Orbit

Abstract: Some eclipsing variables are observed to undergo orbital period modulations of amplitude ΔP/P∼10 −5 over time scales of decades or longer. These modulations can be explained by the gravitational coupling of the orbit to variations in the shape of a magnetically active star in the system. The variable deformation of the active star is produced by variations in the distribution of angular momentum as the star goes through its activity cycle. This mechanism typically requires that the active star be variable at the ΔL/L⇒0.1 level, and be differentially rotating at the ΔΩ/Ω⇒0.01 level

Abstract: The highly elliptical, 16-year-period orbit of the star S2 around the massive black hole candidate Sgr A* is a sensitive probe of the gravitational field in the Galactic centre. Near pericentre at 120 AU, ~1400 Schwarzschild radii, the star has an orbital speed of ~7650 km/s, such that the first-order effects of Special and General Relativity have now become detectable with current capabilities. Over the past 26 years, we have monitored the radial velocity and motion on the sky of S2, mainly with the SINFONI and NACO adaptive optics instruments on the ESO Very Large Telescope, and since 2016 and leading up to the pericentre approach in May 2018, with the four-telescope interferometric beam-combiner instrument GRAVITY. From data up to and including pericentre, we robustly detect the combined gravitational redshift and relativistic transverse Doppler effect for S2 of z ~ 200 km/s / c with different statistical analysis methods. When parameterising the post-Newtonian contribution from these effects by a factor f, with f = 0 and f = 1 corresponding to the Newtonian and general relativistic limits, respectively, we find from posterior fitting with different weighting schemes f = 0.90 +/- 0.09 (stat) +\- 0.15 (sys). The S2 data are inconsistent with pure Newtonian dynamics.

Abstract: An important early prediction of Einstein's general relativity1,2,3 was the advance of the perihelion of Mercury's orbit, whose measurement provided one of the classical tests of Einstein's theory4. The advance of the orbital point-of-closest-approach also applies to a binary pulsar system5,6 and to an Earth-orbiting satellite3. General relativity also predicts that the rotation of a body like Earth will drag the local inertial frames of reference around it3,7, which will affect the orbit of a satellite8. This Lense–Thirring effect has hitherto not been detected with high accuracy9, but its detection with an error of about 1 per cent is the main goal of Gravity Probe B—an ongoing space mission using orbiting gyroscopes10. Here we report a measurement of the Lense–Thirring effect on two Earth satellites: it is 99 ± 5 per cent of the value predicted by general relativity; the uncertainty of this measurement includes all known random and systematic errors, but we allow for a total ± 10 per cent uncertainty to include underestimated and unknown sources of error.

Abstract: About one-quarter of the known hot Jupiter exoplanets are orbiting in the 'wrong direction', or counter to the spin axis of the host star. Attempts to explain this phenomenon have so far failed. It is known that in triple-star systems, retrograde orbits of this type can be produced by the long-term effects of stellar perturbations. An analysis of the motions of planetary bodies, including octupole-order effects and tidal friction, suggest that a similar mechanism may be operating involving planets rather than stars. The new model proposes a mechanism called Kozai capture, in which long-term interactions with a more-distant planet can naturally produce close-in planets with retrograde orbits, through forces familiar in the Kozai mechanism that are thought to cause the high eccentricities observed in the orbits of exosolar planets. About 25 per cent of ‘hot Jupiters’ (extrasolar Jovian-mass planets with close-in orbits) are actually orbiting counter to the spin direction of the star1. Perturbations from a distant binary star companion2,3 can produce high inclinations, but cannot explain orbits that are retrograde with respect to the total angular momentum of the system. Such orbits in a stellar context can be produced through secular (that is, long term) perturbations in hierarchical triple-star systems. Here we report a similar analysis of planetary bodies, including both octupole-order effects and tidal friction, and find that we can produce hot Jupiters in orbits that are retrograde with respect to the total angular momentum. With distant stellar mass perturbers, such an outcome is not possible2,3. With planetary perturbers, the inner orbit's angular momentum component parallel to the total angular momentum need not be constant4. In fact, as we show here, it can even change sign, leading to a retrograde orbit. A brief excursion to very high eccentricity during the chaotic evolution of the inner orbit allows planet–star tidal interactions to rapidly circularize that orbit, decoupling the planets and forming a retrograde hot Jupiter.