When you imagine the Solar System in your head, most people think of the Sun, motionless and stationary in the center, with everything else buzzing around it. But each body in the Solar System also exerts its own gravitational pull on the star, causing it to move only slightly.
Therefore, the precise gravitational center (or barycenter) of the Solar System is not right in the middle of the Sun, but somewhere closer to its surface, just outside it. But it has not been easy for us to discover exactly where this barycenter is, due to the myriad of gravitational influences at play.
Now, using specially designed software, an international team of astronomers has reduced the location of the barycenter of our Solar System to less than 100 meters (328 feet), and could greatly improve our measurements of gravitational waves.
It all has to do with pulsars. These dead stars can spin extremely fast, on millisecond time scales, shooting electromagnetic radiation beams from their poles. If perfectly oriented, these rays flicker past Earth like a very fast cosmic beacon, creating a pulsed signal that is extremely regular.
This regular pulse is useful for all kinds of things, from probing the interstellar medium to a potential navigation system.
In recent years, observatories, including the North American Nanohertz Gravitational Wave Observatory (NANOGrav), have begun to use them to search for low-frequency gravitational waves, as gravitational waves should cause very subtle disturbances at the time of a great variety of pulsars in the sky. .
“Using the pulsars we observe in the Milky Way galaxy, we are trying to be like a spider sitting in stillness in the middle of its web,” explained astronomer and physicist Stephen Taylor of Vanderbilt University and the NANOGrav Collaboration.
“How well we understand that the barycenter of the Solar System is critical when we try to feel even the smallest tingling on the web.”
This is because errors in calculating Earth’s position relative to the barycenter of the Solar System can affect our measurements of pulsar time, which in turn can affect our searches for low-frequency gravitational waves.
Part of the problem is Jupiter. By a very large margin, it has the strongest gravitational effect on the Sun – the jerks of the other planets are tiny in comparison. We know how long it takes for Jupiter to orbit the Sun, about 12 Earth years, but our understanding of this orbit is incomplete.
Previously, estimates of the barycenter’s location were based on Doppler tracking, how the light of objects changes as we (or our instruments) zoom in or out of them, to calculate the orbits and masses of the planets. But any error in these masses and orbits can introduce errors that could closely resemble gravitational waves.
And when the team used these existing data sets to analyze the NANOGrav data, they kept getting inconsistent results.
“We were not detecting anything significant in our searches for gravitational waves between models in the solar system, but we were obtaining large systematic differences in our calculations,” said astronomer Michele Vallisneri of NASA’s Jet Propulsion Laboratory.
“Typically, more data gives a more accurate result, but there was always a shift in our calculations.”
This is where the computer software comes into the picture. It’s called BayesEphem, and it’s designed to model and correct those uncertainties in the orbits of the Solar System that are most relevant to gravitational wave searches using pulsars, particularly Jupiter.
When the team applied BayesEphem to the NANOGrav data, they were able to place a new upper bound on the bottom of gravitational waves and detection statistics. And they were able to calculate a new, more precise location for the barycenter of the Solar System that, in the future, could allow much more accurate low-frequency gravitational wave detections.
“Our accurate observation of pulsars scattered across the galaxy has located us in the cosmos better than ever,” said Taylor.
“By finding gravitational waves in this way, in addition to other experiments, we get a more holistic view of all the different types of black holes in the Universe.”
The research has been published in The astrophysical journal.
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