How to Find the Smallest Stars in the Universe

While looking for exoplanets, astronomers discovered the tiniest star ever found, just slightly bigger than Saturn.

A thin diagonal line of light is shown against a starry sky.
Gene Blevins / Reuters

For astronomer Alexander von Boetticher, false positives in data are actually a good thing.

That’s how von Boetticher and his colleagues at the University of Cambridge discovered the smallest star ever measured. The star showed up in data from a British-led mission known as the Wide Angle Search for Planets, or WASP. WASP looks for potential exoplanets by looking for dimming in the brightness of stars, a signal that an object is passing in front and blocking the light. The mission has detected dozens of planets this way. Sometimes, the source of the dimming turns out to be something else entirely. “Every now and then, instead of finding an exoplanet orbiting some star, it actually finds a very small star that’s orbiting a star,” von Boetticher said.

The newly discovered star, known as EBLM J0555-57Ab, is located about 600 light-years away and is slightly bigger than Saturn. The star is likely as small as stars get; it has just enough mass to fuse hydrogen into helium in its core. The process is known as nuclear fusion, and it’s what makes stars stars. Any smaller, and EBLM J0555-57Ab would no longer be able to sustain this reaction. It would become a brown dwarf, a substellar object that emits mostly infrared, not visible, light. The discovery is described in a recent paper in the journal Astronomy & Astrophysics.

The newly discovered star is one half of a binary system, a common configuration in which two stars closely orbit each other. The nature of this system helps researchers make precise measurements of the smaller star, von Boetticher said. In a binary system, the smaller object exerts a gravitational pull that makes the larger object move back and forth ever so slightly. Scientists can observe this wobbling effect through telescopes and use it to determine the properties of the smaller object, like its mass and radius. These figures can help determine the nature of the object. “If it’s really heavy and it’s really small, it’s likely to be a star,” von Boetticher said. “If it’s light and small, then it’s likely a planet.”

Small stars like EBLM J0555-57Ab are difficult to observe directly, even with powerful telescopes, because they emit so little light. “But when they orbit in a binary system, then you can use indirect detection methods,” von Boetticher said.

Small stars make good candidates in the search for exoplanets, von Boetticher said. Red dwarfs, smaller and cooler than stars like our sun, are the most common type of star in the universe. When you’re looking for exoplanets—and potentially habitable ones—the more star candidates, the better. The transit of a planet in front of a star—the phenomenon that WASP looks out for—is also more pronounced and easier to spot when the star is smaller in size. Earlier this year, scientists discovered seven Earth-sized planets orbiting a red dwarf called TRAPPIST-1, located about 40-light years away. Some of the planets orbit in the star system’s habitable zone, where conditions for liquid water could arise, but some scientists think TRAPPIST-1 may emit enough ultraviolet radiation to wipe out their atmospheres.

The discovery of EBLM J0555-57Ab illustrates the variety of stars in the universe. From our vantage point on Earth, all stars look the same—tiny pinpoints of light against a dark sky. That picture doesn’t do justice to the stars out there, which come in all kinds of different sizes, from dwarfs the size of Saturn to hypergiants hundreds of times larger than our sun.

“I think many people know that stars can be really, really big, much larger than the sun, but not many people are aware that they can also be as small as some of the large planets in our solar system,” von Boetticher said.