In 1995, astronomers first confirmed the discovery of a brown dwarf, a body that was too small to be a star and too large to be a planet – sort of an astronomical tweener. But it turns out that wasn’t the whole story.
Researchers have now taken a fresh look at that brown dwarf and discovered that it is not actually one brown dwarf, but two of them orbiting a smaller star surprisingly close to each other. It was documented in two new studies using telescopes in Chile and Hawaii.
These two brown dwarfs are gravitationally bound to each other in what is called a binary system, an arrangement commonly seen between stars. So the brown dwarf that was named Gliese 229b three decades ago is now identified as Gliese 229ba, which has a mass 38 times that of Jupiter, the largest planet in our solar system, and Gliese 229bb, which has a mass of . 34 times more than Jupiter.
They are located 19 light years from our solar system – quite close in cosmic terms – in the constellation Lepus. A light year is the distance that light travels in one year, 5.9 trillion miles (9.5 trillion km).
An artist’s illustration shows the brown dwarf closest to Earth. ESO-I. Crossfield-N. Risinger/Handout via Reuters
Binary brown dwarfs are rare. Both of them orbit each other every 12 days at a distance of only 16 times the distance between the Earth and the Moon. Only one other pair of brown dwarfs is known to orbit as close to each other as this pair.
Brown dwarfs are neither stars nor planets, but something in between. They can be considered to be aspiring stars that did not reach the mass required to ignite nuclear fusion in their core like a star during their early stages. But they are more massive than even the largest planets.
Sam Whitebook said, “A brown dwarf is an object that fills the gap between a planet and a star. They are formally defined as objects that can burn the heavier form of hydrogen, Which is called deuterium, but is not the most common basic form of hydrogen.” graduate student in Caltech’s Division of Physics, Mathematics, and Astronomy and lead author of a study published in Astrophysical Journal Letters.
“In practice, this means that their mass ranges from about 13 to 81 times the mass of Jupiter. Because they cannot fuse hydrogen, they cannot ignite the fusion channels that power most stars. This dulls their shine. They become quieter,” Whitebook said.
The year 1995 was a big one for astronomers, when the discovery of the first planet beyond our solar system – an exoplanet – was also announced. Until the discovery of Gliese 229b, the existence of brown dwarfs was only hypothesized. But there were discrepancies about Gliese 229b, especially when its mass was measured to be about 71 times that of Jupiter.
“It doesn’t make sense because an object of that mass would be much brighter than Gliese 229b,” said Caltech astronomer Jerry Juan, lead author of a study published in the journal Nature. “In fact, some models predict that objects with masses greater than 70 Jupiter masses will fuse hydrogen and become stars, which was clearly not happening here.”
The new observations were able to identify two different brown dwarfs. They orbit a common type of star called a red dwarf, whose mass is about six-tenths that of our Sun. While both brown dwarfs are more massive than Jupiter, their diameter is actually smaller than that of the gas giant planet because they are more dense.
“We still don’t really know how different brown dwarfs form, and what the transition between a giant planet and a brown dwarf is. The boundary is unclear,” Xuan said. “This discovery also shows us that brown dwarfs can come in strange configurations that we didn’t expect. It shows how complex and messy the star formation process is. We should always be open to surprises.”
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