into “super-Earths” or “sub-Neptunes,” which are sizes commonly found in the galaxy but not present in our own solar system.

“These planets have already undergone a dramatic transformation,” said co-author Prof. James Owen. He noted that over the next few billion years, they will continue to shrink and evolve into the more compact systems seen elsewhere.

The astronomer’s discovery was a blend of careful calculation and good fortune. While the team was able to determine the planets’ sizes by analyzing “transits”—the dips in starlight that occur as a planet passes in front of its star—fixing the orbits of the outer planets proved challenging. Professor Erik Petigura likened successfully locating these outer planets on the initial attempt to “getting a hole-in-one in golf.”

By examining the gravitational push and pull between the planets, researchers estimated their masses, providing the first observational evidence of this “puffy” phase in planetary evolution.