Two Discoveries Reveal Diversity of Galaxy's
Extrasolar Planets
(From the May/June
2006 issue of StarDate magazine)
Radio astronomers studying a disk of planet-forming material
around a star 500 light-years from Earth have found that the
inner disk rotates in the opposite direction of the outer disk.
This never-before-seen behavior means that planets that form
close to this star will orbit it one direction, and planets formed
farther out in the other.
A five-Earth-mass planet orbits a red dwarf star.
Anthony Remijan of the National Radio Astronomy Observatory
and Jan Hollis of NASA's Goddard Space Flight Center used the
Very Large Array radio telescope in New Mexico to track molecules
of silicon monoxide as they orbited in the disk of this star.
The molecules emit radio waves at known frequencies. These frequencies
shift higher or lower, depending on the molecules' motions toward
or away from Earth. These "Doppler shifts" revealed
the opposite rotations of the inner and outer disk.
Though never seen around a star, "similar structures and
dynamics commonly occur on small and large scales throughout
the universe," Hollis said.
The two astronomers speculate that this star was formed by the
merger of two large gas clouds rotating in opposite directions.
A large group of astronomers has found the least massive extrasolar
planet yet. Only five times more massive than Earth, it orbits
a small, cool red dwarf star every 10 years at about three times
the Earth-Sun distance.
"This finding means that Earth-mass planets are not that
uncommon," team member Kailash Sahu of the Space Telescope
Science Institute said. "If we found one, there must be
more."
Astronomers took advantage of a phenomenon called "gravitational
microlensing" to discover the planet. When a massive star
passes precisely in front of a background star, the gravity of
the foreground star acts like a lens, amplifying the light coming
from the background star. The more massive the foreground star,
the longer this "light boost" lasts. And if the foreground
star has an unseen planetary companion, that lengthens the boost,
too. In this case, the lensing event lasted a dozen hours longer
than expected — revealing the presence of the unseen planet.
— Rebecca Johnson
» More information about extrasolar planets
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