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Brown dwarf as cool as coffee found

ESO / L. Calcada

This artist's impression shows the pair of brown dwarfs named CFBDSIR 1458+10. Observations with ESO's Very Large Telescope and two other telescopes have shown that this is the coolest pair of brown dwarfs found so far. The cooler of the two components (in the background) is a candidate for the brown dwarf with the lowest temperature ever found - the surface temperature is similar to that of a cup of hot coffee. The two components are both about the same size as Jupiter.

Astronomers have found a star that's only as hot as a cup of coffee, making it a candidate for the coldest star known. That is, assuming it's a star.

While a cup of coffee may sound hot — the newly discovered object is about 200 degrees Fahrenheit (100 degrees Celsius) — our sun is about 10,000 degrees F (5,500 degrees C). So, by comparison, it really is quite cold.

The object is considered a brown dwarf, a cosmic misfit that's cold enough to blur the lines between small cold stars and big hot planets. Astronomers consider brown dwarfs failed stars because they lack the mass and gravity to trigger the nuclear reactions that make stars shine brightly.

The newly discovered brown dwarf, identified as CFBDSIR 1458+10B, is the dimmer member of a binary brown dwarf system located about 75 light-years from Earth.

"In terms of its physical properties, it is really much closer to typical gas planets that are being found by radial velocity surveys than most brown dwarfs we know about," Michael Liu of the University of Hawaii's Institute for Astronomy told me.

Unique properties
Indeed, with a coffeelike temperature, astronomers expect the brown dwarf to exhibit the properties of a gas giant planet, such as the presence of water clouds in its atmosphere.

"We think as we accumulate more data on its different colors and its spectra we should be able to learn more about its atmosphere, and that will be very unique," Liu added. For example, the color of hotter brown dwarfs is largely shaped by the presence of sodium and potassium atoms in the atmosphere.

At cooler temperatures, according to theory, the sodium and potassium will lock themselves into molecules such as potassium chloride and be removed from the atmosphere. Liu and colleagues have asked for time on the Hubble Space Telescope to observe the system in optical wavelengths.

"The optical colors of this object should be very different than any previously known brown dwarf," Liu said.

Dwarf vs. planet
But is it a brown dwarf? One definition astronomers use to differentiate between stars, brown dwarfs and planets is mass. Anything below about 13 Jupiter masses doesn't get hot enough or dense enough to fuse anything in its interior, so it considered a planet.

The smaller member of the binary system is estimated to be between six and 15 Jupiter masses. "So, most likely it is below the line, and so by that measure one could call it a planet," Liu said.

"But at the same time, I think most of us (astronomers) feel that the way in which something forms, which is difficult to measure, probably has something to do with the way we should classify them," he added.

And this is without a doubt a binary star system – that is two stars orbiting around a common center of mass. Liu and colleagues think it likely formed as low-mass tail of the star forming process, which is different than the way a gas giant planet forms in a disc of material around a star.

"Even though it formed in a different way, it is very low mass, very little energy is coming out and it is very, very cool," Liu said. "And it is a significant leap over the previous coolest known brown dwarf."

More cool dwarfs
Today's announcement comes days after another cool brown dwarf, CFBDS J005910.83-011401.3 located about 40 light years from our solar system, was pronounced as possibly the coldest. This free floating star is about 660 degrees F (350 C).

In addition, two recent brown dwarf discoveries from the Spitzer Space Telescope are also contenders for the coolest objects known, though their temperatures are less well constrained.

Preliminary measurements put the coolest of the Spitzer discoveries at 86 degrees F – about perfect beach weather – and orbiting a white dwarf star at a distance of about 2,500 times that of Earth and the sun, Discovery News reports.

Further observations of the Spitzer object with other telescopes, Liu said, should help pin down its temperature.

"My bet is that will turn out to be slightly colder than our object," he noted. "But I think the real interesting thing … is that all three objects may be in this completely new regime that people have been trying to get into for several years."

New tools
Key to the discovery of the brown dwarf Liu is reporting was adaptive optics instrumentation on the Keck II Telescope in Hawaii, which essentially cancels out much of Earth's atmospheric interference.

This allowed the astronomers to confirm the double dwarfs were linked. The Canada-France-Hawaii Telescope was used to determine distance to the duo.

The binary system was originally detected in 2010 with the European Southern Observatory's Very Large Telescope in Chile, though it was thought to be a single object – albeit with a temperature cool enough to make it the third coldest star known.

"We were very excited originally to see this object had such a low temperature, but we never guessed that it would turn out to be a binary star and have an even more interesting, even colder companion," Phillipe Delorme of the University of Grenoble, said in a press release.

The researchers are reporting their discovery in an upcoming issue of Astrophysical Journal. The paper is available online.

More stories on cool brown dwarfs:

John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).