When you buy through links on our articles, Future and its syndication partners may earn a commission.An artist’s illustration of a brown dwarf. | Credit: Nazarii Neshcherenskyi/iStock / Getty ImagesMany astronomical objects play by clear rules and fit into neat categories, but brown dwarfs (celestial objects too massive to be mere planets, but too small to be real stars) continue to refuse to cooperate.Astronomers recently studied a sample of 70 objects, ranging from Jupiter-mass planets to brown dwarfs that are right on the brink of stardom. By looking for a relationship between the mass of these objects and certain features of their star systems (like whether the host star contained elements heavier than helium, or how round the objects’ orbits were), the researchers hoped to draw a clear line that divides massive objects that form like stars and smaller ones that form like planets. But they were destined for disappointment, because the actual universe is messy and complicated.AdvertisementAdvertisementAs it turns out, the line between stars and planets might be more of a gray, fuzzy continuum, according to University of California Los Angeles astrophysicist Gregory Gilbert and his colleagues in a recent paper published in The Astronomical Journal.Planets and stars form differently — except for that group in the middleStars, by definition, boast at least 80 times the mass of Jupiter, and they form from the outside in. When a clump of gas in a molecular cloud collapses under its own gravity, the densely-packed atoms at its core start fusing together, releasing heat and light; a star is born.Giant gas planets of sizes up to about Jupiter’s mass, on the other hand, form from the inside out. First, a few grains of dust clump together in the disk of material around a newborn star, and their combined gravity is enough to start attracting even more dust. Material keeps piling on, faster and faster, building up a rocky core surrounded by thick layers of gas.In between, however, there’s a whole slew of objects that astronomers aren’t sure whether to classify as “failed stars” or “overgrown planets.”A size comparison between planets, brown dwarfs and the smallest stars. | Credit: NASA/JPL-CaltechAt between 13 and 80 times the mass of Jupiter, brown dwarfs aren’t quite massive enough to fuse hydrogen into helium like a real star, but they’re just big enough to fuse deuterium, an isotope of hydrogen that includes a neutron along with the standard proton and electrons. (Weirdly, deuterium requires less pressure to fuse into helium than straight hydrogen does.) And then there are “sub-brown dwarfs,” gas giants which are truly gargantuan by planet standards, but they’re not quite large enough to be proper brown dwarfs.AdvertisementAdvertisementIdeally, there should be a clear line: objects above a certain mass should be failed stars that formed from collapsing gas clouds, and objects b …
