First author

Astronomers believe that planets form in 'protoplanetary disks' — swirling masses of gas, dust and other particles that surround newborn stars. But direct proof of this theory has been lacking, and the timescales over which planets form, as well as the process by which they do so, are still up for debate. On page 38, Johny Setiawan and his colleagues at the Max Planck Institute for Astronomy in Heidelberg, Germany, reveal their discovery of a giant planet orbiting a star young enough to still be surrounded by a protoplanetary disk. This is a key piece of evidence in the endeavour to understand planet formation.

Were you determined to prove that the protoplanetary disk is deservedly named?

Yes. By studying planet formation we hope to understand the origin of planetary systems and put our solar system in a universal context. To do so, we have to look among the more than 100 young stars with documented circumstellar disks, in which we believe planets are born. Previous work drew attention to TW Hydrae, an 8 million to 10 million-year-old star. There was speculation that variations in its disk structure could be due to a planet forming. So we decided to take a closer look.

Why has no one found this evidence before?

Previous work focused on the quickest way to discover extrasolar planets — using radial velocity, which measures changes in an object's velocity along the line of sight over time. Most researchers excluded young stars from such surveys because they are rife with noisy data resulting from stellar activity. Now that more than 270 extrasolar planets have been found, attention is turning to the physics of young stars to help us understand the birth of planetary systems. We used radial velocity to search young stars one by one and extracted information carefully. We were lucky that the planet we found is big enough for us to detect around a young star.

Do your findings change our understanding of planet formation?

Our work gives an observational upper limit for the timescale of giant planet formation. Statistical studies of young stars suggested that disk lifetime can be a few tens of millions of years. More recent studies put a typical disk lifetime at about 10 million years. Our work indicates that planet formation should be complete within 8 million years.

Do you intend to search for other planet-forming protoplanetary disks?

Yes. But we are also continuing to observe TW Hydrae. A companion planet could be forming in the disk around it.