Using the 10-m Keck II telescope at the W.M. Keck Observatory and Subaru Telescope, a team of astronomers has studied the orbits of 27 long-period giant planets and brown dwarfs in their planetary systems. Combined with modeling of the orbits, the data allowed the researchers to determine that the brown dwarfs in these systems formed like stars, but the gas giants formed like planets.
An artist’s impression of a brown dwarf and its parent star. Image credit: Sci-News.com.
Brown dwarfs are cool, dim objects that have a size between that of a gas giant, such as Jupiter or Saturn, and that of a Sun-like star.
Sometimes called failed stars, they are too small to sustain hydrogen fusion reactions at their cores, yet they have star-like attributes.
Typically, they have masses between 11-16 Jupiters (the approximate mass at which deuterium fusion can be sustained) and 75-80 Jupiters (the approximate mass to sustain hydrogen fusion).
Dr. Brendan Bowler from the University of Texas at Austin and colleagues wanted to settle the question: are gas giant planets on the outer fringes of planetary systems the tip of the planetary iceberg, or the low-mass end of brown dwarfs?
Using the Near-Infrared Camera, second generation (NIRC2) instrument on the Keck II telescope, as well as the Subaru Telescope, they took images of giant planets and brown dwarfs as they orbit their parent stars.
They combined their new observations of 27 systems with all of the previous observations published by other astronomers or available in telescope archives.
At this point, computer modeling comes in. They created an orbit-fitting code called Orbitize!’ which uses Kepler’s laws of planetary motion to identify which types of orbits are consistent with the measured positions, and which are not.
The code generates a set of possible orbits for each companion. The slight motion of each giant planet or brown dwarf forms a ‘cloud’ of possible orbits. The smaller the cloud, the more the researchers are closing in on the companion’s true orbit. And more data points — that is, more direct images of each object as it orbits — will refine the shape of the orbit.
“Rather than wait decades or centuries for a planet to complete one orbit, we can make up for the shorter time baseline of our data with very accurate position measurements,” said Dr. Eric Nielsen, an astronomer at Stanford University.
“A part of Orbitize! that we developed specifically to fit partial orbits, OFTI (Orbits For The Impatient), allowed us to find orbits even for the longest period companions.”
Finding the shape of the orbit is key: objects that have more circular orbits probably formed like planets. That is, when a cloud of gas and dust collapsed to form a star, the distant companion (and any other planets) formed out of a flattened disk of gas and dust rotating around that star.
On the other hand, the ones that have more elongated orbits probably formed like stars.
In this scenario, a clump of gas and dust was collapsing to form a star, but it fractured into two clumps.
Each clump then collapsed, one forming a star, and the other a brown dwarf orbiting around that star.
This is essentially a binary star system, albeit containing one real star and one brown dwarf.
“Even though these companions are millions of years old, the memory of how they formed is still encoded in their present-day eccentricity,” Dr. Nielsen said.
“Eccentricity is a measure of how circular or elongated an object’s orbit is.”
“The punchline is, we found that when you divide these objects at this canonical boundary of more than about 15 Jupiter masses, the things that we’ve been calling planets do indeed have more circular orbits, as a population, compared to the rest. And the rest look like binary stars,” Dr. Bowler said.
The findings were published in the Astronomical Journal.
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Brendan P. Bowler et al. 2020. Population-level Eccentricity Distributions of Imaged Exoplanets and Brown Dwarf Companions: Dynamical Evidence for Distinct Formation Channels. AJ 159, 63; doi: 10.3847/1538-3881/ab5b11
