Building a skull with physics not biology

Physics and geometry have been used to simulate how the human skull grows. At birth, the skull is a series of bone plates connected by soft fibrous boundaries called sutures. The arrangement means the skull can grow and remodel around the increasing volume of the brain. To understand the driving force behind the growth, biologists have focused on genetics and biochemistry, but some believe the mechanical stresses induced by the evolving brain are equally important. Johannes Weickenmeier from Stanford University in the US and colleagues have built a computational model that is based purely on the mechanical processes. In the simulation, the cranial vault holding the brain is treated as a semi-ellipsoid, separated into segments that represent the plates. Using existing estimates of pressures, stresses and strains related to the developing brain and bones, the team incorporated two modes of bone growth. Suture growth refers to the accretion of new bone between the skull plates, compensating for the growing volume of brain. Meanwhile, surface growth thickens the bone plates and also allows for any changing curvature by removing bone on the inside surface and producing new bone on the outer surface. As well as depicting the growth of a normal skull, the simulation successfully modelled the development of known skull deformities, confirming that it accurately represents biological processes. Exactly how the brain and skull communicate in order to grow in sync remains a mystery, but the researchers hope that incorporating biochemical processes into the model will provide an insight. With further development, the work described in Physical Review Letters could help surgeons treat infants with skull growth problems.

Most exoplanets come in two distinct sizes

Most exoplanets fall into two distinct groups – rocky Earth-like bodies and larger "mini-Neptunes". That's the conclusion of a team of astronomers in the US and Canada, who have classified 2000 of the nearly 3500 exoplanets that are known to exist in the Milky Way. The 2000 exoplanets had been discovered using NASA's Kepler space telescope, and the team used spectral data from the Keck Observatory to determine the sizes of exoplanets' host stars. This allowed the astronomers to measure the radii of the exoplanets at four-times higher precision than before – thus revealing the two distinct size groups (see figure). The Earth-like exoplanets have radii up to about 1.75 that of Earth, while the mini-Neptunes measure-up between 2–3.5 Earth radii. There is also a clear dearth of exoplanets between 1.75–2 Earth radii, according to a paper by the team to be published in The Astronomical Journal. "In the solar system, there are no planets with sizes between Earth and Neptune," says team-member Erik Petigura of Caltech. "One of the great surprises from Kepler is that nearly every star has at least one planet larger than Earth but smaller than Neptune," he adds. "We'd really like to know what these mysterious planets are like and why we don't have them in our own solar system." In a separate development, astronomers working on Kepler have released their latest survey catalogue of exoplanets, which covers the mission's first four years of observing. Kepler has so far identified over 4000 candidate exoplanets, of which 2335 have been confirmed. These include more than 30 Earth-sized exoplanets that are in the habitable zones of their stars – which means that they could harbour life.

South Korea to phase out nuclear energy

South Korean president Moon Jae-in has announced that the country will begin to phase out its nuclear-energy programme. South Korea has 25 reactors that generate around a third of the country's electricity, and in a speech yesterday at an event to mark the closure of the Kori-1 nuclear power plant, he declared that no new reactors would be built and existing units will not operate beyond 40 years. Moon says that the country would now focus on developing renewable sources of energy. "An era of clean energy that puts first the safety of the people is what our energy policies must pursue," he notes. Kori-1, which came online in 1978, is the country's oldest nuclear power plant and will now be decommissioned – the first South Korean nuclear power unit to do so.


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