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Caltech News Release
Contact:        Robert Tindol
(626) 395-3631
tindol{at}caltech.edu

Embargoed for Release at 8 a.m. PDT, Thursday, May 29, 2003

Astronomers "weigh" pulsar's planets

NASHVILLE, Tenn. -- For the first time, the planets orbiting a pulsar 
have been "weighed" by measuring precisely variations in the time it 
takes them to complete an orbit, according to a team of astronomers 
>from the California Institute of Technology and Pennsylvania State 
University.

Reporting at the summer meeting of the American Astronomical Society, 
Caltech postdoctoral researcher Maciej Konacki and Penn State 
astronomy professor Alex Wolszczan announced today that masses of two 
of the three known planets orbiting a rapidly spinning pulsar 1,500 
light-years away in the constellation Virgo have been successfully 
measured.  The planets are 4.3 and 3.0 times the mass of Earth, with 
an error of 5 percent.

The two measured planets are nearly in the same orbital plane.  If 
the third planet is co-planar with the other two, it is about twice 
the mass of the moon.  These results provide compelling evidence that 
the planets must have evolved from a disk of matter surrounding the 
pulsar, in a manner similar to that envisioned for planets around 
sun-like stars, the researchers say.

The three pulsar planets, with their orbits spaced in an almost exact 
proportion to the spacings between Mercury, Venus, and Earth, 
comprise a planetary system that is astonishingly similar in 
appearance to the inner solar system. They are clearly the precursors 
to any Earth-like planets that might be discovered around nearby 
sun-like stars by the future space interferometers such as the Space 
Interferometry Mission or the Terrestrial Planet Finder.

"Surprisingly, the planetary system around the pulsar 1257+12 
resembles our own solar system more than any extrasolar planetary 
system discovered around a sun-like star," Konacki said.  "This 
suggests that planet formation is more universal than anticipated."

The first planets orbiting a star other than the sun were discovered 
by Wolszczan and Dale Frail (of the National Radio Astronomy 
Observatory) around an old, rapidly spinning neutron star, PSR 
B1257+12, during a large search for pulsars conducted in 1990 with 
the giant, 305-meter Arecibo radio telescope. Neutron stars are often 
observable as radio pulsars, because they reveal themselves as 
sources of highly periodic, pulse-like bursts of radio emission. They 
are extremely compact and dense leftovers from supernova explosions 
that mark the deaths of massive, normal stars.

The exquisite precision of millisecond pulsars offers a unique 
opportunity to search for planets and even large asteroids orbiting 
the pulsar. This "pulsar timing" approach is analogous to the 
well-known Doppler effect so successfully used by optical astronomers 
to identify planets around nearby stars.  Essentially, the orbiting 
object induces reflex motion to the pulsar which result in perturbing 
the arrival times of the pulses.

However, just like the Doppler method, the pulsar timing method is 
sensitive to stellar motions along the line-of-sight, the pulsar 
timing can only detect pulse arrival time variations caused by a 
pulsar wobble along the same line. The consequence of this limitation 
is that one can only measure a projection of the planetary motion 
onto the line-of-sight and cannot determine the true size of the 
orbit.

Soon after the discovery of the planets around PSR 1257+12, 
astronomers realized that the heavier two must interact 
gravitationally in a measurable way, because of a near 3:2 
commensurability of their 66.5- and 98.2-day orbital periods. As the 
magnitude and the exact pattern of perturbations resulting from this 
near-resonance condition depend on a mutual orientation of planetary 
orbits and on planet masses, one can, in principle, extract this 
information from precise timing observations.

Wolszczan showed the feasibility of this approach in 1994 by 
demonstrating the presence of the predicted perturbation effect in 
the timing of the planet pulsar. In fact, it was the first 
observation of such an effect beyond the solar system, in which 
resonances between planets and planetary satellites are commonly 
observed. In recent years, astronomers have also detected examples of 
gravitational interactions between giant planets around normal stars.

Konacki and Wolszczan  applied the resonance-interaction technique to 
the microsecond-precision timing observations of PSR B1257+12 made 
between 1990 and 2003 with the giant Arecibo radio telescope. In a 
paper to appear in the Astrophysical Journal Letters, they 
demonstrate that the planetary perturbation signature detectable in 
the timing data is large enough to obtain surprisingly accurate 
estimates of the masses of the two planets orbiting the pulsar.

The measurements accomplished by Konacki and Wolszczan remove a 
possibility that the pulsar planets are much more massive, which 
would be the case if their orbits were oriented more "face-on" with 
respect to the sky. In fact, these results represent the first 
unambiguous identification of Earth-sized planets created from a 
protoplanetary disk beyond the solar system.

Wolszczan said, "This finding and the striking similarity of the 
appearance of the pulsar system to the inner solar system provide an 
important guideline for planning the future searches for Earth-like 
planets around nearby stars."

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