By Paul Rincon
Science reporter, BBC News, Austin
New work by a team of US astronomers has shown that wherever there is room for a planet to form around a young star, it does.
It may now be possible to find all planets within a given system
The researchers predicted the existence of an unknown planet circling a star more than 200 light-years from Earth.
This prediction was based on a study of the orbits of two planets already known to orbit the star HD 74156.
They then observed the system and confirmed there was a new planet just where they predicted it would be.
The find represents the first time astronomers have successfully forecast the existence of an unknown planet since Neptune was predicted in the 1840s.
Co-author Dr Rory Barnes from the University of Arizona in Tucson presented his results here at the 211th meeting of the American Astronomical Society (AAS) in Austin, Texas.
He and his colleagues studied orbits in several planetary systems beyond that of our own Sun.
They found that planets' orbits tend to be as closely packed together as they can be. There is a limiting factor on packing: if planets huddle too closely together, gravity will start to destabilise their orbits.
The researchers reasoned that there must be universal rules applying to how planets form around young stars.
When they looked at two planets, called "b" and "c", already identified orbiting the star HD 74156, they found that there was a large gap between them.
If the scientists' "Packed Planetary Systems" theory was correct, there should be a planet between "b" and "c", and in a particular orbit around its parent star.
When Dr Jacob Bean and his colleagues from the University of Texas observed the system, they confirmed that there was indeed a new planet where Dr Barnes had predicted.
"They found it to have a mass a little bit larger than we expected, but in exactly the zone where we expected it to be," Dr Barnes told reporters.
The new planet has been named, by convention, HD 74156 d.
"For whatever reason, it seems that planetary formation is a somewhat efficient process. Wherever there is room for a planet to form, it is there; and observers can try and find it," said Dr Barnes.
"This is a new way to find planets. And more speculatively, it might be a new way to try and search for life. It may be a coincidence, but this new planet was found inside the habitable zone (around its parent star)."
Another group of astronomers has since found another planet where Dr Barnes and his colleagues had predicted one to be in another system they had considered during their analysis - 55 Cancri.
The team also forecast the presence of a planet in a stable region around the star HD 38529, and Dr Barnes said he hoped this would be confirmed soon, too.
Sara Seager, professor of astronomy at the Massachusetts Institute of Technology (MIT), commented: "This field is littered with predictions that are wrong, so I would congratulate Rory on being one of the few people to make a successful one in advance."
She added: "It will be great to see if we can find all planets within a given system, because it will tell us something about planet formation around other stars."