The team used an array of radio telescopes in Chile to close in on a nascent planetary system lying 176 light-years from Earth - distant to us, but nearby in astronomical terms.
The forming planet is thought to be an ice giant, similar to Uranus or Neptune in our Solar System.
The findings are to be published in Astrophysical Journal Letters.
In the two decades since the first exoplanets were found, astronomers have discovered that planetary systems do not necessarily follow the familiar template set by the eight planets which orbit the Sun.
Scientists from all over the world try to understand the great diversity in the configuration of planetary systems and in the characteristics of exoplanets themselves.
Takashi Tsukagoshi at Ibaraki University, Japan, and colleagues used the Atacama Large Millimeter/submillimeter Array (Alma) in northern Chile to take a close look at the planet formation site.
TW Hydrae is estimated to be about 10 million years old and is one of the closest young stars to Earth.
Thanks to its proximity and the fact that its axis of rotation points in Earth's direction, astronomers are able to get a face-on view of the developing planetary system.
The young star is surrounded by a disc made of tiny dust particles. Variations in the signal received by Alma allow researchers to estimate the size of these dust grains.
Gravitational interactions and friction between gas and dust has probably pushed the larger dust out of the gap, say the researchers.