Earlier this month at a conference in Aspen, Colo., Penn State professor of astronomy and astrophysics Alex Wolszczan announced the confirmation of the smallest planet ever discovered outside of our solar system.

The planet is the size of a large asteroid, Wolszczan said.

He said it is one of four rocky planets, all found in the same system, orbiting a neutron star more than 1,500 light-years away in the constellation Virgo.

The star at the center of the system is very different from our sun, he said. A neutron star is created when a super-massive star burns out. The star is so heavy and its gravity is so strong that the atoms inside collapse into incredibly dense neutrons.

"The neutron star is very different in every aspect from a normal star," Wolszczan said.

Because it is so dense, it curves space more than an ordinary star like the sun, and it also emits thousands of times more radiation, he said.

"It's really weird," said Maciej Konacki, a postdoctoral researcher at the California Institute of Technology and a former student of Wolszczan's who worked on the system with him.

The differences between the neutron star and normal stars were exploited to detect planets by timing regular pulses of light that are constantly emitted by the neutron star, Wolszczan said.

This is far more accurate than other techniques, and it was concluded that the small planet was the last object left to be found in that system, he said.

"It seems...that we have reached the outskirts of that planetary system," Wolszczan said.

Planets around neutron stars of this type are pretty rare, so the timing technique of observing cannot be applied to most extra-solar planets, which are typically larger and made of gas, Wolszczan said.

Konacki said the planets in the neutron star's system have similar orbits to the inner planets of our solar system and were probably formed in the same way.

The story behind the system actually started in 1992, when Wolszczan discovered two Earth-sized planets. These were the first planets ever discovered outside of the solar system, he said. At the time of the discovery, it was suspected that the star had more than just two planets orbiting it.

But Wolszczan said it took a few more years to confirm the existence of the third, and more than a decade more to confirm the fourth.

Even though the fourth planet is probably the last object in the system, there is still a great deal more to learn about it.

For one thing, astronomers are wondering how the planets even got there, Konacki said.

"First of all, we would never expect to find planets around" a neutron star, he added.

Wolszczan and Konacki both said the ultimate goal is to improve techniques of observing planets around regular stars.

"The next thing beyond that is to try to find life," Wolszczan said, although the planets around the neutron star are bombarded with way too much radiation to ever harbor life.

Currently, Konacki is studying giant gas planets around normal stars.

While technology still has to catch up with scientific ambitions, he said his hopes are high.

"If you could create an Earth-sized planet around such a strange type of star, why not around a solar-type star?" he said.