In their quest to find signals of the largest black holes in the Universe, astronomers have had to first divert their attention to a different cosmic quandary – where the center of gravity in our Solar System lies.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) monitors a set of rapidly spinning neutron stars (pulsars) that reliably emit regular flashes of radio waves. Described as “outstanding galactic clocks,” timing deviations seen across the incoming light of pulsars could signify that gravitational waves are warping our galaxy. Longer than those observed by LIGO, these gravitational waves could originate from black holes that are billions of times more massive than the Sun.
“Using the pulsars we observe across the Milky Way galaxy, we are trying to be like a spider sitting in stillness in the middle of her web,” Stephen Taylor, assistant professor of physics and astronomy at Vanderbilt University, said in a statement. “How well we understand the Solar System barycenter is critical as we attempt to sense even the smallest tingle to the web.”
The Solar System’s center of gravity, its barycenter, is the center of mass of every object in the system combined. At this point, the planets, moons, and asteroids would be able to “balance” with the Sun. Therefore if our knowledge of these celestial objects is off, then our “spidey-senses” will be scuppered.
“The catch is that errors in the masses and orbits will translate to pulsar-timing artifacts that may well look like gravitational waves,” Joe Simon, an astronomer at NASA’s Jet Propulsion Laboratory (JPL) explained.
In fact with the NANOgrav data, the researchers were finding large systematic differences in their calculations. “Typically, more data delivers a more precise result, but there was always an offset in our calculations,” Michele Vallisneri, also of JPL, said.
The problem comes back to uncertainties in the Solar System’s barycenter. Despite the comparatively humungous mass of the Sun, our system’s gravitational center is above the surface of the star, rather than in its core. That is because of another rather large influence, Jupiter. Indeed, as the center of mass between these two objects alone is at a point in space above the Sun’s surface, Jupiter doesn’t exactly orbit the Sun.
However, our imperfect knowledge of Jupiter’s orbit, in part because of technical faults on JPL’s Galileo probe that studied the planet between 1995 and 2003, has caused uncertainty in the location of the Solar System’s barycenter. As described in their paper, published in The Astrophysical Journal, by modeling these uncertainties the team was able to hone in on this sought-after point to within 100 meters (328 feet). If the Sun was scaled to the size of a football pitch, this distance would equate to the width of a strand of hair.
Over time, as NASA’s Juno spacecraft continues to provide further measurements of Jupiter, this figure will become more accurate and allow for a clearer search of gravitational waves.
“Our precise observation of pulsars scattered across the galaxy has localized ourselves in the cosmos better than we ever could before,” Taylor said. “By finding gravitational waves this way, in addition to other experiments, we gain a more holistic overview of all different kinds of black holes in the Universe.”