The scientists studied a disc of gas and rocky particles around HL Tau, which is 520 light-years away and thought to be less than 100,000 years old. The disc is unusually massive and bright, making it an excellent place to search for signs of planets in the process of formation. Dr Greaves, from the University of St Andrews, Scotland, said the discovery of a forming planet around such a young star was a major surprise. "It wasn't really what we were looking for. And we were amazed when we found it," she told BBC News. The next youngest planet confirmed is 10 million years old."
Dr Ken Rice, from the Institute of Astronomy in Edinburgh, said the discovery shed new light on theories of planet formation. According to one model, planets form from the bottom up. Under this scenario, particles of rocky material collide and "stick" to one another, forming a bigger and bigger object. But he thinks the proto-planet in HL Tau formed relatively quickly when a region of the disc collapsed to form a self-contained structure. This could occur because of gravitational instability in the disc itself.
Intriguingly, another young star in the same region called XZ Tau may have made a close pass of HL Tau about 1,600 years ago. Although not required for planet formation, it is possible that this flyby perturbed the disc, making it unstable. This would be a very recent event in astronomical terms. "It's possible it gave a 'yank' to one side of the disc around HL Tau, making it unstable, and that this was a 'trigger' for the planet to form," Dr Greaves explained. "If the planet formed in the last 1,600 years, that would be incredibly recent."