Over the past few years, astronomers have found more than a dozen extrasolar
planets by looking for stars that 鈥渨obble鈥� due to the gravity of planets that
orbit them. But this technique only picks up very massive planets鈥攇as
giants like Jupiter. These are unlikely to be hospitable to life. 鈥淲e live on a
low-mass planet,鈥� says David Bennett, an astrophysicist at the University of
Notre Dame in Indiana. 鈥淭hose are the ones you need to look for.鈥�

In their quest for low-mass planets, Bennett and his colleagues have been
watching the way the gravity of stars magnifies the light from objects behind
them, an effect called gravitational lensing. When two stars line up with the
Earth, the gravity of the one nearest us can focus and brighten the one behind
for weeks. If the nearer star has an orbiting planet, it could add a short extra
brightness blip as it passes across our line of sight.

That鈥檚 what Bennett鈥檚 team thinks it saw on 4 July last year. In a paper
submitted to The Astrophysical Journal, they conclude that a
planet caused a 2.5-hour-long blip in the brightness of a star near the centre
of our Galaxy. From the size and timing of the blip, they calculate that the
planet may be as light as a few Earths, and orbits its star in the inner zone
where rocky planets are likely to form.

鈥淭he method is fantastic,鈥� says Keith Horne, an astrophysicist at the
University of St Andrews. But he cautions that Bennett鈥檚 finding must be
corroborated by other observations of the event: 鈥淚t鈥檚 an intriguing finding but
I wasn鈥檛 quite convinced.鈥�

Another way to search for Earth-sized planets is to look for tiny changes in
the brightness of a star as a planet passes in front of it
(New 女生小视频, 18 September, p 32).
Since 1994, a team led by Laurance Doyle of
the SETI Institute in Mountain View, California, and Hans-J枚rg Deeg of the
Institute of Astrophysics in the Canary Islands, has been attempting to catch
this via a network of telescopes round the world. The team has created computer
simulations of the way a faint double star system 55 light years away called CM
Draconis should brighten and fade when planets with various sizes and orbits
pass in front of it.

Observed brightness changes in CM Draconis matched one of these patterns,
suggesting the system may host a planet 2.5 times the size of Earth. 鈥淚t would
receive the equivalent energy that the Earth receives from the Sun,鈥� says Doyle.
鈥淚t鈥檚 smack in the middle of the habitable zone.鈥� The astronomers say the
brightness of the system should change in a predictable way in early October.
Then they will be able to say for certain whether the discovery stands up.

鈥淜nowing that neutrons are around is a big deal if you鈥檙e looking for
plutonium,鈥� says Mary Bliss, principal investigator on the project. 鈥淥ther than
a few applications in geology, there鈥檚 no reason why someone would walk through
an airport with a neutron source.鈥� However, traditional detectors鈥攚hich
consist of a metal tube filled with pressurised gas鈥攁re bulky, cannot
safely be shipped by air, and can be damaged by vibrations.

The new fibre-optic detector is light and flexible. It is made from layers of
plastic sandwiched between layers of the special fibres, which are impregnated
with cerium(III) ions and the isotope lithium-6. When a neutron hits the
sandwich, the plastic slows it down. It then collides with a lithium-6 atom,
smashing it apart and releasing a shower of electrons.

The electrons excite nearby cerium(III) ions, which emit photons of visible
light that travel to the ends of the fibres, where they can be detected. If four
or more photons are detected within 200 nanoseconds, a neutron is almost
certainly responsible.

鈥淧eople have been trying since the early 1960s to use this stuff as radiation
detectors,鈥� says Bliss. But making the necessary glass has proved to be the
stumbling block.

The trouble is that cerium(III) is easily oxidised to cerium(IV). And if
the fibres contain any cerium(IV) the detector won鈥檛 work, because cerium(IV)
will absorb any photons that are produced. To avoid contamination, the fibre
must be made in a low-oxygen atmosphere, which requires precise control of the
manufacturing process.

The laboratory has licensed the technology to Canberra Industries of Meriden,
Connecticut. The company has produced a prototype plutonium detector, and the
International Atomic Energy Agency has installed a unit at the border between
Austria and Hungary.

So far, it has not uncovered any plutonium. It was, however, triggered by a
woman on a bus who had recently received radiation therapy and was emitting
gamma rays, to which the detector is also sensitive.

Glenn Knoll, a nuclear engineer at the University of Michigan is impressed by
Bliss鈥檚 technical accomplishment, but is less convinced of the practical value
of the detector. 鈥淎lthough I can certainly recognise the novelty of what has
been done,鈥� says Knoll, 鈥淚 cannot, in all honesty, point to an important
application where I think it has made a big difference.鈥�

But plutonium detection is not the only use for these fibres. They are also
being tested at the University of Washington鈥檚 Nuclear Radiation Center in
Pullman as a way of monitoring the dose delivered to a brain tumour during
radiation therapy. Bundles of the fibre are taped to the patient鈥檚 skull and
placed in the mouth and sinuses, allowing medical staff to deliver a precise
dose of radiation.