May 28, 2007
The finds were among 37 objects seen orbiting distant stars by a US and Anglo-Australian team in the last year.
Other objects reported by the group, at an American Astronomical Society meeting in Honolulu, included five failed stars, known as brown dwarfs.
The finds increase the total number of known exoplanets to 236, more than half of which were discovered by the team.
"The more we look, the more we find planets," said Professor Tinney of the University of New South Wales, head of the Australian part of the Anglo-Australian Planet Search.
Among the finds were at least four multiple-planetary systems. All of the planets were so-called gas giants, similar to Jupiter, with no solid surface.
"Something like 10 to 15% of stars host gas giants," said Professor Tinney. "A larger fraction of stars may host planets too small for us to detect."
These could include Earth-sized objects, which could harbour life.
Earlier this year, scientists using the European Southern Observatory (ESO) 3.6m Telescope in Chile discovered the smallest exoplanet - as astronomers call planets that orbit a star other than the Sun - yet.
The "super-Earth" orbited the faint star Gliese 581, 20.5 light-years away in the constellation Libra. Its radius was just 1.5 times that of the Earth.
Intriguingly, the planet could have liquid water on its surface, a key ingredient of life.
It was discovered using a sensitive instrument that can measure tiny changes in the velocity of a star as it experiences the gravitational tug of a nearby planet.
It was the same method used to detect the latest batch of extra solar planets.
According to Dr Jason Wright of the University of California, Berkeley, one of the members of the US California and Carnegie Planet Search, the technique has become more sophisticated, improving detection rates dramatically.
"We're just now getting to the point where, if we were observing our own Solar System from afar, we would be seeing Jupiter," he said.
These techniques allow scientists to detect changes in the motion of stars as small as one metre per second.
Astronomers are stuck with such indirect methods of detection because current telescope technology struggles to image very distant and faint objects - especially when they orbit close to the glare of a star.
The discovery of the 28 new planets has given astronomers new targets to analyse in more detail.
For example, a planet discovered two years ago has already yielded a mass of "extraordinarily rich" information, according to Professor Geoff Marcy of the University of California, Berkeley.
Circling the star Gliese 436, 30 light years from Earth, was an ice-giant planet that was calculated by Swiss and Belgian scientists to be at least 22 Earth masses, slightly larger than the mass of Neptune.
Their studies had also revealed its density.
"It must be 50% rock and about 50% water, with perhaps small amounts of hydrogen and helium," said Professor Marcy.
"This planet has the interior structure of a hybrid super-Earth/Neptune, with a rocky core surrounded by a significant amount of water compressed into solid form at high pressures and temperatures."
However, the planet is not thought to be capable of supporting life.