Astronomers have finally figured out how the most common types of planets in our galaxy are formed—by watching a set of baby planets grow up in real time.
These worlds, found in a young star system called V1298 Tau, are in the process of transforming into “super-Earths” and “sub-Neptunes,” planets that are larger than Earth but smaller than Neptune.
Although such planets are extremely common around other stars, our own solar system doesn’t have one, making them harder to study—until now.
An international team of scientists led by researchers from UCLA observed four giant young planets orbiting V1298 Tau, a star only about 20 million years old.
That may sound ancient, but compared with our 4.5-billion-year-old Sun, the star is practically a newborn.
In human terms, it would be like studying a five-month-old baby to understand how adults develop.
Planets form from a swirling disk of gas and dust surrounding a newborn star. Over time, clumps of material collide and merge, building larger bodies.
But scientists have long struggled to explain why so many mature planets end up between the sizes of Earth and Neptune. The four planets around V1298 Tau appear to hold the answer.
Right now, these planets are huge—between the sizes of Neptune and Jupiter—but they are not staying that way.
Observations show they are shrinking as they lose their thick outer atmospheres. In other words, they started out as bloated gas giants and are gradually slimming down into the smaller planets commonly seen across the galaxy.
To make this discovery, scientists watched the planets pass in front of their star, an event called a transit. Each transit causes a tiny dip in the star’s brightness.
By tracking these dips over many years using both space-based and ground telescopes, the team calculated the planets’ orbits, sizes, and masses.
It wasn’t easy, especially for the outermost planet, whose transits were rare and hard to predict. A lucky observation from a ground telescope finally revealed its orbit, allowing the team to complete the picture.
When researchers measured the planets’ masses, they were surprised. Even though the planets are currently five to ten times wider than Earth, they are only about five to fifteen times heavier. This means they are extremely low-density—more like fluffy foam than solid rock. Such “puffy” planets are expected to lose gas over time, becoming smaller and denser.
Scientists believe these worlds have already shed a large portion of their original atmospheres and will continue to shrink over billions of years. Eventually, they will resemble the compact super-Earths and sub-Neptunes seen around many other stars.
This rare glimpse of planets in their early stages provides a missing link between newborn planetary systems and the mature ones astronomers observe today. By catching these worlds in the act of transformation, researchers now have strong evidence explaining how the galaxy’s most common planets come to exist—and why our own solar system may be unusual for lacking them.
