Two brown dwarf-sized objects orbiting a giant old star show that planets may assemble around stars more quickly and efficiently than anyone thought possible, according to an international team of astronomers.

"We have found two brown dwarf-sized masses around an ordinary star, which is very rare," said Alex Wolszczan, Evan Pugh professor of astronomy and astrophysics, Penn State and lead scientist on the project.

The star, BD +20 2457, is a K2 giant - an old bloated star nearing the end of its life. Seeing a pair of brown dwarfs around a K-type giant is a first for astronomers and offers a unique window into how they can be produced. The researchers from the Torun Center for Astronomy, Poland and the Center for Exoplanets and Habitable Worlds, Penn State report their findings in the current issue of the Astrophysical Journal.

Brown dwarfs are dim, elusive objects that straddle the dividing line between planets and stars. They are too massive to be planets, but not massive enough to generate the fusion-powered energy of a star. These stellar cousins represent a kind of "missing link" between planets and stars, but little is known about how they are made.

"If we find one brown dwarf, we are not sure where it came from," Wolszczan explained. "It could be either from the process of planet formation or it could be a direct product of star formation."

Seeing two of them around a parent star means they must have originally formed from the enormous supply of raw materials that surrounded the star when it was young. Astronomers call this thick, solar system-sized pancake of gas and dust the "circumstellar disk."

"If that is the case," he continued, "then if we add up the minimum masses of these two objects, we know the disk had to be extremely massive."

To find these faint companions, the astronomers used the High Resolution Spectrograph on the Hobby-Eberly Telescope in west Texas to split up the light of BD +20 2457. This technique is similar to the way a prism breaks light into a rainbow - spectrum - of colors. They looked for shifts in color of certain features in the spectrum, called spectral lines, as the dwarfs moved around the star and caused the star to wobble back and forth from their gravitational tugs.

When the brown dwarfs' gravitational influence causes BD +20 2457 to move towards Earth slightly, its spectral lines decrease in wavelength, becoming slightly bluer. As it moves away, the wavelengths increase, becoming slightly redder. By noting how quickly and strongly the spectral lines shift, astronomers can infer the objects' masses, as well as the sizes and shapes of their orbits.

"Artist's rendition of a brown dwarf and its moon orbiting a triple star system. (Credit: NASA)"

Source: Penn State