Astronomers have used radio signals from distant objects called pulsars to calculate the masses of five of the planets in our own solar system. Scientists think the technique might someday reveal the paths of previously unseen asteroids and comets—such as the one that hit Jupiter just a few days ago. And the pulsar signals could help astronomers detect one of most sought-after phenomena in science: gravity waves—assuming they exist.
Pulsars, tiny but superdense remnants of giant stars that exploded in supernovae, generate extremely regular radio blips. They're like "very, very massive flywheels that spin really, really smoothly," explains theoretical astrophysicist Charles Horowitz of Indiana University, Bloomington, who was not involved in the research. Scientists want to employ the regularity of pulsar signals to help probe the secrets of the universe.
One of those secrets is gravity waves, ripples in the fabric of spacetime. Einstein predicted that they're caused whenever something enormous happens, such as two supermassive black holes smashing together. So far, no one has detected gravity waves, but several countries, including the United States, have built huge instruments to try to catch one as it passes by Earth. One problem is that a gravity wave's effects are extremely subtle—measured as a tiny fraction of the width of an atomic nucleus—so finding one depends on ultraprecise data.
That's where pulsar timing comes in. Each radio blip from a pulsar reaches Earth at exactly the same time interval, usually a small fraction of a second. But the big radio dishes on the ground receiving the signals don't always read them that way. That's because Earth's motion in its orbit, either toward or away from the pulsar, stretches out or bunches up the wavelength of the radio signal, in an effect much like a Doppler shift.
Scientists can correct for these signal distortions, but there's an extra complication: Other nearby planetary bodies also affect the pulsar signals because their gravitation tugs at Earth's motion. Researchers must identify all these factors so they can correct for them. Then, if Earth's motion is disturbed ever so slightly by a gravity wave, the pulsar signals might reveal it.
David Champion of the Max Planck Institute for Radio Astronomy in Bonn, Germany, explains that to identify all of the pulsar signal distortions caused by solar system bodies, scientists will need to collect and coordinate data from 20 pulsars for at least 10 years. That much data should reduce all distortions to 100 nanoseconds (billionths of a second) over that entire time period. "This is a tall order," says Champion, lead author of a paper in press in the Astrophysical Journal.
Part of the challenge is that scientists must know the exact masses of all of the other planets to be able to calculate how much they distort pulsar signals. So Champion's team looked at the signal histories of four pulsars collected over 22, 13, 10, and 5 years, respectively. In those histories, they first subtracted out the effect of Earth's motion and the motion of the moon. Then they isolated signal distortions caused by Earth's five nearest planetary neighbors and, based on how often the distortions appeared, pinpointed which planet was causing which distortion.
Finally, the researchers used the size of each distortion to infer the mass of the planet causing it, and they compared their calculations with the best figures available: from instruments aboard spacecraft that have swung by those planets. To their surprise, they found that the two sets of figures matched almost perfectly. Mercury's mass derived from pulsar timing was off by only 0.1%. For Venus, the two measurements were exactly the same. For Mars, the variation was 0.03%. For Jupiter, the difference was a mere 0.00002%, and for Saturn, only 0.0005%.
Based on those results, Champion says, scientists eventually could use pulsar timing to determine the masses of a host of solar system bodies, even asteroids and comets. The longer the time period covered by the signal database, he explains, the more precisely it can reveal distortions caused by all kinds of solar system denizens.
Donald Yeomans, senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, foresees another new and potentially useful application for the data. Space missions supply accurate mass measurements for the individual planets, he says, but "we do not have masses for entire planetary systems," including the planets and their moons. Given enough years, he says, pulsar timings could provide them.
Not only that, says Horowitz, but "you can use the data to search for unknown [bodies] in the solar system and set limits on what can be discovered in the future." Using pulsar-timing data, he says, "You don't even have to see the object, or even know it is there, to feel its gravitational effects."