* Astronomy

Astronomers using NASA's Spitzer Space Telescope have discovered of the smallest known brown dwarf.

The brown dwarf contains only about 8 times the mass of Jupiter, and is surrounded by a dusty disk. It is even smaller than several planets around other stars, leading to the question of whether any objects that form from the disk around it should be considered planets or moons.


This artist's conception compares a hypothetical solar system centred around a tiny "sun" (top) to a known solar system centred around a star about the same size as our Sun called 55 Cancri. Astronomers using a combination of ground-based and orbiting telescopes, including NASA's Spitzer Space Telescope and the Hubble Space Telescope, discovered the beginnings of such a miniature solar system 500 light-years away in the Chamaeleon constellation.
The tiny system consists of an unusually small "failed" star, or brown dwarf called Cha 110913-773444, and a surrounding disk of gas and dust that might one day form planets. At a mass of only eight times that of Jupiter, the brown dwarf is actually smaller than several known extrasolar planets. The largest planet in the 55 Cancri system is about four Jupiter masses.
The astronomers, lead by Kevin Luhman at Penn State, speculate that the disk around Cha 110913-773444 might have enough mass to make a small gas giant and a few Earth-sized rocky planets, as depicted here around the little brown dwarf.
Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA
Position(2000): RA = 11h07m45s, Dec = 77°40'00"

"There are two camps when it comes to defining planets versus brown dwarfs. Some go by size, and others go by how the object formed. For instance, this new object would be called a planet based on its size, but a brown dwarf based on how it formed" - Giovanni Fazio, Harvard-Smithsonian Centre for Astrophysics (CfA), a member of the discovery team.

The tiny brown dwarf, called Cha 110913-773444, is the smallest known brown dwarf to harbour what appears to be a planet-forming disk of rocky and gaseous debris.
A team led by Kevin Luhman of Penn State University discusses this finding in the December 10 issue of The Astrophysical Journal Letters. Luhman led a similar observation in 2004 that uncovered a 15-Jupiter-mass brown dwarf with a protoplanetary disk .
If the protoplanetary disk surrounding Cha 110913-773444 does form into planets, the whole system would be a miniaturized version of our solar system-with the central "sun," the planets and their orbits all roughly 100 times smaller.


GNIRS spectrum of the candidate young brown dwarf Cha 1109-7734 compared to spectra of the known young brown dwarf OTS 44 and the old cool dwarf LHS 2065 (M9V). Like the latter two objects, Cha 1109-7734 exhibits broad, deep absorption in H2O,demonstrating that it has a late spectral type. The weak K I and Na I absorption lines and the triangular shape of the continuum between1.5 and 1.8 µm in the spectrum of Cha 1109-7734 indicate a low surface gravity, and hence young age, like that of OTS 44. The spectra are displayed at a resolution of R = 200 and are normalized at 1.68 µm.

"Our goal is to determine the smallest `sun' with evidence for planet formation. Here we have a sun that is so small it is the size of a planet. The question then becomes, what do we call any little bodies that might be born from this disk: planets or moons?" - Kevin Luhman.

Brown dwarfs are born like stars, condensing out of thick clouds of gas and dust. But unlike stars, brown dwarfs do not have enough mass to sustain nuclear fusion. They remain relatively cool objects visible in lower-energy wavelengths such as infrared.


This graph of data from NASA's Spitzer Space Telescope shows that an extraordinarily low-mass brown dwarf, or "failed star," is circled by a disc of planet-building dust. The brown dwarf, called OTS 44, is only 15 times the mass of Jupiter, making it the smallest known brown dwarf to host a planet-forming disc. Spitzer was able to see this unusual disc by measuring its infrared brightness. Whereas a brown dwarf without a disc (red dashed line) radiates infrared light at shorter wavelengths, a brown dwarf with a disc (orange line) gives off excess infrared light at longer wavelengths. This surplus light comes from the disc itself and is represented here as a yellow dotted line. Actual data points from observations of OTS 44 are indicated with orange dots.
Credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

With Spitzer, the science team spotted Cha 110913-773444 about 500 light-years away in the southern constellation Chamaeleon.
This brown dwarf is young, only about 2 million years old.
The team studied properties of the brown dwarf with infrared instruments at other observatories, but the cool, dim protoplanetary disk was only detectable with Spitzer's Infrared Array Camera, which was developed at Harvard-Smithsonian Centre for Astrophysics .

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