Chinese astronomers find real 3-body system. It is not what Liu Cixin thinks

In author and computer engineer Liu Cixin’s science fiction hit The Three-Body Problem, three suns throw a doomed world into chaos. Chinese astronomers say they have found a real three-star system that is far better behaved.

Using FAST, the world’s biggest single-dish radio telescope in southwestern China, the researchers found two dead stars and a “living” one locked in a steady cosmic dance about 7,000 light years from Earth.

One of the dead stars is a pulsar that spins more than 300 times a second, about as fast as the blades of a kitchen blender. It whirls around its partner, a white dwarf – the dim, burnt-out core of a smaller star – every eight days.

Meanwhile, a star much like the sun circles the pair from afar, taking 73½ years to complete each lap.

The team, led by Han Jinlin from the National Astronomical Observatories in Beijing, reported the finding in The Astrophysical Journal Letters earlier this month. It is only the second such system known, after one found by a US-led team in 2014 in which all three stars were already dead.

“The detailed evolution of triple-star systems is complicated and poorly known,” the researchers wrote in their paper. The newly identified system, they added, was “unique” for studying how such star families changed over time.

Triple-star systems are fairly common, but those containing a millisecond pulsar – the fast-spinning core left behind when a massive star explodes at the end of its life – are “extremely rare”, according to the team, because the star’s explosion can easily tear the system apart.

Such systems give astronomers a rare chance to test theories of gravity. For instance, this one could help check a key idea in Einstein’s theory of gravity, known as the strong equivalence principle: that everything falls the same way, whether it is a feather or a city-sized dead star heavier than the sun.

Pulsars are some of the most reliable clocks in the universe. As they spin, they sweep beams of radio waves across space like a lighthouse and the pulses reach Earth so regularly they can rival atomic clocks.

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So astronomers were puzzled when this one, first spotted by FAST in 2020, seemed to be running slow. Five years of monitoring by a team from the Xinjiang Astronomical Observatory showed its spin was losing speed about 100 times faster than other pulsars of its kind.

In a paper published in Nature Astronomy in April, the Xinjiang team said the finding challenged a standard theory of how such pulsars formed. They also noted hints that something else might be lurking in the system.

The researchers behind the new study went looking. Combining FAST data with observations from other telescopes, including 18 years of gamma-ray data from Nasa’s Fermi space telescope, they found the culprit: a sun-like star hiding in plain sight.

As the star slowly circles the pair, its gravity tugs the pulsar back and forth, making it look as if the pulsar’s spin is slowing down.

The three stars were likely born together from the same cloud of gas, like triplets, the team said. Because they started out with very different masses, the siblings have aged at very different speeds.

The biggest, about 20 times the mass of the sun, burned out first and exploded, leaving behind the pulsar, while the second faded into the white dwarf.

The smallest, about the mass of the sun, sits far enough away to survive its siblings’ deaths and is still shining. Eventually it too will burn out, according to the researchers, leaving behind a pulsar and two white dwarfs, much like the system found in 2014.

A European team, with scientists from Germany, the Netherlands, France and Britain, independently reached the same conclusion in a paper posted online in July.

Both teams found that the sun-like star will swing closest to the pair in 2036. As it does, its pull on the pulsar will change quickly, allowing scientists to measure the system much more precisely.