Astronomers in the past few years have discovered many planets orbiting distant stars beyond our solar system. Now, they have found another type of object around a distant star - a celestial body halfway between a planet and a star called a "brown dwarf."
A twin to our sun at the relatively close cosmic distance of 60 light years is host to a type of failed star known as a brown dwarf. Brown dwarfs are a class of dim stars 50 to 75 times more massive than the gas giant Jupiter, but too small to ignite the nuclear fires that drive a real star. Because they lack intensity, sightings are rare.
But University of Hawaii astronomer Michael Liu led a team that photographed a brown dwarf around a star in the constellation Sagitta. He told the American Astronomical Society convention in Washington that the object is about two billion kilometers from the star, roughly the same distance between the planet Uranus and our sun. "Our finding," he said, "shows that brown dwarf companions do exist at the same distances that giant planets do around our own solar system. This tells us that there is a diversity of other solar systems possible around stars like the sun."
The photograph of the brown dwarf is a triumph of a new technology called adaptive optics, which sharpens the view of ground telescopes normally blurred by the movement of air currents.
Astronomers have found dozens of planets around distant stars since 1995, but this is the closest body observed directly. The other objects were not detected by direct visual observation. Rather, their presence was inferred by measuring the subtle effect of their gravitational pull on the orbit of the star. So far, this technique has detected only massive objects relatively near their star.
However, adaptive optics allows direct observation of objects further away from their stars, raising hope that smaller, habitable, Earth-like planets can be found. Mr. Liu's team used the adaptive optics on two of the world's biggest telescopes, the Keck and Gemini North observatories in Hawaii.
Astronomer Ray Jayawardhana of the University of California at Berkeley is employing the same instruments to track down a possible brown dwarf in a quadruple star system 900 light years from Earth. "What adaptive optics does for us from the ground is that it corrects for this blurring so that you get much sharper images than ever before," he said. "The technique works by actually having a secondary mirror, which is a very thin mirror, that flexes into different shapes a few times every second to correct for the atmosphere's blurring."
Michael Liu's brown dwarf discovery is like detecting a firefly next to a bright lamp. He said it could mean that planetary systems form in many ways, not just by the collapse of a swirling disk of gas around young stars. Mr. Liu said, "Many of the standard models for how we think giant planets form in our own solar system probably cannot explain something like this new brown dwarf companion. It's probably too heavy to have formed at least in the standard picture that we think." Mr. Liu says he is searching other stars to learn how frequent brown dwarfs are.
At the Carnegie Institution in Washington, astronomer Alan Boss says that both brown dwarf searches are only a tantalizing appetizer for what is to yet come as scientists continue the hunt for planets. He said, "These adaptive optics techniques in the next few years most likely are going to achieve another major discovery in the era of extra-solar planets, which is they are likely to find the first evidence for a newly forming planet. They have now demonstrated that the have the ability to do it. It's just a question of time to see which ones are going to be the right ones [and] which ones turn out to be background stars."
Mr. Boss says it would be interesting to find if planets cohabit with the newly discovered brown dwarf.