NASA WISE Discovers Coldest Brown Dwarf Neighbour of the Sun

This artist's conception shows a newfound object named WISE J085510.83-071442.5, the coldest known brown dwarf. 

Credit: Penn State University/NASA/JPL-Caltech

A brown dwarf as cold as the North Pole has been discovered lurking remarkably close to our solar system, and it appears to be the coldest of its kind yet found, scientists say.

Using NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope, astronomers discovered the dim, "failed star" lurking just 7.2 light-years away, making it the fourth closest system to our sun.

"It's very exciting to discover a new neighbor of our solar system that is so close," Kevin Luhman, an astronomer at Pennsylvania State University's Center for Exoplanets and Habitable Worlds, said in a statement.

"And given its extreme temperature, it should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures."

This diagram illustrates the locations of the star systems closest to the sun. 

Credit: Penn State University

Brown dwarfs are sometimes called failed stars because they have many of the elements of that make up stars, but they lack the huge mass needed to kick off nuclear fusion in their core.

As a result, these objects don't radiate starlight and they sometimes resemble planets.

Some are even cool enough to have atmospheres much like gas giants.

While brown dwarfs are hidden in images taken in the visible spectrum, infrared telescopes like WISE can pick up the meager glow of brown dwarfs.

Luhman and colleagues first spotted the object in WISE data. It appeared to be moving quite fast, hinting that it was close by.

The team then investigated the object using Spitzer and the Gemini South telescope on Cerro Pachon in Chile to measure its distance and temperature.

"It is remarkable that even after many decades of studying the sky, we still do not have a complete inventory of the sun's nearest neighbours," Michael Werner, the project scientist for Spitzer at NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a statement.

Dubbed WISE J085510.83-071442.5, our newfound neighbor is now the record-holder for the coldest brown dwarf, with a temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius), Luhman and colleagues say.

The previous record holders were more tepid, chilling only to room temperature.

At 3 to 10 times the mass of Jupiter, the object also may be one of the least massive brown dwarfs ever found, the astronomers say.

Because it is so small, the scientists say it's possible that the body is actually a planet ejected from its star system, but brown dwarfs are known to be quite common cosmic objects.

The findings were described April 21 in The Astrophysical Journal.

NASA WISE: Coldest brown dwarfs blur lines between stars and planets

This artist's conception portrays a free-floating brown dwarf, or failed star. 

A new study shows that several of these objects are warmer than previously thought with temperatures about 250-350 degrees Fahrenheit. 

Credit: NASA/JPL-Caltech

Astronomers are constantly on the hunt for ever-colder star-like bodies, and two years ago a new class of such objects was discovered by researchers using NASA's WISE space telescope.

However, until now no one has known exactly how cool their surfaces really are - some evidence suggested they could be room temperature.

A new study shows that while these brown dwarfs, sometimes called failed stars, are indeed the coldest known free-floating celestial bodies, they are warmer than previously thought with temperatures about 250-350 degrees Fahrenheit.

To reach such low surface temperatures after cooling for billions of years means that these objects can only have about 5 to 20 times the mass of Jupiter.

Unlike the Sun, these objects' only source of energy is from their gravitational contraction, which depends directly on their mass.

Trent Dupuy

"If one of these objects was found orbiting a star, there is a good chance that it would be called a planet," says Trent Dupuy, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.

But because they probably formed on their own and not in a proto-planetary disk, astronomers still call these objects brown dwarfs even if they are "planetary mass."

Characterizing these cold brown dwarfs is challenging because they emit most of their light at infrared wavelengths, and they are very faint due to their small size and low temperature.

To get accurate temperatures, astronomers need to know the distances to these objects.

"We wanted to find out if they were colder, fainter, and nearby or if they were warmer, brighter, and more distant," explains Dupuy.

Using NASA's Spitzer Space Telescope, the team determined that the brown dwarfs in question are located at distances 20 to 50 light-years away.

Locations of brown dwarfs: The locations of brown dwarfs discovered by NASA's Wide-field Infrared Survey Explorer, or WISE, and mapped by NASA's Spitzer Space Telescope, are shown here in this diagram. 

The view is from a vantage point about 100 light-years away from the sun, looking back towards the constellation Orion. 

At this distance our sun is barely visible as a speck of light. The vastly fainter brown dwarfs would not even be visible in this view. The red lines all link back to the location of the sun. 

Credit: NASA/JPL-Caltech

To determine the distances to these objects the team measured their parallax - the apparent change in position against background stars over time.

As the Spitzer Space Telescope orbits the Sun its perspective changes and nearby objects appear to shift back and forth slightly.

The same effect occurs if you hold up a finger in front of your face and close one eye and then the other. The position of your finger seems to shift when viewed against the distant background.

The new data also present new puzzles to astronomers that study cool, planet-like atmospheres. Unlike warmer brown dwarfs and stars, the observable properties of these objects don't seem to correlate as strongly with temperature.

This suggests increased roles for other factors, such as convective mixing, in driving the chemistry at the surface.

This study examined the initial sample of the coldest brown dwarfs discovered in the WISE survey data.

Additional objects discovered in the past two years remain to be studied and will hopefully shed light on some of these outstanding issues.