Discovery of a starless binary system of brown dwarfs

An international research team have discovered an exotic binary system composed of two young planet-like objects, orbiting around each other from a very large distance. Although these objects look like giant exoplanets, they formed in the same way as stars, proving that the mechanisms driving star formation can produce rogue worlds in unusual systems deprived of a Sun.

Star-forming processes sometimes create mysterious astronomical objects called brown dwarfs, which are smaller and colder than stars, and can have masses and temperatures down to those of exoplanets in the most extreme cases. Just like stars, brown dwarfs often wander alone through space, but can also be seen in binary systems, where two brown dwarfs orbit one another and travel together in the galaxy.

The research was led by Clémence Fontanive from the University of Bern (who completed her PhD at the School’s Institute for Astronomy in 2019), Prof Beth Biller from the School’s Institute for Astronomy and scientists from Bucknell University.

They discovered a curious starless binary system of brown dwarfs. The system CFHTWIR-Oph 98 (or Oph 98 for short) consists of the two very low-mass objects Oph 98 A and Oph 98 B. It is located 450 light years away from Earth in the stellar association Ophiuchus. The researchers were surprised by the fact that Oph 98 A and B are orbiting each other from a strikingly large distance, about 5 times the distance between Pluto and the Sun, which corresponds to 200 times the distance between the Earth and the Sun. The study has just been published in The Astrophysical Journal Letters.

Extremely low masses and a very large separation

The pair is a rare example of two objects similar in many aspects to extra-solar giant planets, orbiting around each other with no parent star. The more massive component, Oph 98 A, is a young brown dwarf with a mass of 15 times that of Jupiter, which is almost exactly on the boundary separating brown dwarfs from planets. Its companion, Oph 98 B, is only 8 times heavier than Jupiter. Components of binary systems are tied by an invisible link called gravitational binding energy, and this bond gets stronger when objects are more massive or closer to one another. With extremely low masses and a very large separation, Oph 98 has the weakest binding energy of any binary system known to date.

Discovery thanks to data from Hubble

Researchers discovered the companion to Oph 98 A using images from the Hubble Space Telescope. Brown dwarfs are only visible in infrared light.  This is because they are very cold and emit very little light, and the stellar association in which the binary is located, Ophiuchus, is embedded in a dense, dusty cloud which scatters visible light.

Because brown dwarfs are cold enough, water vapor forms in their atmospheres, creating prominent features in the infrared that are commonly used to identify brown dwarfs. However, these water signatures cannot be easily detected from the surface of the Earth. Located above the atmosphere in the vacuum of space, Hubble allows to probe the existence of water vapor in astronomical objects.

An atypical result of star formation

The Oph 98 binary system formed only 3 million years ago in the nearby Ophiuchus stellar nursery, making it a newborn on astronomical timescales. The age of the system is much shorter than the typical time needed to build planets. Brown dwarfs like Oph 98 A are formed by the same mechanisms as stars. Despite Oph 98 B being the right size for a planet, the host Oph 98 A is too small to have a sufficiently large reservoir of material to build a planet that big. Therefore Oph 98 B, like its host, must have formed through the same mechanisms that produce stars.