Coldest brown dwarf ever discovered just around the corner

Astronomers are constantly pushing our knowledge of galaxies farther toward the edge of the visible Universe, and yet we tend to forget that our local neighbourhood has not yet been fully mapped.  Just last month astronomers discovered the sixth-closest system to us, the brown dwarf WISE J085510.83-071442.5, which is also the record holder for the coolest star around.

Brown dwarfs are sub-stellar objects with masses between those of stars and planets (roughly between 10 and 80 times the mass of Jupiter). Like stars, brown dwarfs are thought to form from the collapse of molecular clouds, but their low mass prevents them from igniting nuclear reactions in their cores and shining brightly like our own Sun. Because they are much cooler than stars, the atmospheres of brown dwarfs contain  complex molecules like methane, which is seen in the giant gas planets of our Solar System. This makes brown dwarfs unique tools to investigate both the low-mass end of star formation and the chemistry of planetary atmospheres.

At the time of their formation, brown dwarfs have a typical surface temperature around 1,000 K and will cool down as they get older.  This means that they cannot be seen at optical wavelengths and infrared telescopes are needed to detect brown dwarfs.  In the last few years, the Wide-field Infrared Survey Explorer (WISE) has nearly doubled the number of known brown dwarfs within 30 light-years from the Sun. By surveying the entire sky several times, WISE has not only been able to detect a large number of brown dwarfs, but also measure their proper motion (which is their angular velocity as they are seen to move across they sky). By following the motion of brown dwarfs over several years, astronomers can then determine their distance from the Earth using parallax.

WISE J085510.83-071442.5 is the latest brown dwarf discovered by WISE. Not only is it the closest brown dwarf to us at a distance of just 7.2 light years (compared to the closest star system alpha Centauri at a distance of about 4 light-years), but it’s also the coldest with a temperature of about 240 K, or about -30 C.

Kevin Luhman of Pennsylvania State University’s Center for Exoplanets and Habitable Worlds noticed that WISE J085510.83-071442.5 was moving pretty fast when comparing images from WISE that were six months apart.  The fast motion compared to the other stars in the images suggested that the object was very close to the Earth.  Luhman analysed images from the Spitzer Space Telescope and the Gemini North telescope in Hawaii to further track the motion of WISE J085510.83-071442.5, as well as help determine its temperature.  Combined the detections from WISE and Spitzer, taken from different positions around the Sun, enabled the measurement of its distance through parallax.

The coldest and closest brown dwarf yet known, WISE J085510.83-071442.5 was discovered through its rapid motion across the sky. It was first noticed in two infrared images taken six months apart in 2010 by NASA’s Wide-field Infrared Survey Explorer, or WISE (orange triangles). Two additional images taken with NASA’s Spitzer Space Telescope in 2013 and 2014 (green triangles) show its continued motion across the sky. The four images were used to measure a distance of 7.2 light-years using the parallax effect. (Credit: ASA/JPL-Caltech/Penn State)

Its incredibly low temperature implies that WISE J085510.83-071442.5 is very old, as it would take several billion years for a brown dwarf to cool from a thousand degrees to its current chilly temperature. The estimated mass is extremely low, between 3-10 Jupiter masses.  This makes WISE J085510.83-071442.5  the closest, coldest, oldest and lowest mass sub-stellar objects detected to date!  Such a low mass puts it in the mass range of giant planets, which opens the intriguing possibility that WISE J085510.83-071442.5 may not be a brown dwarf, but might in fact be a giant planet that originally formed around a star and was later ejected by gravitational interactions with other planets in the system. Unfortunately, there is no way to directly test this scenario and Luhman excluded the hypothesis on a statistical basis, given that there brown dwarfs are expected to be common while the frequency of ejected planets is unknown.

Whatever the real nature of WISE J085510.83-071442.5, this exciting discovery shows that, even after many decades of studying the sky, our local neighbourhood is still full of surprises.

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[Luca Cortese and Sarah Maddison]





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