Ultra-Cool Star Detected

Robert Minchin and Barbara Kennedy
June 13, 2012

Astronomers using the world’s largest radio telescope, at Arecibo, Puerto Rico, have discovered flaring radio emissions from an ultra-cool star, shattering the previous record for the lowest stellar temperature at which radio waves were detected and possibly boosting the odds of discovering life elsewhere in the universe.

Alex Wolszczan

Alex Wolszczan

The team, led by Alex Wolszczan, Evan Pugh Professor of Astronomy and Astrophysics at Penn State, has been using the 305-meter (1000-foot) telescope to look for radio signals from a class of objects known as brown dwarfs: small, cold stars that bridge the temperature gap between Jupiter-like giant planets and hotter, more massive, hydrogen-fusing stars. The astronomers hit the jackpot with a star named J1047+21, a brown dwarf 33.6 light years away in the constellation Leo.

“This object is the coolest brown dwarf ever detected emitting radio waves—it’s half the temperature of the previous record holder, making it only about five times hotter than Jupiter,” according to team member Matthew Route, a graduate student at Penn State. That means it’s much smaller and colder than our Sun, Route adds. With a surface temperature not much higher than that of a giant planet, and a size comparable to Jupiter’s, it is scarcely visible in optical light. Yet the radio flares detected at Arecibo show it must have a strong magnetic field, implying that the same could be true of other similar stars.

glowing magenta orb with background of dark starry sky
R. Hurt/NASA

An artist’s impression of a brown dwarf similar to J1047+21.

The discovery dramatically broadens the window through which astronomers can study the atmospheres and interiors of these tiny stars, using the radio detection of their magnetic fields as a tool. At the temperature of this brown dwarf, the researchers say, its atmosphere must be made of neutral gas, which would not give off radio signals like those seen. The energy to drive the signals is likely to come from magnetic fields deep inside the star, similar to the field that protects the Earth from dangerous high-energy particles. By monitoring the radio flares from J1047+21, astronomers will be able to tell how stable the magnetic field is over time, and, from flare duration, they can infer the size of the emitter itself.

The possibility that young, hot planets around other stars could be detected in the same manner—because they still maintain strong magnetic fields—has implications for the chances of finding life elsewhere in our Milky Way galaxy, Wolszczan explains. “The Earth’s field protects life on its surface from harmful particles of the solar wind. Knowing whether planetary magnetic fields are common or not throughout the galaxy will aid our efforts to understand chances that life may exist beyond the solar system.”

Adds Wolszczan, discoverer of the first planets ever found outside our solar system: “We hope that in the future we’ll be able to detect yet colder brown dwarfs, and possibly even giant planets around other stars.”

Alex Wolszczan, Ph.D., is Evan Pugh Professor of Astronomy and Astrophysics and director of the Center for Exoplanets and Habitable Worlds, alex@astro.psu.edu. Matthew Route, M.A., is a Ph.D. student in astronomy and astrophysics.

Last Updated June 14, 2012