It's possible to study how gravity affects small objects here on Earth, but most alternatives to general relativity predict that there will be differences between gravity's influence on small and large objects. This means that if we want to test these alternatives, we need to be able to make precision measurements of how gravity affects massive objects, which requires something outside our Solar System.

Generally, scientists have relied on pulsars, rapidly rotating neutron stars that sweep a beam of intense light toward the Earth every few milliseconds. Any nearby sources of gravity can subtly shift the neutron star's rotation, altering the timing of the pulses and providing a way to measure gravity itself. All you need is a sufficiently large source of gravity in the neutron star's neighborhood.

Now, scientists have discovered not one but two nearby sources. A survey of pulsars identified an unusual system that resides a bit over 4,000 light years from Earth: A neutron star circled by a white dwarf that completes an orbit in 1.6 days, with the inner system orbited by a second white dwarf that takes 327 days to complete an orbit. This strange system hints at a past its discoverers call "complex and exotic," and it raises hopes for a high-precision test of gravity.

The discovery began with a large survey of pulsars performed using three different instruments, including the giant radio dish at Arecibo. Once the new system, PSR J033 +1715, was spotted, the team behind the survey used Arecibo to pin down the timing of its pulses. With a half an hour of observations, they were able to measure the timing with an error of only 100 nanoseconds, which allowed them to determine if there were any variations in the interval between pulses. There very clearly were, which suggests that there were other massive objects influencing its rotation.

In fact, the data indicated two separate objects. At very short time scales, the pulse timing showed a sine-wave pattern, oscillating up and down as a very nearby object tugged at the neutron star. But that was superimposed on a much longer sine wave, indicating a second object much farther out. The authors attempted to model the system using a three-body solution with parameters chosen at random, seeking a solution that got a close match to the observations. This told them things like the mass of the objects and orbital periods.

Observations in optical wavelengths suggested that there were no normal stars nearby, and the apparent masses of the objects (0.2 and 0.4 times the mass of the Sun, respectively) indicated that the neutron star's companions are white dwarfs. The orbits of these bodies appear to be nearly circular, and they're all aligned on the same plane. That's exceedingly rare—nothing of the sort has been observed before, and the authors estimate that fewer than 100 of them exist in the entire galaxy.

It's presumably rare because it is extraordinarily difficult to form a system like this. After all, a neutron star is formed in a supernova, yet there are two close-in stars that appear to have survived the blast. The authors suspect that the stars were once in long, elliptical orbits that kept them at a safe distance when their massive companion exploded. As the stars interacted with the debris and the outermost star shed its outer layers, however, the orbits became more regular, and the infalling debris spun up the neutron star, turning it into a pulsar. When the last remaining star expanded, it probably engulfed the neutron star, which orbited within the star's envelope. Continued gravitational interactions have since made the orbits nearly circular and the system very compact.

This isn't the first neutron star orbited by multiple companions, but in all the other cases, at least one of the companions is a planet-sized mass. This is the first chance we have of studying a system where all the bodies are in the stellar mass range, and the authors think it will have gravitational interactions that are six or seven orders of magnitude larger than anything we've seen before. This should provide a very high-precision test of the gravitational interactions of massive objects, and it might enable us to catch any differences between what we see there and what we can see with smaller objects.

Nature, 2013. DOI: 10.1038/nature12917 (About DOIs).

Listing image by NSF