Tilted Orbits

Within the family of celestial orbs in the universe, brown dwarfs are somewhat like misfits. They are less massive and cooler than stars but are 10 to 80 times more massive than Jupiter. Brown dwarfs are sometimes called "failed stars," because they lack the mass to ignite nuclear fusion and shine with starlight.

One mystery that surrounds these oddballs is how they formed: Some theories propose that they form like stars do, out of collapsing clouds of material, while others suggest they form like planets, taking shape within rotating dusty disks that circle young stars. It is also possible, scientists propose, that brown dwarfs may form both like stars and planets.

Steven Giacalone, a National Science Foundation (NSF) Postdoctoral Scholar Fellowship Trainee in Astronomy at Caltech, and his colleagues are addressing the mystery by studying the orbital tilts of brown dwarfs that circle very closely around companion stars. Brown dwarfs, as well as some other exoplanets, can have orbits that are tilted to varying degrees relative to the rotational direction of their host stars. If a brown dwarf has an orbital tilt, then it is out of whack with its partner star: the brown dwarf will loop above and below a plane that aligns with the star's equator. This is unlike the planets in our own solar system that orbit in a plane that aligns with the Sun's rotational direction.

Using the Keck Planet Finder (KPF), a new planet-hunting instrument at the W. M. Keck Observatory in Hawaiʻi, Giacalone and his colleagues wanted to assess whether a brown dwarf named GPX-1b has an orbital tilt. They say that a tilt would indicate that the object probably formed like a star and not like a planet.

"For a brown dwarf to have made its way into a tilted close-in orbit, it would have had to have been knocked around by a larger planetary body or captured by the star as the brown dwarf passed by," explains Giacalone, who works in the group of Andrew Howard, a professor of astronomy at Caltech and the principal investigator of KPF. "That would mean it started out like a star."

On the other hand, if the brown dwarf has an orbit aligned with the equatorial plane of its central star, then "it most likely migrated inward similar to planets via interactions with the disk in which it formed," Giacalone says.

The results revealed GPX-1b is not tilted in its orbit, but that it circles in a plane that aligns with the host star's equator.

"This is only one data point, and preliminary, but it suggests that the brown dwarf migrated close to its companion star in a similar manner to planets," says Giacalone, who presented the results at the 243rd meeting of the American Astronomical Society (AAS) in New Orleans on January 10, 2024. "Theory has predicted that brown dwarfs should be able to form like planets, but observational evidence is only just beginning to be gathered to support that idea."

The result contrasts with what is known about brown dwarfs with wide separations from their companion stars. "The wide-separation brown dwarfs are known to have high orbital tilts and do not form in a disk, but rather, like stars," Giacalone says. "The short-separation ones like GPX-1b, on the other hand, probably do form in the disk if they have low orbital tilts, meaning they form like planets. In other words, we think brown dwarfs can form either like stars or planets."

KPF, a high-precision spectrograph, was able to determine the orbital inclination of the object by watching it pass in front of, or transit, its star. The brown dwarf was discovered by NASA's TESS (Transiting Exoplanet Survey Satellite) mission and the Galactic Plane eXoplanet Survey (GPX) in 2021. It is one of a small number of brown dwarfs known to pass in front of, or transit, its host star.

The researchers hope to use KPF to study the orbital inclinations of more brown dwarfs in the future. "We have demonstrated the power of KPF for studying these systems," Giacalone says. "Because close-in brown dwarfs are so rare, they are mostly found around relatively faint and distant stars. That means we need large telescopes like Keck and advanced instruments like KPF to study them accurately."