Object near sunlike star caught on camera

This brown dwarf, identified by the arrow, orbits close to a star like our sun.

By Richard Stenger
CNN Sci-Tech

(CNN) -- Astronomers have taken optical images of a giant planetlike body orbiting a sunlike star, making it the closest ever observed around a star through direct imaging.

The distance between the substellar object known as brown dwarf and its parent star is less than that between the planet Uranus and the sun.

The observation is the latest in a flurry of star system discoveries, some using ground-based technology that in specific situations can make sharper pictures than space telescopes.

"This discovery implies that brown dwarf companions to average, sunlike stars exist at a separation comparable to the distance between the sun and the outer planets in our solar system," said University of Hawaii astronomer Michael Liu, who with colleagues announced the findings Monday.

Brown dwarfs, middling objects between planets and stars, are considerably more massive than the largest planets but do not have enough mass to ignite the thermonuclear reactions necessary to become stars.

This one, located 58 light-years away in the constellation Sagitta, contains more than 12 times the mass of all the planets in our solar system, astronomers estimate. Its parent star is roughly 2 billion years old, slightly younger than our sun.

In this composite image, light from the brighter primary star is removed from the region of the brown dwarf, lower left.

"This companion is probably too massive to have formed the way we believe that planets do, namely from a circumstellar disk of gas and dust when the star was young," Liu said in a statement. "This finding suggests that a diversity of processes act to populate the outer regions of other systems."

In recent years, dozens of distant planets have been found, but only through indirect means, such as observing the gravitational tug on their parent stars.

That technique has failed to turn up brown dwarfs, only working within the first 4 astronomical units (AU) of a star system, or four times the distance of the Earth and sun.

In our solar system, the giant planets reside in the planetary outskirts. Saturn orbits the sun from a distance of about 10 AUs. Uranus floats around at 19.

The newly discovered brown dwarf is 14 AUs from its parent star, known as 15Sge.

Liu and his partners made the discovery using the Gemini North and twin Keck telescopes, which stand atop Hawaii's Mauna Kea, where observatories take advantage of the volcanic mountain's thin, clean and dry air.

Even so, atmospheric turbulence usually blurs images taken by observatories on the ground. But new optic techniques compensate for the effect.

"Only by using adaptive optics to produce very sharp images could we have found this companion. It is too faint and too close to its parent star to be seen otherwise," said Liu, who presented the images at a meeting of the American Astronomical Society.

In 1995, astronomers detected evidence of the first brown dwarf, a substellar companion to a cool red dwarf star about 19 light-years from Earth.