dead center

Scientists found the center of the Solar System, and it's not where you think

The Sun wobbles out of place.

Deep space planets, awesome science fiction wallpaper, cosmic landscape. Elements of this image furn...

When we think of Earth and its neighboring planets orbiting around our common host star, we picture the center of the Solar System as smack in the middle of the Sun. However, that's not entirely true, according to new research.

The planets and the Sun actually orbit around a common center of mass. And for the first time, a team of astronomers has pinpointed the center of the entire Solar System down to within 100 meters, the most precise calculation yet.

Their findings are detailed in a study published in April in The Astrophysical Journal, and will help astronomers in their quest to hunt for gravitational waves given off in the universe by objects such as supermassive black holes.

The entire Solar System, including the Sun, has a barycenter, or a common center of mass of all of the Solar System's objects, around which they orbit.

Despite popular belief, the barycenter of the Solar System is not the center of the Sun. That's because planets and other bodies of the Solar System enforce a gravitational tug on the star, causing it to wobble around a little bit.

Instead, the barycenter of the Solar System lies a little outside of the Sun's surface. However, scientists have not been able to pinpoint exactly where this center lies.

The reason why it's difficult to do so is partly because of Jupiter, the Solar System's largest planet. Due to its large mass, Jupiter has the strongest gravitational pull on the Sun by a long shot.

However, the team of scientists behind the new study were able to narrow down the location of the barycenter within 100 meters, a very small margin considering the colossal size of the Solar System, and found that it lies right above the surface of the Sun.

The secret for their accurate measurements — pulsars. Pulsars are a fast-rotating neutron star, or the super dense remains of a star that exploded in a supernova. These stars emit electromagnetic radiation in the form of bright, narrow beams that sweep across the cosmos in a round motion as the star itself spins, sort of like a lighthouse.

If you are observing the stars from a distance, it will look as though they are pulsating in regular flashes of light, which is how they got their name.

"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, a physicist and astronomer at Vanderbilt University, and lead author of the study, 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."

From Earth, the beams given off by the pulsars are detected as pulse signals that appear on a regular basis. Using these signals, the team of astronomers was able to more accurately measure Earth's distance from other objects in the Solar System, including the barycenter.

Now that astronomers have a more accurate measurement of where the barycenter of the Solar System lies, they can in turn make much more accurate detections of low-frequency gravitational waves.

Gravitational waves are ripples in space and time caused by objects of accelerated masses such as supermassive black holes, which emit these waves outwards at the speed of light.

"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."

Abstract: The regularity of pulsar emissions becomes apparent once we reference the pulses' times of arrivals to the inertial rest frame of the solar system. It follows that errors in the determination of Earth's position with respect to the solar system barycenter can appear as a time-correlated bias in pulsar-timing residual time series, affecting the searches for low-frequency gravitational waves performed with pulsar-timing arrays. Indeed, recent array data sets yield different gravitational-wave background upper limits and detection statistics when analyzed with different solar system ephemerides. Crucially, the ephemerides do not generally provide usable error representations. In this article, we describe the motivation, construction, and application of a physical model of solar system ephemeris uncertainties, which focuses on the degrees of freedom (Jupiter's orbital elements) most relevant to gravitational-wave searches with pulsar-timing arrays. This model, BayesEphem, was used to derive ephemeris-robust results in NANOGrav's 11 yr stochastic-background search, and it provides a foundation for future searches by NANOGrav and other consortia. The analysis and simulations reported here suggest that ephemeris modeling reduces the gravitational-wave sensitivity of the 11 yr data set and that this degeneracy will vanish with improved ephemerides and with pulsar-timing data sets that extend well beyond a single Jovian orbital period.
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