This animation shows the coldest brown dwarf yet seen, and the fourth closest system to our sun. Credit: NASA/JPL-Caltech/Penn State University
WISE 0855, a tiny brown dwarf, first made headlines in 2014 for being the coldest known object located outside of our solar system. Two years later, WISE 0855 has a new claim to fame that is even more astounding—it’s the first known object located outside of our solar system to have water. Though computational models suggested that astronomers could expect to find water in the atmosphere of an object this cold, its discovery is no less remarkable. WISE 0855’s water abundance is strikingly similar to Jupiter’s, leaving many of us wondering:
If celestial bodies orbit WISE 0855 like the moons of Jupiter, could there be life around WISE 0855?
An artist’s rendition of WISE 0855. The blue color isn’t real. We don’t know what color it is because it’s barely visible in optical light. Credit: NASA/JPL-Caltech/Penn State University
To answer this question, we have to understand the nature of a planet’s habitable zone. A habitable zone is the ideal distance of a planet from its host star, and it’s ruled by a number of factors concerning how heat stabilizes liquid water. If a planet is too close to its sun, it will receive too much heat and its water will evaporate, if it is too far away, the water will freeze. A planet’s orbital path is also a factor: if the orbit keeps the planet at a comfortable distance from its sun some of the time but not others, then it won’t receive energy consistently throughout its year, and thus, won’t be able to maintain the presence of liquid water. Size is another factor: the smaller the host star, the smaller the habitable zone.
With all these factors at play, it may seem unlikely that we’d find life anywhere else in our solar system—especially on cold moons surrounding other planets–but there’s one other key factor to consider—planet heat. Moons obtain their heat not only from the Sun, but also their host planet—and in the case of both gas giant planets and brown dwarfs like WISE 0855, we’re talking enough heat to possibly stabilize water. That’s because both brown dwarfs and gas giants have chemical reactions happening in their core causing them to radiate heat.
An artist’s rendition of a debris disk surrounding a brown dwarf where a planet might form. Credit: NASA/JPL
The amount of heat depends on the mass of the object. Our Sun obviously radiates the most heat in our solar system, and a gas giant like Jupiter produces a fraction of that. So how much heat does WISE 0855 radiate? Considering it’s about 300 degrees Fahrenheit warmer than Jupiter, it will produce more heat, but it doesn’t have a Sun to add to the total. WISE 0855 is its own sun.
Is its own heat enough? Unlike Jupiter, WISE 0855 is not a planet. But it’s also not a star. Brown dwarfs don’t have enough mass to fuse hydrogen within their cores, the way that “real” stars do. Without hydrogen fusion, the brown dwarf lacks a constant heat source in its center, causing it to continually cool over time.
As WISE 0855 loses its heat, its habitable zone becomes smaller—and smaller and smaller until it doesn’t exist anymore. While we stay safe in our Sun’s habitable zone for its entire lifetime, a planet around WISE 0855 would meet a very cold end. So if there is a planet orbiting this dwarf brown at the perfect distance, the chances that it could sustain life for long look pretty slim. But that doesn’t stop astronomers from looking closer anyway. With larger and more sensitive telescopes on the way, we may soon see WISE 0855 making headlines again.