Record-breaking White Dwarf Stars That Circle Each Other Every Seven Minutes Have Fastest Orbit in Known Universe

A team of astronomers has discovered an unusual star system which represents the "fastest eclipsing white dwarf binary" in the known universe. The find could have have significant implications for the study of gravitational waves—or ripples in the fabric of space-time.

White dwarfs are the (relatively) small, dense remnants of stars—such as our sun—which have shed their outer layers, spent most of their hydrogen and helium fuel, and are gradually cooling as they reach the end of their lives. Ninety-seven percent of stars in our galaxy will meet their end in this way.

The star system that the team discovered—dubbed ZTF J1539+5027 or J1539 for short—comprises two of these white dwarfs, according to a study published in the journal Nature.

"White dwarfs are very dense stars—they have as much mass as the sun stuffed into a size comparable to Earth," Lori Allen, Director of the Kitt Peak National Observatory, told Newsweek. "A teaspoon of white dwarf matter would weigh about 15 tons!"

"If two white dwarfs are orbiting each other in a binary, and we are looking at the system in the plane of their orbit, one star will pass in front of the other and temporarily block—that is, eclipse— their light. That makes it an eclipsing white dwarf binary," she said.

White dwarf binaries are rare objects so scientists know little about them. But according to the study, J1539 is particularly intriguing because of its very short orbital period—just 6.91 minutes.

"One amazing thing about this binary is that the two stars orbit very closely—their orbit could fit within the planet Saturn," Allen said. "Because they are so close together, they also orbit each other very quickly—that is, these two Earth-sized stars are locked in a whirling dance where they circle each other roughly every seven minutes! It makes my head spin to think how fast that is."

"Close white dwarf binaries like this are expected to be common, but only a few are known to date," she said. "Of the known binaries, this one is notable, because it has the shortest period."

The latest discovery was made as part of a new survey of the night sky known as the Zwicky Transient Facility (ZTF) which uses the Palomar Observatory in California to identify objects that move or vary in brightness before following up promising candidates with a new instrument at the Kitt Peak National Observatory in Arizona. The study was led by Kevin Burdge from Caltech.

"This story is also about how astronomers can combine the old with the new to make exciting discoveries," Allen said. "[Burdge] and his team were able to identify this rare system using a new instrument at the Kitt Peak 2.1-meter telescope that is designed to make rapid, sensitive measurements of the changing brightness of stars. Their discovery illustrates how a venerable telescope like the 2.1-meter, now more than 50 years old, remains a platform for exciting discoveries."

The findings may have implications for the study of gravitational waves, which were predicted by Albert Einstein in 1916 as part of his general theory of relativity. However, they were not directly observed until 2015 when the LIGO ground-based observatory made a historic measurement by detecting the gravitational waves created by a pair of colliding black holes.

"Close binaries are predicted to spiral together by emitting gravitational waves—they lose energy by creating ripples in space-time," Allen said. "This binary will be one of the best targets for a future space mission to detect gravitational waves, both because it should be a strong source of gravitational waves and because we now know a lot about this source."

One of the future space missions designed to detect gravitational waves is known as LISA (Laser Interferometer Space Antenna)—a European-led mission expected to launch in 2034.

"I think the really exciting element here to emphasize is that right now in the era of LIGO, we are talking about dozens of sources that emit gravitational waves, and those are already teaching us exciting things," Burdge told Newsweek. "In the LISA era, there should be tens of thousands of binary white dwarfs emitting gravitational waves strong enough to be detected. This system is really just a hint of all the exotic and crazy objects that await us when we can finally see them all because of LISA."

Kitt Peak National Observatory
The 2.1-meter telescope at Kitt Peak National Observatory. P. Marenfeld & NOAO/AURA/NSF