When is something too small to be a star, yet too massive to be a planet? When it's a brown dwarf, otherwise known as a "failed star." But if you think the failed star moniker is a little pessimistic, you'll be excited to hear that astronomers have discovered a special brown dwarf that's more starlike than we ever thought a brown dwarf could be.
Brown dwarfs are an exotic kind of celestial object. Thought to have approximate masses between 13 and 80 Jupiters, they can be defined neither as massive planets nor tiny stars; they're entirely different substellar curiosities that possess qualities of both. They're the bridge between the most massive planets and the tiniest of stars.
Stars are stars because they're massive enough (and therefore have strong enough gravities) to sustain fusion in their dense cores. Our sun, for example, is a "yellow dwarf" star that is approximately halfway through its 10-billion-year life span, fusing 600 million tons (544 million metric tons) of hydrogen per second.
Astronomers classify stars according to their luminosity (brightness) and their surface temperature on the Hertzsprung-Russell Diagram. Starting at the brightest and hottest (a surface temperature of around 30,000 Kelvin) are "O" class stars, then "B", "A", "F", "G", "K" to "M" in descending order of temperature. Brown dwarfs start at class "M6.5" (known also as late-M dwarfs, less than 3,000 Kelvin) and continue through "L", "T" and "Y" – Y being the coolest. The coolest class Y dwarfs can have temperatures as low as 250 Kelvin (negative 23 degrees C).
Brown dwarfs are not considered stars because they are too small to fuse hydrogen in their cores – they don't have the gravitational oomph in their core to sustain hydrogen fusion, but, depending on how massive they are, they do have enough mass to sporadically fuse elements like lithium and deuterium.
Supermassive Jupiters? Supersmall Stars?
Our buddy Jupiter is a massive planet that has a thick atmosphere with a core and a layered differentiation of chemicals in its gaseous atmosphere. But if Jupiter were 13 times more massive and considered a small brown dwarf, it would start to exhibit some star-like qualities. For example, brown dwarfs exhibit convection in their atmospheres. Like boiling water in a kettle, the material is heated near the brown dwarfs' cores, causing it to rise. When convection currents reach the surface, they emit infrared radiation, cool and sink back to the interior. Planets like Jupiter do not exhibit this behavior; their atmospheric chemicals form layers where large-scale convection isn't possible.
But brown dwarfs don't just exhibit starlike convection currents, they also have pretty impressive magnetic fields. Case in point: A brown dwarf called LSR J1835+3259 was studied and found to be magnetically active, according to a September 2017 study published in the Astrophysical Journal. In fact, it's so active that it rivals our sun's magnetism.
Located around 18.5 light-years away, LSR J1835+3259 is estimated to be 55 times the mass of Jupiter. During the observation campaign, the researchers noted the polarization of infrared light being emitted from the brown dwarf. This technique can reveal the magnetic conditions near the surface of the brown dwarf.
What they found came as a surprise: As the object rotated, a powerful magnetic region came into view, more powerful than the magnetic fields associated with sunspots that we observe on the sun. Sunspots are magnetically active regions that can trigger coronal mass ejections, solar flares and produce powerful streams of solar wind — all of which can generate powerful geomagnetic storms on Earth.
In an interview with New Scientist, the researchers point out that LSR J1835+3259 is very young (approximately 20 million years old) and the powerful magnetic field could be interacting with the object's protoplanetary disk (if it has one). But if this active magnetic region is long-lasting and representative of its global magnetic field, LSR J1835+3259 is way more "star-like" than we give brown dwarfs credit for.
So rather than calling brown dwarfs "failed stars," perhaps we should call them overachieving planets or magneto-dwarfs.