Observation of Luhman 16A (credit: H.M.J. Boffin et al./arXiv)

Precise measurements of the closest pair of failed stars to the Sun suggest that the system harbors a third, planetary-mass object.

Failed stars are known as brown dwarfs and have a mass below 8% of the mass of the Sun — not massive enough to burn hydrogen in their centers.

This system, Luhman 16AB, was discovered earlier this year and is only 6.6 light-years away. Both objects have a mass between 30 and 50 Jupiter masses. By comparison, the Sun has a mass of about 1,000 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,” explained Carnegie’s Yuri Beletsky.

The team used the FORS2 instrument on ESO’s Very Large Telescope at Paranal to image the brown dwarf couple. The instrument enabled the observers to make very precise measurements so the scientists were already able to detect tiny displacements of the two objects in their orbit during only this the two-month period, with ten times better accuracy than before and thereby detect even small perturbations of their orbit.

“We have been able to measure the positions of these two objects with a precision of a few milli-arcseconds,” said H.M. J. Boffin of European Southern Observatory, Chile. “That is like a person in Paris being able to measure the position of someone in New York with a precision of 10 centimeters.”

The measurements were so fine that the astronomers were able to see some very small deviations from the expected motion of the two brown dwarfs around each other. The fact that the deviations appear correlated is a strong indication that a companion perturbs the motion of one of the two brown dwarfs. This companion is most likely a planetary-mass object, which has an orbital period between two months and a year.

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

Astronomers at Université Libre de Bruxelles, Belgium, and Universidad de Valparaiso, Chile were also involved.

Abstract of arXiv paper

Using FORS2 on the Very Large Telescope, we have astrometrically monitored over a period of two months the two components of the brown dwarf system WISE J104915.57-531906.1, the closest one to the Sun. Our astrometric measurements – with a relative precision at the milli-arcsecond scale – allow us to detect the orbital motion and derive more precisely the parallax of the system, leading to a distance of 2.020+/-0.019 pc. The relative orbital motion of the two objects is found to be perturbed, which leads us to suspect the presence of a substellar companion around one of the two components. We also perform VRIz photometry of both components and compare with models. We confirm the flux reversal of the T dwarf.