Are brown dwarfs stars, planets or not?

The universe is filled with billions of massive celestial objects, from stars to planets, comets to asteroids. But what happens when lines are blurred between these classifications and we simply can not place an object in a major category?

Well, brown dwarves know it better than anyone. They are much too massive to be planets, but not so massive that the hydrogen atoms fuse in their nuclei and become stars. They have straddled the border between the planet and the star since their first confirmation in 1995, and it looks like they will not come out soon.

New search, published in the The astrophysical journal, shows that two recently surveyed brown dwarfs exceed the mass limit known for their classification, leading researchers to guess their current theoretical models. Their results further complicate the ongoing debate to define the differences in mass, brightness and temperature between stars and brown dwarfs.

Blurred lines

In terms of their training, both are actually set up to live a similar life. Stars form when huge clouds of dust and gas collapse on themselves and condense into hot nuclei that absorb matter that surrounds it. If the nucleus becomes dense enough, nuclear fusion is triggered and the hydrogen atoms begin to convert to helium, causing it to burst into the sky for billions of years. In order for this life-changing fusion to take place, it is believed that the stars must be at least 70 to 73 times the mass of Jupiter, about 7% of the mass of our Sun.

Brown dwarves are thought to be formed in the same way, but their nuclei simply do not become dense enough for nuclear fusion to occur – excluding them from the official star classification. And since they form differently and exceed the maximum weight of a planet, which is about 13 times the mass of Jupiter, they are also excluded from the status of the planet. These middlemen have since been dubbed brown dwarfs, but creating a clear line around their classification is still outstanding.

"Understanding the boundary between stars and brown dwarfs will enhance our understanding of shape and evolution, as well as their potential ability to accommodate habitable planets," said lead researcher Serge Dieterich of the Carnegie Institution for Science. in a press release.

To further explore their nature, Dieterich and his team of researchers focused on Epsilon Indi B and Epsilon Indi C, two brown dwarfs located just 12 light-years away from Earth. Using data from Carnegie Astrometric Planet astronomical research and the research consortium of the Cerro Tololo Inter-American Observatory parallel observatory, they were able to observe the perceived movements of brown dwarves in front of distant stars, illuminating them and allowing the team to determine their masses.

They discovered that Epsilon Indi C was just at the theoretical threshold of nuclear fusion, reaching 70 times the mass of Jupiter, and that Epsilon Indi B actually exceeded it, weighing 75 times the mass of Jupiter. But even if they fall into star status on paper, none of these brown dwarfs unleashed the fusion of hydrogen that would turn them into real stars.

"Taken together, our results mean that existing models need to be revised," said Dieterich. "We have shown that the heaviest brown dwarfs and the lightest stars have only slight differences in mass. But despite this, they are destined for different lives – one is racing to cool off and the other for billions of years.

And so the saga continues. There is no doubt that clear statistics on brown dwarfs would help us understand how they form and evolve, how many of them could be scattered throughout the universe, and whether habitable planets could exist in their orbit. But in the end, would it be so bad to simply crush the debate between the stars and the planet and let the brown dwarves settle in a class of their own? Only time will tell.