It's a star, but not much of one.

Astronomers have discovered the smallest star known, and it's hardly bigger than Jupiter.

Researchers found the tiny star indirectly, through its interaction with a much larger star that it closely orbits. As seen from Earth, the small body passes in front of the big star every 7.3 days, periodically blocking about 1.5 percent of the larger star's light.

From this dimming alone, Frederic Pont of the Geneva Observatory in Sauverny, Switzerland, and his colleagues couldn't determine whether the closely orbiting object was a small star or a large planet. But by monitoring the back-and-forth motion of the large star in response to the gravitational tug of its minute partner, the scientists determined that the object was about one-tenth as massive as the sun. That's much too heavy to be a planet, the researchers say.

The orbiting object is 96 times as massive as Jupiter, yet has a radius only 1.16 times as large as the planet's, the team reports in a mid-April Astronomy & Astrophysics.

The star's small size suggests that it won't always be easy for planet hunters to discern whether an eclipsing body is a hot Jupiter--a large planet that lies within roasting distance of its parent star--or a diminutive stellar partner orbiting at the same distance.

The newfound object has barely enough mass to qualify as a star, a body that shines by burning hydrogen at its core. If it weighed less than 80 Jupiters, it would have fizzled out relatively soon after it formed. Such failed stars are known as brown dwarfs.

Pont's team measured the motion of the large star using a spectrograph on one of the quartet of 8.2-meter telescopes in Paranal, Chile, collectively known as the Very Large Telescope. --R.C.