Record-breaking photo reveals a planet-sized object as cool as Earth
Astronomers classify this object as a brown dwarf, an object that formed just like a star out of a massive cloud of dust and gas.
October 20, 2011
The photo of a nearby star and its orbiting companion — whose temperature is like a hot summer day in Arizona — will be revealed by Kevin Luhman from Penn State in State College, Pennsylvania, during a presentation at the Signposts of Planets conference at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
An artist's impression of the coldest imaged companion, named WD 0806-661 B, (right foreground) orbiting at a large distance from a white dwarf — the collapsed-core remnant of a dying star. Credit: NASA Goddard Space Flight Center/Francis Reddy
“This planet-like companion is the coldest object ever directly photographed outside our solar system,” said Luhman. “Its mass is about the same as many of the known extrasolar planets — about six to nine times the mass of Jupiter — but in other ways it is more like a star. Essentially, what we have found is a very small star with an atmospheric temperature about as cool as the Earth’s.”
Luhman classifies this object as a brown dwarf, an object that formed just like a star out of a massive cloud of dust and gas. But the mass that a brown dwarf accumulates is not enough to ignite thermonuclear reactions in its core, resulting in a failed star that is cool. In the case of the new brown dwarf, the scientists have gauged the temperature of its surface to be between 80° and 160° Fahrenheit (27° and 71° Celsius) — possibly as cool as a human.
Ever since brown dwarfs first were discovered in 1995, astronomers have been trying to find new record holders for the coldest brown dwarfs because these objects are valuable as laboratories for studying the atmospheres of planets with Earth-like temperatures outside our solar system.
Astronomers have named the brown dwarf WD 0806-661 B because it is the orbiting companion of an object named WD 0806-661 — the white dwarf core of a star that was like the Sun until its outer layers were expelled into space during the final phase of its evolution. “The distance of this white dwarf from the Sun is 63 light-years, which is very near our solar system compared with most stars in our galaxy,” Luhman said.
“The distance of this white dwarf from its brown-dwarf companion is 2,500 astronomical units (AU) — about 2,500 times the distance between the Earth and the Sun — so its orbit is very large as compared with the orbits of planets, which form within a disk of dust swirling close around a newborn star,” said Adam Burgasser from the University of California, San Diego. Because it has such a large orbit, the astronomers say this companion most probably was born in the same manner as binary stars, which is known to be separated as far apart as this pair, while remaining gravitationally bound to each other.
To make their discovery, Luhman and his colleagues searched through infrared images of more than 600 stars near our solar system. They compared images of nearby stars taken a few years apart, searching for any faint points of light that showed the same motion across the sky as the targeted star. “Objects with cool temperatures like the Earth are brightest at infrared wavelengths,” Luhman said. “We used NASA’s Spitzer Space Telescope because it is the most sensitive infrared telescope available.”
Luhman and his team discovered brown dwarf WD 0806-661 B moving in tandem with white dwarf WD 0806-661 in two Spitzer images taken in 2004 and 2009.
In a related new discovery involving a different cool brown dwarf, John Bochanski and his colleagues from Penn State have made the most detailed measurement yet of ammonia in the atmosphere of an object outside our solar system. “These new data are much higher quality than previously achieved, making it possible to study, in much more detail than ever before, the atmospheres of the coldest brown dwarfs, which most closely resemble the atmospheres that are possible around planets,” Bochanski said.
“Brown dwarfs that are far from their companion stars are much easier to study than are planets, which typically are difficult to observe because they get lost in the glare of the stars they orbit,” Burgasser said. “Brown dwarfs with Earth-like temperatures allow us to refine theories about the atmospheres of objects outside our solar system that have comparatively cool atmospheres like that of our own planet.”