Space Briefs
Cold brown dwarfs blur line
between stars and planets
In 2011, astronomers on the hunt for the coldest star-like celestial bodies
discovered a new class of such objects using NASA’s Wide-Field Infrared
Survey Explorer (WISE) space telescope. But until now, no one knew exactly
how cool the bodies’ surfaces really are. In fact, some evidence suggested
they could be at room temperature.
A new study using data from NASA’s Spitzer Space Telescope shows that
while these so-called brown dwarfs are indeed the coldest known freefloating celestial bodies, they are warmer than previously thought, with
surface temperatures ranging from about 250 to 350 degrees Fahrenheit (125
to 175 degrees Celsius). By comparison, the sun has a surface temperature of
about 10,340 degrees Fahrenheit (5,730 degrees Celsius).
To reach these surface temperatures after cooling for billions of years, these
objects would have to have masses of only five to 20 times that of Jupiter.
Unlike the sun, the only source of energy for these coldest of brown dwarfs is
from their gravitational contraction, which depends directly on their mass. The
sun is powered by the conversion of hydrogen to helium; these brown dwarfs
are not hot enough for this type of “nuclear burning” to occur.
The findings help researchers understand how planets and stars form.
“If one of these objects were found orbiting a star, there is a good chance
that it would be called a planet,” said Trent Dupuy, a Hubble Fellow at the
Harvard-Smithsonian Center for Astrophysics and a co-author of the study,
appearing online in the journal Science Express. But because they probably
formed on their own and not in a planet-forming disk orbiting a more massive
star, astronomers still call these objects brown dwarfs even if their mass is of
planetary size.
Characterizing these cold brown dwarfs is challenging because they emit
most of their light at infrared wavelengths and are very faint due to their small
size and low temperature.
To get accurate temperatures, astronomers need to know the distances to
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