Nearby Star System May Have a Mystery Planet

Just under 7 light years away from the Sun, a pair of neighborhood brown dwarf stars may be harboring a small planetary system, astronomers reported this week in a letter appearing in the journal Astronomy & Astrophysics.

Brown dwarfs are failed stars that only have about 8 percent of the mass of the Sun. As expected, an analysis of the two stars in the nearby binary system, known as Luhman 16AB, revealed them to be small compared to the Sun. While our star has a mass about 1,000 times that of Jupiter, the stars in Luhman 16AB were found to only have a mass between 20 and 50 Jupiter masses.

"The two brown dwarfs are separated by about three times the distance between the Earth and the Sun. Binary brown dwarf systems are gravitationally bound and orbit about each other. Because these two dwarfs have so little mass, they take about 20 years to complete one orbit," said Yuri Beletsky of the Carnegie Institution for Science.

The Luhman 16AB binary system presents a unique research opportunity for astronomers as the system is the closest pair of failed stars to the Sun.

Upon observing the details of the binary system with the FORS2 instrument on European Southern Observatory's Very Large Telescope, the researchers were able to view the brown dwarfs at an unprecedented level of detail.

"We have been able to measure the positions of these two objects with a precision of a few milli-arcseconds," said the ESO's Henri Boffin, who led the study of the binary system. "That is like a person in Paris being able to measure the position of someone in New York with a precision of 10 centimeters."

Orbiting brown dwarf binary systems are gravitationally bound to one another. But because the distance separating them is about three times the distance between the Earth and the Sun, it can take about two decades for the completion of one orbit.

But the latest measurements of the system were so detailed that the team was able to detect small deviations from the expected motion of the two brown dwarfs as they orbited.

Because these deviations appear to be correlated, the astronomers suspect that the deviation is caused by another celestial body also orbiting the system. It is most likely a planetary-mass object with an orbital period between two months and a year, the researchers said.

"Further observations are required to confirm the existence of a planet," Boffin said. "But it may well turn out that the closest brown dwarf binary system to the Sun turns out to be a triple system!"