Astronomers discover local star's cool companion

Washington, April 21: An international team, led by astronomers at the University of Hertfordshire in the UK, has discovered one of the coolest sub-stellar bodies ever found outside our own solar system.

The stellar companion orbits the red dwarf star Wolf 940, some 40 light years from Earth.

According to Dr Ben Burningham, of the University of Hertfordshire, "Although it has a temperature of 300 degrees Celsius, which is almost hot enough to melt lead, temperature is relative when you study this sort of thing, and this object is very cool by stellar standards."

"In fact, this is the first time we've been able to study an object as cool as this in such detail. The fact that it is orbiting a star makes it extra special," he said.

The object is thought to have formed like a star, but has ended up looking more like Jupiter.

It is roughly the same size, despite being between 20 and 30 times as heavy, and when the infrared spectral "fingerprints" of the two objects are compared, their resemblance is striking.

The new object orbits its star at about 440 times the distance at which the Earth orbits the sun. At such a wide distance, it takes about 18,000 years to complete a single orbit.

Too small to be stars, so-called "brown dwarfs" have masses lower than stars but larger than gas giant planets like Jupiter.

Due to their low temperature, these objects are very faint in visible light, and are detected by their glow at infrared wavelengths.

In most cases, astronomers don't initially know much about the age and composition of brown dwarfs and this can make it hard to tell where the models are right, and where they are going wrong.

"What's so exciting in this case, is that we can use what we know about the primary star to find out about the properties of the brown dwarf, and that makes it an extremely useful find," explained Dr Burningham.

"You can think of it as a Rosetta Stone for decrypting what the light from such cool objects is telling us," he said.

The object has been named Wolf 940B, after the red dwarf star that it orbits.

"Red dwarfs are the most populous stars in the Galaxy, and systems like this may be more common than we know," said Dr David Pinfield of the University of Hertfordshire.

"As the generation of ongoing large scale surveys continues, we may discover a pack of Wolf-940B-like objects in our solar back yard," he added.

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