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Close, Cold Neighbour of Sun

By David Reneke

The coldest brown dwarf ever known has been discovered only 7.2 light-years away.

NASA’s Wide field Infrared Survey Explorer (WISE) and Spitzer Space Telescope have discovered what appears to be the coldest brown dwarf known – a dim, star-like body that is as frosty as the Earth’s North Pole. At a distance of only 7.2 light-years away, it is the fourth closest system to our Sun. The closest system, a trio of stars, is Alpha Centauri, a little over 4 light years away.

“It’s very exciting to discover a new neighbour of our solar system that is so close,” said Kevin Luhman, an astronomer at Pennsylvania State University’s Centre for Exoplanets and Habitable Worlds. “And given its extreme temperature, it should tell us a lot about the atmospheres of planets, which often have similarly cold temperatures.”

Brown dwarfs start their lives like stars, as collapsing balls of gas, but they lack the mass to burn nuclear fuel and radiate starlight. The newfound coldest brown dwarf, WISE J085510.83-071442.5, has a chilly temperature between –48°C to –13°C. The previous record for the coldest brown dwarf was about room temperature.

WISE was able to spot the rare object because it surveyed the entire sky twice in infrared light, observing some areas up to three times. Cool objects like brown dwarfs can be invisible when viewed by visible light telescopes, but their thermal glow, even if feeble, stands out more in infrared light.

After noticing the fast motion of this enigmatic body in March 2013, Luhman analysed additional images taken with Spitzer and the Gemini South telescope in Chile. Spitzer’s infrared observations helped determine the frosty temperature of the brown dwarf. Combined detections from WISE and Spitzer, taken from different positions around the Sun, enabled the measurement of its distance through the parallax effect.

WISE J085510.83-071442.5 is estimated to be 3–10 times the mass of Jupiter. It could be an ejected gas giant similar to Jupiter, but scientists estimate it’s probably a brown dwarf as they’re fairly common. If so, it is one of the least massive brown dwarfs known.

SKA to Chase Stellar Bursts

It is hard to imagine that any astronomical phenomenon could escape our latest and most powerful telescopes, but an international research team has now forecast some of the exotic discoveries that will only be able to be studied with the forthcoming Square Kilometre Array (SKA).

The team, including Dr Davide Burlon and Dr Tara Murphy from the University of Sydney’s School of Physics, has calculated that the SKA will reveal the lingering footprints of tens of thousands of enigmatic cosmic explosions known as gamma-ray bursts.

“With current telescopes, we see a bright gamma-ray burst somewhere in the universe around once per day, but new radio telescopes will soon be able to see an afterglow of the explosion after the initial burst has faded away,” explains Burlon. “This afterglow can generally take weeks to gradually decay, and teaches us incredible amounts about both the initial explosion and its neighbourhood.”

The catch is that a gamma ray burst is not an explosion that can be seen from all directions, but comprises a very narrow and energetic jet. Therefore an observer needs to be looking down the barrel of the jet at the right time or it will be invisible.

However, the radio afterglow should be visible from any direction and for long periods of time, even if the burst is missed. These afterglows without a burst are a phenomenon that astronomers have been looking for without success – until now. The unprecedented sensitivity and wide field of view of the SKA means that orphan afterglows should be visible for months or even years before eventually disappearing, bubbling across the sky more than 10,000 times per year.

Of course, the SKA’s view of the sky will be full of all sorts of objects and events, such as supernova explosions and flaring black holes that are more common than orphan afterglows. The SKA will join the Australian SKA precursor telescope ASKAP and the South African SKA precursor MeerKAT in painting an entirely new picture of the “radio sky”.

The SKA will not only allow us to finally see these orphan afterglows but help us understand how gamma ray bursts produce such powerful, narrow jets, and will cast new light on the big question of just what causes gamma-ray bursts in the first place.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com