Meet the Sun’s Chilly New Neighbor

Infrared observations have uncovered a cool brown dwarf that’s only about 7 light-years away. The object is one of the closest stellar systems to the Sun and the coolest brown dwarf yet discovered.

closest stars to the Sun

This diagram illustrates the locations of the star systems closest to the Sun, shown with the year when the distance to each system was determined. Two of the four are brown dwarf systems.
NASA / Penn State University

Astronomy is a science pursued at a distance. Most of the light we see from distant stars and galaxies takes thousands to millions of years to reach us. That makes our solar neighborhood a valuable place for detailed observations: the closest companions to the Sun are benchmarks, because they are the easiest stars to study in detail.

While the census of the solar neighborhood has tallied more stellar citizens over time, most of the newly discovered neighbors have been relatively distant, usually at least 30 to 60 light-years away. But recently, an astronomer from Penn State discovered a solar neighbor about 7 light-years away, and it’s a "cool" result in more ways than one!

Using data from NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope, Kevin Luhman recently discovered an object known as WISE J085510.83−071442.5. The object is special for quite a few reasons. First, it is right next door, in astronomical terms. At 7.2 light-years away (6.5 to 8 is the error range), this is likely the fourth closest stellar system ever detected, farther only than the Alpha Centauri triple system (4.2 light-years), Barnard’s star (5.9 light-years) and the brown dwarf binary WISE J104915.57−531906.1 (6.6 light-years). (It displaces Wolf 359, which lies 7.8 light-years away).

Second, it’s moving fast. Using infrared images obtained by WISE and Spitzer, Luhman noticed the object was traveling extremely quickly across the sky in between images. Part of this motion is from parallax, the apparent back-and-forth change in position with respect to background objects. This motion is caused by Earth orbiting around the Sun: the closer a star is to the Sun, the larger its apparent shift in position as we look at it from different sides of our orbit. It’s the same effect you see if you hold your finger up at arm's length and blink your eyes one at a time: you will notice your finger appears to move back and forth as you blink each eye. If you move your finger closer to your face, that effect increases.

proper motion of brown dwarf

This animation shows WISE J085510.83−071442.5, the coldest brown dwarf yet seen and the fourth closest system to our sun. Kevin Luhman (Penn State) discovered this dim object through its rapid motion across the sky, shown here in a compilation of images spanning four years. The object was first seen in two infrared images taken 6 months apart in 2010 by WISE. Two additional images of the object were taken with NASA's Spitzer Space Telescope in 2013 and 2014.
NASA / JPL-Caltech / Penn State University

WISE J0855−0714’s parallax allowed Luhman to infer that WISE J0855−0714 was close to the Sun. But the object’s parallax is small compared with its proper motion, which is its apparent motion across the sky from point A to point B over time. The object is traversing 8.1 arcseconds per year, the third largest proper motion of any object outside the solar system (second only to Barnard’s star and Kapteyn’s star). In comparison, most of the brightest stars have a proper motion of a few tenths of an arcsecond per year or less — for example, Rigel only moves 0.004 arcsecond per year.

The other thing that makes WISE J0855−0714 "cool" is that it really is cold! Using images of the object taken in different filters, Luhman estimated its temperature to be about 250 kelvin, or about 10 degrees below zero in Fahrenheit. This makes WISE J0855−0714 not only the coldest neighbor to the Sun but also the coldest brown dwarf ever discovered.

This combination of close, fast, and cold makes WISE J0855−0714 unique among all of the solar neighborhood members. As Luhman states in a press release, “It is very exciting to discover a new neighbor of our solar system that is so close. In addition, its extreme temperature should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”

The discovery of WISE J0855−0714 points out just how important large-scale surveys of the sky, such as WISE, really are. This cold brown dwarf was discovered relatively close to the plane of our Milky Way, which astronomers often avoid because "crowding" can occur — that is, there are so many stars along the galactic plane that it can be tough to tell one from the other, especially when they are moving. (Luhman actually had to use multiple filters to separate WISE J0855−0714 from the signals of two stationary background objects in order to study it.) But as Luhman has shown, this may be a fertile hunting ground for finding more close companions to the Sun.

 

Reference: K. Luhman. "Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun." Astrophysical Journal Letters, May 10, 2014.

CATEGORIES
News, Stellar Science
John Bochanski

About John Bochanski

John Bochanski is a physics professor at Rider University in Lawrenceville, NJ. John uses large surveys of the sky to study its coolest members, from nearby low-mass stars and brown dwarfs, to some of the most distant red giant stars ever discovered.

4 thoughts on “Meet the Sun’s Chilly New Neighbor

  1. Anthony Barreiro

    Yes, very cool! If I’m interpreting the coordinates correctly, this brown dwarf is in the western end of Hydra, about 11 degrees south of the head of the snake, very near the fifth magnitude double star 15 Hydrae. I like knowing where things are in the sky. Maybe I’ll have a look in that direction after dark this evening, and wave to our new neighbor.

  2. Alderamin

    Some say, at 3-10 Jupiter masses, it’s actually a rogue planet. It’s definitely below the limit for ever fusing even deuterium, but it may have formed like a star from a collapsing cloud. It depends on what definition of a brown dwarf is adopted, what you might call this object.

  3. PeteDeGraff

    The article identifies proper motions but, I feel, should make clear that such motions are in reference to earth – which makes me wonder who is heading where and how fast? Some articles have spoken to that but usually only where earth is heading.
    What ever happened to THE GREAT ATTRACTOR?
    (Alan Dressler, Borzoi Book, Alfred A. Knopf, 1994)
    And of course, when you get way out there, what do you use as a “ground” or “fixed” reference?

  4. BDMayfield

    I agree Dieter, this is a rogue planet. Per IAU’s definition brown dwarfs start at about 13 Jupiter masses. It shouldn’t matter how it formed, which might be extremely hard to discover. This makes this object the first rogue planet, which is bigger news than the discovery of just another “common” brown dwarf.

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