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Unveiling Gliese 229B: The First Brown Dwarf Binary System Discovered

Introduction to Gliese 229B

Gliese 229B, the first identified brown dwarf, has been the subject of extensive research since Caltech scientists discovered it in 1995. Yet, the perplexing question of why it is dimmer than expected for its mass endures.

What are Brown Dwarfs?

Brown dwarfs have masses between those of stars and gas giants like Jupiter. Despite astronomers estimating Gliese 229B's mass to be 70 times that of Jupiter, its observed brightness is unexpectedly low for an object of this size.

The Breakthrough Discovery

An international team of astronomers led by Caltech has finally resolved the mystery: Gliese 229B is, in fact, a close binary system of two brown dwarfs, with masses of 38 and 34 times that of Jupiter, orbiting each other every 12 days. Their combined luminosity aligns with the expected brightness for two small brown dwarfs of this size.

Insights from Researchers

"Gliese 229B was regarded as the quintessential brown dwarf," notes Jerry W. Xuan, a graduate student collaborating with Dimitri Mawet, the David Morrisroe Professor Astronomy. "Now, we realize our previous assumptions were incorrect. It's not a single object but two, and only now are we capable of detecting such close separations."

Xuan is the lead author of a study published in Nature, entitled "The cool brown dwarf Gliese 229B is a close binary," which reports these findings.

Independent Studies Supporting the Discovery

An independent study published in The Astrophysical Journal Letters, led by Caltech graduate student Sam Whitebook and Space Telescope Science Institute associate astronomer Tim Brandt, also determined that Gliese 229B consists of a brown dwarf pair.

Implications of the Discovery

This discovery raises new questions about the formation of closely bound brown dwarf pairs and suggests the potential existence of additional brown dwarf binaries---or even exoplanet binaries--yet to be discovered. (Exoplanets are planets that orbit stars other than the sun.)

"The revelation that Gliese 229B is a binary system not only resolves the longstanding discrepancy between its mass and brightness but also enhances our broader understanding of brown dwarfs, which occupy the transitional zone between stars gas giants," says Mawet, a senior research scientist at JPL, operated by Caltech for NASA.

Historical Context of Gliese 229B

The discovery of Gliese 229B in 1995 was made by a Caltech team, featuring Rebecca Oppenheimer, who was a graduate student at the time, Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science, Keith Matthews, an instrument specialist, and other team members.

Early Discoveries

The astronomers utilized Palomar Observatory to discover methane in the atmosphere of Gliese 229B--a trait typical of gas giants, such as Jupiter, but not found in stars. This discovery signified the first confirmed observation of a category of cool star-like objects known as brown dwarfs, which had been hypothesized to bridge the gap between planets and stars roughly 30 years prior.

"Seeing an objects smaller than a star orbiting a sun for the first time was absolutely electrifying," notes Oppenheimer, co-author of the current study and an astrophysicist at the American Museum of Natural History. "It triggered widespread efforts to discover other unusual objects, but this one remained an enigma for decades."

Investigative Techniques and Observations

Nearly three decades after its discovery and following hundreds of observations, Gliese 229B continued to perplex astronomers due to its unusual dimness. Scientists suspected it might be a binary system, but, as Xuan explains, "for two brown dwarfs to go unnoticed for 30 years, they would need to be extremely close together."

Advanced Instrumentation

To distinguish Gliese 229B as two separate objects, the team employed two instruments at the European Southern Observatory's Very Large Telescope in Chile. The GRAVITY interferometer, which merges light from four telescopes, was used to spatially resolve the binary system, while the CRIRES+ instrument detected distinct spectral signatures from each object.

Analyzing Motion

The second technique measured the motion, or Doppler shift, of molecules within the atmospheres of the brown dwarfs, revealing that one body was moving toward Earth while the other was receding, alternating as they orbited one another.

"It's amazing to witness a new breakthrough nearly 30 years later," says Kulkarni, who is not an author of the current paper. "This binary system continues to astonish."

Characteristics of the Brown Dwarf Pair

Over five months of observations, it was determined that Gliese 229Ba and Gliese 229Bb, the brown dwarf pair, orbit each other every 12 days with a separation 16 times the distance between Earth and moon. Together, they orbit an M-dwarf star, smaller and redder than our sun, once every 250 years.

"These two bodies, orbiting each other at high speed, are smaller in radius than Jupiter. If we had something like them in our solar system, they would appear quite unusual in our night sky," says Oppenheimer. "This is the most thrilling and significant discovery in substellar astrophysics in decades."

Future Research Directions

The origin of this fast-spinning pair of cosmic bodies remains unknown. Some hypotheses suggest that brown dwarf pair could arise within the dense disks of material surrounding a young star. These disks might fragment into two brown dwarf seeds, which would become gravitationally linked after a close interaction. It is still uncertain whether similar processes form planetary pairs around other stars.

Looking Ahead

Looking ahead, the team aims to discover more tightly bound brown dwarf binaries using advanced instruments like the Keck Planet Imager and Characterizer (KPIC), developed by Mawet's team at the W.M. Keck Observatory in Hawai'i, and the forthcoming High-resolution Infrared SPectrograph for Exoplanet Characterization (HISPEC), currently under construction at Caltech and other institutions under Mawet's leadership.

The discovery that the first identified brown dwarf companion is a binary offers promising prospects for current efforts to locate more, says Xuan.

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