Even though the stars are millions of light years away, they shine in the night sky because they are incredibly hot. Planets are much colder. Brown dwarfs are an astronomical conundrum: They’re larger than planets but smaller than stars and don’t quite fit into either category.

Sometimes astronomers refer to these objects as “failed stars.” So if you fail as a star, can you succeed as a planet? In other words, could it become a star or a brown dwarf planet? It’s an interesting idea, but for many astronomers the answer is no. “Stars and planets are two different things depending on how they form,” said Covey Rose, a postdoctoral researcher in astronomy at the University of Sydney in Australia.

Stars (and possibly brown dwarfs) form when clouds of gas coalesce under influence gravity. According to Rose, stars are supportive thanks to their massive mass. nuclear fusionit turns hydrogen into heavier elements: first helium, then carbon, then iron. According to the International Astronomical Union, brown dwarfs aren’t big enough to fuse ordinary hydrogen, but they can briefly fuse a heavier version known as deuterium.

Like planets, brown dwarfs cool as they age. But Rose said that many stars explode when they die, “scattering a whole lot of other material into the universe.” New stars are formed from the remains of gas. At the same time, the heavier elements rotate around them in disks. “This leftover material will form pebbles and planetesimals and eventually planets,” Rose said.

Stars and brown dwarfs are made up of gases, while planets are made up of heavier elements. So by definition, a star can never turn into a planet.

But definitions are changing. For most of history, stars and planets were defined by patterns of motion in the sky. Later, people learned that the planets revolved around the stars. Astronomers now know that stars (and brown dwarfs) can orbit each other, and that some planets orbit nothing.

Brown dwarfs, hypothesized only in the 1960s and observed in the 1990s, further complicate conventional definitions. Evgenia Shkolnyk, professor of astrophysics at Arizona State University, said these were “something their own, not failed stars or planets created with the help of soup.”

Rose agreed. But even the third category cannot fully capture the nuances of these celestial bodies. “The better we begin to understand things, the better we understand that things exist on a spectrum,” Rose said.

Often stars, brown dwarfs, and planets are defined by their size: stars are at least 80 times larger than themselves. Jupiter (roughly the mass needed to fuse hydrogen) Brown dwarfs are 12 to 80 times larger than Jupiter (roughly the mass needed to fuse deuterium), Rose said.

But many celestial bodies complicate these categories. Some planets, such as gas giant Jupiter, may become large enough to gravitationally collect gas in their rocky cores, and some of these gas planets are large enough to fuse deuterium. Meanwhile, brown dwarfs lose their ability to synthesize deuterium over time. At the other end of the spectrum, Rose recently spotted the coolest brown dwarf known to emit radio waves, a feature more commonly associated with stars.

Because of these coincidences, many astronomers prefer to describe these objects by their origins rather than their masses. As long as these celestial bodies are described by how they formed, brown dwarfs will never become planets; but stay tuned.

“I think you could compare that to the debate over Pluto’s planetary status,” Rose said. Pluto was downgraded to “dwarf planet” in 2006 as the International Astronomical Union changed its definitions; Some brown dwarfs may also be reclassified as planets in the future.

And while a star may never be a single planet, it can become part of many stars; for the same reason people say “we are all made of stardust”. When a star explodes, Shkolnyk said, “that star will eventually be transformed into next-generation planets and everything else.” “I think it’s wonderful.” Source