Brown dwarfs are stellar objects small enough that they never end up fusing hydrogen atoms and becoming fully-fledged stars. They generally have masses between 13 and 75 times the mass of Jupiter, making these objects a sort of missing link between giant planets and stars. And here lies a mystery. Is there a smooth transition from giant planet to star? A new study says no, strengthening the evidence researchers have been collecting for years.
As reported in The Astronomical Journal, a team of astronomers looked at the formation processes of giant planets and brown dwarfs. They studied 27 stellar systems where these faint companions orbit stars. The observations were combined with models to reveal that brown dwarfs and giant planets are different because they form in different ways.
The difference is evident in the orbits of the two companions. Giant planets are formed in more circular orbits primed for the steady but slow accretion of material, while brown dwarfs form through the collapse of a cloud of gas and dust, just like a star. This collapse puts them on more peculiar orbits around their star.
“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,” team leader Brendan Bowler, from the University of Texas at Austin, said in a statement. “And the rest look like binary stars.”
The observations for this project did not come fast. The objects are located very far from their star so the team took pictures every year for decades, in some cases, to work out their orbits. They had to combine the observations with computer simulations that generate possible orbits. By measuring the eccentricity of these orbits (how squished they were), the team could work out which objects formed like planets and which formed like stars.
“Even though these companions are millions of years old, the memory of how they formed is still encoded in their present-day eccentricity,” team member Eric Nielsen of Stanford University added.
The team admits that the current study is limited as it only looked at 27 objects, so they are planning to use the European Space Agency’s Gaia satellite to find more candidates to expand their research.