Strange mini-solar systems revealed

By Robert Roy Britt

Senior science writer

updated 9:35 p.m. ET Feb. 7, 2005

An object smaller than Pluto has been discovered orbiting a dying star in what astronomers said Monday resembles a pint-sized version of our solar system.

In a separate study, a disk of planet-building material was spied circling a dim starlike object just 15 times the mass of Jupiter. The brown dwarf, as it is known, is a cosmically lukewarm ball of gas that straddles the definition between planet and star. The system could evolve into a compact, dim solar system, again with a familiar look.

The two findings suggest that tiny, dim solar systems may be common. They also have scientists wondering anew what really constitutes a planet and what sorts of exotic worlds might harbor life.

The results were presented to reporters in a teleconference from an extrasolar planet meeting held at the Aspen Center for Physics in Colorado.

Smaller than Pluto
In one of the discoveries, an object just one-fifth the size of Pluto was called the smallest planet ever found outside our solar system. It could be viewed as a large asteroid, too. It all depends on how one sees the confusing array of orbital relationships that astronomers are uncovering.

The object orbits a burned out, fast-spinning neutron star known as a pulsar. Three other roughly Earth-sized planets were already known to circle the pulsar. Their orbits are similar to those of Mercury, Venus and Earth.

Though rocky, like Earth, all four objects are considered dead worlds, because the star they circle ended its normal glowing life long ago in a massive explosion. Some astronomers prefer not to even list them in planet catalogues.

The newfound small object orbits the pulsar at a distance equal to that from the sun to the asteroid belt, which is between the orbits of Mars and Jupiter. Observations suggest it marks the outer fringe of material that went into making the miniature solar system.

"Because our observations practically rule out a possible presence of an even more distant, massive planet or planets around the pulsar, it is quite possible that the tiny fourth planet is the largest member of a cloud of interplanetary debris at the outer edge of the pulsar's planetary system," said Penn State researcher Alex Wolszczan, who since 1990 has led the investigation into the system.

Planet or asteroid?
The pulsar is named PSR B1257+12. It is 1,500 light-years away in the constellation Virgo.

"Surprisingly, the planetary system around this pulsar resembles our own solar system more than any extrasolar planetary system discovered around a sunlike star," said Maciej Konacki of the California Institute of Technology, who worked on the latest finding.

INTERACTIVE The search for extrasolar planets

Most of the more than 130 planets detected around sunlike stars are large and gaseous, many of them much more massive than Jupiter. The smallest known planet around a sunlike star is about 14 times the mass of Earth. For the most part, technology has not allowed the discovery of less massive planets, presuming they are out there.

The pulsar planetary system has proved easier to probe, thanks to the central star's rapid spin and clockwork pulsation. Wobbles induced by the gravity of the orbiting objects cause slight perturbations in the pulsations.

Pressed on whether an object so small could really be called a planet, Wolszczan told SPACE.com the term is "just a placeholder." Something that has the mass of one-fifth of Pluto, or twice the mass of asteroid Ceres, could be thought of as a large asteroid, he said.

"I don't think that distinction is really important," Wolszczan said. "What is important that we may have found a large member of the debris orbiting the pulsar at the outer edge of the system."

Whatever it's called, the newfound object and the other three roughly Earth-sized objects all orbit the pulsar in the same plane, just as the planets in our solar system — with the exception of Pluto — circle the sun in much the same plane.

That is "solid evidence," Wolszczan said, that the dead worlds evolved out of a protoplanetary disk, the same type of flat, dusty disk that is thought to have given rise to the comets, asteroids and planets in our solar system.

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