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."
