58.7
Earth days
time it takes
mercury to
rotate on its axis
88
Earth days
time it takes
mercury to travel
around the sun
New scenario for
planetary births
theory offers way to explain
diversity of extrasolar orbs
By Ron Cowen
A radical new theory that planets are
born within a massive veil of gas may
help explain how recently discovered
extrasolar planets developed their stunning diversity of sizes and locations.
In the theory, planets are born under
wraps, hidden at the centers of giant
gas clouds far from their parent stars. A
star’s gravity then reels in the planetary
cloud, stripping away some or all of the
gas to reveal the planet inside.
Depending on how much of the gas
is removed in the process, the unveiled
planet would resemble a gas giant like
Jupiter, a solid core with a layer of gas like
Neptune, or a solid body like Earth. Sergei
Nayakshin of the University of Leicester
in England describes the process in an
upcoming Monthly Notices of the Royal
Astronomical Society as well as in several
papers posted online at arXiv.org.
Such a beginning might explain
the abundance of small-to-middling
extrasolar planets spotted by NASA’s
Kepler spacecraft in orbits within roast-
ing distance of their stars (SN: 2/26/11,
p. 18). Standard planet-formation models
can’t easily account for
the many types of exo-
planets described since
Mercury viewed close-up
naSa’s meSSenger spacecraft has returned the first images
ever taken by a probe orbiting the solar system’s innermost
planet. recorded on march 29, the historic close-ups show
parts of mercury’s south polar terrain never before viewed by a
spacecraft. naSa released the first image taken from orbit on
march 29 and several others march 30. the first image (left)
shows two craters and part of the south polar region, resolving
features as small as about 5 kilometers across. other portraits
show details of a surprisingly high number of secondary craters,
small pockmarks created during the formation of larger craters.
“that’s the barest hint of what we’ll have on a regular basis,”
says meSSenger lead scientist Sean Solomon of the Carnegie
institution for Science in Washington, D.C. by the mission’s end
in march 2012, the craft will have taken some 75,000 images
that will create a global mosaic of mercury. — Ron Cowen
www.sciencenews.org
april 23, 2011 | science news | 9
