First Planets Found Outside Solar System
An astronomer has found “irrefutable evidence†of at least two planets orbiting a nearby star--the first confirmed observation of planets outside the solar system humanity calls home.
What has scientists most excited, however, is that the finding suggests that planets can form around almost any star and that the galaxy may well be crowded with planets.
Astronomer Alexander Wolszczan at Pennsylvania State University confirmed the existence of the planets in orbit around an unusual neutron star located 1,200 light-years away in the constellation Virgo. The star is one of just 21 known stellar objects, called millisecond pulsars, that spin thousands of times faster than typical stars, broadcasting powerful radio pulses as they revolve.
The star, known as PSR B1257+12, is an extremely small, very dense body barely the size of Los Angeles, that weighs as much as the sun and spins faster than a kitchen blender. It is the cinder from an ancient supernova that once burned more brightly than any other object in the sky.
“The proof that objects of planetary size do exist outside the solar system indicates that our planets are not unique and uncommon anymore,†Wolszczan said. “The evidence is such that there is no need to question the reality of this.â€
Wolszczan and his colleagues determined that one planet is about 3.4 times the size of Earth and is orbiting the star every 66.6 days; the second planet-sized object appears to be about 2.8 times the size of Earth and is orbiting the pulsar every 98.2 days.
Scientists Thursday said it was extremely unlikely that either planet could support life.
Wolszczan also reported the possibility that a smaller, moon-sized object is orbiting closer to the star, but cautioned that those observations were not yet confirmed. That object appears to circle the pulsar every 25 days.
More than one scientific reputation has foundered on such announcements of new planetary systems.
At least five times in the past 30 years, news of planet discoveries has been followed quickly by retractions when errors in the data were discovered or when others could not confirm the initial sighting. In 1992, for example, a respected British astronomer had to tell his colleagues that the planet he thought he had discovered the year before “just evaporated.â€
But Wolszczan’s findings, which are being published today in the journal Science, appear to convince many skeptical astronomers. He spent three years confirming the existence of the planets, which he detected in the fall of 1991 with the help of Dale Frail of the National Radio Astronomy Observatory after 16 months of monitoring the unusual star.
Shrinivas Kulkarni, the Caltech astronomer who discovered millisecond pulsars in 1982, said Thursday that he found the evidence very persuasive, calling it “a wonderful thing.
“There were some healthy doubts that maybe the pulsar was doing some bizarre stuff that we were incorrectly interpreting as planets,†he said. “But we can all be sure now that we are (detecting) planets.â€
The star would be considered remarkable under any circumstances, astronomers said, but it is an especially unlikely place for planets to form because it is the collapsed remnant of a supernova explosion that should have destroyed any existing orbiting bodies.
For that reason, the discovery suggests that planets may be far more common in the universe than any astronomer had ever dared hope, authorities said.
“The fact that planets have been found orbiting a neutron star is a strong piece of evidence that planets are easy to form,†Caltech astronomer Stephen Thorsett said. “This is certainly the best case I’ve seen and I am personally confident things have been done right and this is correct.â€
The discovery apparently culminates a search that began with the invention of the first telescope centuries ago. But no one has laid eyes on the two planets with any optical instrument.
Instead, to find the planets, researchers used statistical analyses and detailed observations of the regular radio signals the pulsar emits. Wolszczan was able to detect the infinitesimal wobble caused by the gravitational pull of the planets whirling around the central star. The star sends out a radio signal 160 times a second with a precision greater than the most accurate atomic clock on Earth.
Wolszczan used the 305-meter-diameter radio telescope at the Arecibo Observatory in Puerto Rico to measure the arrival times of the pulsar’s energy pulses. Unlike many of the other 550 known pulsars in the universe, these signals contained a subtle, periodic hesitation that strongly suggested that the position of the star was changing in response to one or more large objects in orbit around it.
Further meticulous analysis revealed the effect of the planets’ changing orbits as they passed each other in their journeys around the strange sun, bearing out theoretical predictions. That was the test that other astronomers found most convincing.
There are two theories on how the planets may have formed.
Any planets already around that star should have been destroyed in the supernova blast. “It is the last place you’d look for planets,†Kulkarni said.
But Thorsett at Caltech and Rachel Dewey at the Jet Propulsion Laboratory in Pasadena last year calculated that planets could survive a cataclysmic stellar explosion, depending on where they were in their orbits when the supernova was triggered.
But the supernova happened so long ago that planets could have formed since then. Scientists believe that in that scenario, the neutron star was orbited by a companion star, which eventually swelled into a red giant, a cool star typically 100 times larger than our sun. As it did so, it dumped matter into the neutron star, causing it to spin rapidly. The red giant somehow came apart, and from its ruins the seeds of the newly discovered planets formed, astronomers theorize.
“We have to take news of the first two planets very seriously,†said Stephen P. Maran, a senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. and spokesman for the American Astronomical Society. “I think we now have much greater confidence that there really are planets of other stars.
“But the confirmation comes from the person who made the initial discovery,†he cautioned. “Everything I have heard indicates this is good work, but . . . he already knew what he wanted to find.â€
