Of the hundreds of billions of stars that throng the Milky Way, only one is closest to the sun: a little red dwarf named Proxima Centauri, a star so dim it was unknown a century ago. Now this stellar neighbor is about to betray some of its secrets, because in October it will pass in front of another star. As the light from the distant star skirts past Proxima, the red star's gravity will bend the beam, divulging our neighbor's mass and perhaps even its planets.

A gravitational deflection by a star "has never ever been seen outside of the solar system," says astronomer Kailash Sahu of the Space Telescope Science Institute, who discovered the remarkable upcoming alignment. In 1919, during a solar eclipse, observers watched the sun's gravity alter the apparent positions of stars in a way that confirmed Albert Einstein's new general theory of relativity. Isaac Newton's theory of gravity also predicted a deflection, but by only half the amount seen. Since then astronomers have discovered cases in which a galaxy's gravity splits the light of a distant quasar into multiple images, and observers have watched unseen stars cause background stars to brighten by passing before them and magnifying their light, a phenomenon known as gravitational microlensing. But no one has ever seen a star near the solar system cross in front of a much more distant star.

To find out whether any such alignments would occur, Sahu examined the paths of 5,000 nearby stars when he struck gold with Proxima Centauri. "Just by observing the distant star's deflection you will know precisely how massive Proxima is," says Sahu, who plans to track the event with the Hubble Space Telescope. "Low-mass stars are the most common stars in the universe, but there is a lot of uncertainty in measuring their mass." Mass is a crucial stellar parameter, because it dictates how a star ages and how long it lives.

Even by red dwarf standards Proxima Centauri is feeble. At the time of its discovery, in 1915, it was the least luminous star known. Scottish-born South African astronomer Robert Innes spotted the star traveling 2.2 degrees from Alpha Centauri A and B, a pair of brilliant stars that together shine as the third-brightest star in the night sky. Unfortunately, they lie so far south that most Americans can't see them. Alpha Centauri A, a yellow star like the sun, and Alpha Centauri B, an orange star somewhat fainter, are 4.37 light-years from us. Proxima Centauri orbits them roughly every million years. It is 4.24 light years from Earth, making it slightly closer, hence its name.

Alpha Centauri A and B exemplify how astronomers normally measure stellar masses. As the two stars orbit one another every 80 years, one tugs the other, revealing that relative to the sun the brighter star is 10 percent more massive whereas the other is 8 percent less massive.

But Proxima Centauri is 13,000 times farther from its bright mates than Earth is from the sun. As a result, the little red star's gravity barely perturbs its partners, making a mass measurement impossible—or so it seemed until Sahu's discovery. As his team will report in a future issue of The Astrophysical Journal, Hubble observations in October and again in February 2016, when the star passes even closer to another star, should yield Proxima's mass to an accuracy of 5 percent. Astronomers have already sized up the star— its diameter is 15 percent of the sun's —so the mass measurement will provide Proxima's density.

"It's the first time that such an event has ever been identified," says Andrew Gould, an astronomer at The Ohio State University who is unaffiliated with the researchers. "It's opening up a new domain that people have thought about for 50 years." In 1964, astronomer Sjur Refsdal published calculations describing how a star's gravity deflects a background star's light.

Proxima Centauri's passage promises more. "This is definitely a chance to detect planets around Proxima," Sahu says. In 2012 astronomers reported an Earth-mass world circling Alpha Centauri B, the orange star, but Proxima Centauri has no known planets. The more massive a planet, the more its gravity will alter the distant starlight, making the world easier to discern. Unfortunately, Hubble probably won't look for planets, because that would require too much of its time; instead, ground-based observatories will take up the search. Sahu says prospects for finding Proxima's planets are low—he puts the odds from both alignments at only 6 to 10 percent—but the passages may just unveil one of the most exciting extrasolar worlds ever found: a planet circling the very closest star to the sun.