Adam Kraus, of the University of Hawaii's Institute for Astronomy, said the planet is being formed out of dust and gas circling a 2-milion-year-old star about 450 light years from Earth.
The planet itself, based on scientific models of how planets form, is estimated to have started taking shape about 50,000 to 100,000 years ago.
Called LkCa 15 b, it's the youngest planet ever observed. The previous record holder was about five times older.
Kraus and his colleague, Michael Ireland from Macquarie University and the Australian Astronomical Observatory, used Keck telescopes on Mauna Kea to find the planet.
"We're catching this object at the perfect time. We see this young star, it has a disc around it that planets are probably forming out of and we see something right in the middle of a gap in the disc," Kraus said in a telephone interview.
Kraus presented the discovery Wednesday at NASA's Goddard Space Flight Center in Maryland. Kraus and Ireland's research paper on the discovery is due to appear in The Astrophysical Journal.
Observing planets while they're forming can help scientists answer questions like whether planets form early in the life of a star or later, and whether they form relatively close to stars or farther away.
Planets can change orbits after forming, so it's difficult to answer such questions by studying older planets.
"These very basic questions of when and where are best answered when you can actually see the planet forming, as the process is happening right now," Kraus said.
Other planets may also be forming around the same star. Kraus said he'll continue to observe the star and hopefully will see other planets if there are in fact more.
Scientists hadn't been able to see such young planets before because the bright light of the stars they're orbiting outshines them.
Kraus and Ireland used two techniques to overcome this obstacle.
One method, which is also used by other astronomers, was to change the shape of their mirror to remove light distortions created by the Earth's atmosphere.
The other, unique method they used was to put masks over most of the telescope mirror. The combination of these two techniques allowed the astronomers to obtain high-resolution images that let them see the faint planet next to the bright star.
The astronomers found the planet while surveying 150 young dusty stars. This led to a more concentrated study of a dozen stars.
The star LkCa 15 — the planet is named after its star — was the team's second target. They immediately knew they were seeing something new, so they gathered more data on the star a year later.