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Super-Earths —
dense or fluffy
By Nadia Drake
With exoplanets almost as numerous as
fireflies on a midsummer’s eve, two top
planet-finding missions disagree over
the abundance of low-mass planets that
are heavier than Earth but lighter than
Neptune.
The Swiss-led HARPS mission suggests that between 30 and 50 percent of
sunlike stars in the solar neighborhood
host super-Earths and sub-Neptunes.
But NASA’s Kepler mission finds that
these planets circle roughly 15 percent
of the stars in its far-flung field of view.
That discrepancy is of great interest,
because the number of planets in the
weight class just above Earth hints at
how many bodies of terrestrial proportions are likely to be discovered.
But a new report suggests there may
not be a discrepancy at all.
“We know the Geneva team does
a good job observing, and they have
a good technique. And we know the
Kepler telescope is working beautifully.
So we wanted to see if there was a plausible, believable way in which you could
have the difference between those two
surveys,” says Greg Laughlin, an astronomer at the University of California,
Santa Cruz.
In a paper posted online August 30 at
arXiv.org, he and UC Santa Cruz graduate student Angie Wolfgang propose that
there are two kinds of low-mass planets
out there, one of which is more amenable
to discovery by HARPS.
Laughlin and Wolfgang produced a
simulation based on the HARPS data.
In it, they created a population of planets between one and 17 Earth masses
around the more than 100,000 stars
being monitored by Kepler and gave the
Two major exoplanet surveys have
disagreed on the abundance of distant
planets with masses that fall between
Earth’s (left) and Neptune’s (right).
virtual planets varying characteristics
and orbital periods between one and 50
days. When peppered with two distinct
kinds of planets in the size range — one
rocky and dense, the other gaseous and
fluffy — the simulation mimics Kepler’s
data. “You can explain the results of
those surveys if you have these two distinct populations,” Laughlin says.
HARPS searches for distant planets
by looking for the telltale signs of an
orbiting planet tugging on its host star
in what’s called a radial velocity sur-
vey. Kepler uses the transit method,
monitoring a star field for blips in
brightness caused by a planet briefly
blocking some of the star’s light. These
different methods can bias the types of
planets detected — heavier planets tug
more on their stars, and bloated planets
with bigger radii block more light.
Chase continues for a lighter Higgs
Latest estimates narrow target for mass of elusive particle
By Devin Powell
In the hunt for the Higgs boson, the
world’s most powerful particle collider has
tightened the net. Ne w data from CERN’s
Large Hadron Collider near Geneva narrow the range of allowable masses for the
hypothetical particle, whose existence
would confirm the mechanism thought
to give mass to other particles.
To fit with the standard model of particle physics, the Higgs must now be
lighter than 145 billion electron volts, or
GeV, team members from LHC’s ATLAS
and CMS experiments reported August
22 in Mumbai, India, at the International Symposium on Lepton Photon
Interactions at High Energies.
This new limit goes beyond previous
results from the Tevatron at the Fermi
National Accelerator Laboratory in Bat-
avia, Ill. — which directly excluded 156 to
177 GeV by looking for debris left behind
when the Higgs breaks down, and indi-
rectly ruled out masses above 185 GeV.
